Opinion
TRENDS in Biochemical Sciences
Vol.31 No.1 January 2006
Mitochondrial modulation: reversible
phosphorylation takes center stage?
David J. Pagliarini1,2,3 and Jack E. Dixon1
1
Departments of Pharmacology, Cellular and Molecular Medicine, and Chemistry & Biochemistry, University of California San
Diego, 9500 Gilman Drive, La Jolla, CA 92093-0721, USA
2
Biomedical Sciences Graduate Program, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0721, USA
3
Current addresses: Department of Medicine, Harvard Medical School, Boston, MA 02114, USA and Center for Human Genetics
Research, Massachusetts General Hospital, Boston, MA 02114, USA
In the past 1.5 billion years, mitochondria have evolved
from oxygen-scavenging bacterial symbionts into primary control centers for energy production and cellular
life-and-death processes in eukaryotes. This maturation
of mitochondrial function has necessitated the coevolution of various mechanisms of communication with the
rest of the cell. Emerging evidence indicates that
reversible phosphorylation, the most prevalent form of
cellular posttranslational modification, is an important
and largely overlooked means of regulating mitochondrial functions. The steadily increasing number of
reported mitochondrial kinases, phosphatases and
phosphoproteins suggests that phosphorylation is likely
to emerge as a common theme in the regulation of
mitochondrial processes.
Introduction
Mitochondria, once merely considered the ‘powerhouses’
of the cell, are now understood to be central to diverse
cellular functions. Mitochondria have crucial roles in
apoptosis, reactive oxygen species production, numerous
metabolic processes including the production of more than
90% of cellular ATP, and are at the heart of more than 40
known human diseases [1–3]. The complexity and
pervasiveness of mitochondrial activity reflect the need
for a sophisticated system of communication with the rest
of the cell. As cells grow and divide, new mitochondria
have to be made – a process that requires careful
coordination of nuclear and mitochondrial DNA transcription and translation. As cellular energy needs change,
mitochondria must respond rapidly by tuning their ATP
output. In a similar manner, cells must have mechanisms
to detect, and to respond appropriately to, improperly
functioning mitochondria. It is perhaps not surprising,
then, that the means of communication to and from
mitochondria have evolved to include ions, gases,
metabolites, hormones, transcription factors and myriad
other proteins [4–6] (Figure 1). These observations have
fostered a view of mitochondria as true centers for
receiving, integrating and transmitting cellular signals.
A means of signaling that is likely to emerge as a
cornerstone of mitochondrial regulation is reversible
Corresponding author: Dixon, J.E. ([email protected]).
Available online 5 December 2005
phosphorylation [7–9]. The concept that mitochondria
house machinery to perform protein phosphorylation is
certainly not new. In fact, the first demonstration of a
protein kinase event, reported in 1954, was the phosphorylation of casein with a mitochondrial extract (by
what very well might have been mitochondrial casein
kinase!) [10]. Likewise, the fact that a core mitochondrial
function can be regulated by reversible phosphorylation
was discovered nearly four decades ago when Lester Reed
and colleagues [11] identified the E1 subunit of the
pyruvate dehydrogenase complex (PDC) as the first
mitochondrial phosphoprotein. But while the field of
signal transduction burgeoned in the 1980s and 1990s
with the elucidation of phosphorylation cascades that
traverse the plasma membrane and extend through the
cytosol to the nucleus, reports of mitochondrial phosphorylation events were scarce.
For understandable reasons, the mitochondrion has
been viewed as an unlikely site for signaling by reversible
phosphorylation. In 1983, Wong and Goldberg reported
that only w2% of the total cellular tyrosine kinase activity
was found in mitochondrial fractions [12]. A computational analysis predicted that only w5% of yeast protein
kinases are targeted to mitochondria [13]. Similarly, the
closest bacterial relative to mitochondria, Rickettsia
prowazekii, is predicted to have only four protein kinases
encoded in its entire genome [14]. Furthermore, the bulk
of the mitochondrial protein machinery lies behind two
lipid bilayers, the second of which is largely impermeable
even to protons – a fact that seemingly places these
proteins out of the reach of cytosolic signaling cascades. Is
it realistic, then, to consider the mitochondrion as an
active arena for signaling by reversible phosphorylation?
Is the regulation of the PDC by phosphorylation a rare
exception or indicative of a larger overlooked theme? A
gradual stream of reports over the past few decades
implicating new signaling molecules in mitochondrial
functions suggests the latter.
Mitochondrial phosphoproteins
The ubiquitous nature of phosphorylation is perhaps best
captured by the estimation that a third of cellular proteins
are phosphorylated at some point in their lifetime [15].
Many studies have attempted to determine the extent to
www.sciencedirect.com 0968-0004/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibs.2005.11.005
Opinion
TRENDS in Biochemical Sciences
Vol.31 No.1 January 2006
27
Signaling in
Effect
Signaling out
Mediator
Increased respiration
and ATP production
O2
Ca2+
Regulation of dehydrogenases,
carriers and apoptosis
Ca2+
S-nitrosylation, regulation
of COX, DLDH, etc.
Regulation of PDC, OXPHOS,
apoptosis, etc.
Mitochondrial biogenesis
Membrane structure,
mitochondrion-to-ER signaling
Cell survival or cell
death signals
Mediator
NO
Kinases/
phosphatases
ATP
ROS
Transcription
factors
Phospholipids
Cyt c
TCA
intermediates
Target or effect
Calcineurin or
kinases
Insulin secretion,
kinases,
energy-requiring
processes, etc.
Redox regulation;
various signaling
cascades
Apoptosis
Insulin secretion
Apoptotic
proteins
Ti BS
Figure 1. Signaling processes to and from mitochondria. The extensive roles of mitochondria in maintaining cellular homeostasis have created the need for diverse means of
communication. Ions, gases, metabolites, phospholipids, transcription factors and myriad other proteins with signaling roles to or from mitochondria have now been
described [4–6]. Abbreviations: Cyt c, cytochrome c; DLDH, dihydrolipoamide dehydrogenase; ER, endoplasmic reticulum; NO, nitric oxide; OXPHOS, oxidative
phosphorylation; ROS, reactive oxygen species; TCA, tricarboxylic acid cycle.
which this theme is upheld in mitochondria by assessing
the profile of mitochondrial phosphoproteins. Early
studies that took precautions to limit phosphorylation
events to those occurring from matrix-produced ATP in rat
liver mitochondria found only a few obvious phosphoproteins [16]. The development of various new reagents
including phospho-specific antibodies and dyes capable of
detecting steady-state levels of phosphorylation, the
assessment of mitochondria from a broader range of
tissues, and more focused studies of individual mitochondrial proteins, however, challenge these early data. In fact,
a review of the current literature reveals that more than
60 mitochondrial proteins have now been identified as
phosphoproteins [8,11,17–55] (Table 1).
These proteins are found in all mitochondrial compartments – matrix, inner membrane, intermembrane space
and outer membrane, including the cytoplasmic-facing
outer surface – and are implicated in a wide spectrum of
mitochondrial functions. Yet, it remains possible that
several of these phosphorylation events are experimental
artifacts or might not constitute actual in vivo regulation.
Notably, most of the proteins listed are derived from single
reports, and many have been identified only by proteomics
efforts or are the result of an in vitro phosphorylation
event. For certain, the mere identification of phosphoproteins of mitochondrial origin is insufficient to declare that
reversible phosphorylation is a bona fide means of
regulating mitochondrial function. To do so, at least two
other criteria must be met: first, the unambiguous
existence of protein kinases and phosphatases within
mitochondria; and second, clear demonstration that these
phosphorylation events affect mitochondrial function.
Mounting data for each of these initiatives, coupled with
a handful of classic studies, suggest that this objective
could soon be fulfilled.
www.sciencedirect.com
Kinases and phosphatases
The importance of reversible phosphorylation in cellular
function is further exemplified by the extent of the genome
that is dedicated to kinases and phosphatases. The largest
kinase and phosphatase families in the human genome,
the protein kinases (PKs) and protein tyrosine phosphatases (PTPs), possess more than 500 and more than 100
members, respectively [56,57]. Together with smaller
families of kinases and phosphatases, these signaling
molecules comprise nearly 3% of all proteins encoded in
the human genome.
Similar to the phosphoproteins listed earlier, kinases
and phosphatases have been implicated in mitochondrial
functions in a surprising number of studies. So far, at least
25 kinases and eight phosphatases have been reported to
localize to mitochondria [7,8,58–73] (Figure 2). These
kinases and phosphatases are clearly not restricted to one
group or family; rather, they represent nearly every
known mammalian kinase and phosphatase subgroup,
reflecting the range of signaling pathways that are likely
to influence the mitochondrion (Figure 2). These signaling
molecules include kinases and phosphatases varying in (i)
substrate specificity, such as tyrosine kinases, classic PTP
subgroups, serine/threonine kinases and dual-specific
PTPs; (ii) catalytic mechanisms, for example, cysteinebased PTPs, aspartic acid-based PTPs and metal-dependent phosphatases; and (iii) evolutionary conservation,
such as bacterially related pyruvate dehydrogenase
kinases (PDKs) and phosphatases (PDPs), branched
chain ketoacid dehydrogenase kinase (BCKDK) and
phosphatase (BCKDP), and many mammalian-specific
enzymes (Figure 2).
Most of these signaling molecules possess other welldocumented non-mitochondrial roles in the cell and are
primarily found to exist outside mitochondria. The
impetus for, or mechanism of, their translocation to
Opinion
28
TRENDS in Biochemical Sciences
Vol.31 No.1 January 2006
Table 1. Mitochondrial phosphoproteinsa
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
Protein
PDC E1a
PDC E1b
PDC E3
PDK isoform 2b
Aconitase
NAD-isocitrate dehydrogenase
NAD-malate dehydrogenase
NAD-malic enzyme
Succinyl-CoA-ligase a subunit
Succinyl-CoA-ligase b subunit
Formate dehydrogenase
Aconitase
BCKAD
BCKAD kinaseb
HSP22
HSP 90
Chaperonin 60
mthsp75
TRAP-1c
CYP2E1c
CYP2B1c
GSTA 4-4
DBP
MnSOD
EF-Tu
Creatine kinase
MTERFb
Abf2p
MtTBPb
NDK
StAR
Axmitoc
SSATb
Sab
CPT-I
MtGATc
BADc
BCL-2
Bcl-xL
CREB
VDAC
CI: ESSS
CI: 10 kDa
CI 42 kDa (2 sites)
CIII core I
CIII core II
CIV I
CIV II
CIV IIIb
CIV IV
CIV Vbb
CV a
CV b
CV d
CV b
ScIRP
SDH-Fp
bc1 complex, b-MPP subunit
CIII core I
NAD(P) transhydrogenase
ANT
Phosphate carrier protein
Aldose reductasec
a
Location
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M/IM?
M/IM?
M
M
M
M
M
M
M
M
M/IMS
IMS
M?
M?
OM
OM
OM
OM
OM
OM
M/IM
OM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
IM
?
P site
Ser
Tyr
Tyr
Ser/Thr
?
?
?
?
?
?
Ser/Thr
Tyr
Ser
Ser
Ser
?
?
Tyr
Tyr
Ser
Ser
Ser/Thr
?
?
Thr?
?
?
Ser/Thr?
?
Ser/His
Ser
Tyr
Ser?
Ser
Ser
Ser/Thr?
Ser
Ser
Thr
Ser?
Tyr
Ser
Ser
Ser & Thr
Tyr
?
Tyr
Tyr
Ser/Thr?
?
Ser/Thr?
?
Thr
?
?
?
?
?
Tyr
?
?
?
S/T?
Source
Various
Human sperm
Hamster sperm
Rat
Bovine/potato
Bovine/potato
Potato
Potato
Rat/potato
Rat/potato
Potato
Guinea pig synaptosomes
Various
Rat
Corn
Potato
Potato
Rat hepatoma cells
Human sperm
COS
COS
COS
Yeast
Potato
Rabbit heart
Bovine
Rat
Yeast
Yeast
Pisum sativum
COS-1
Rat
Rat
Rat cardiac myocytes
Rat
Rat
FLS.12 cells
Jurkat
U-937 cells
Rat
Guinea pig synaptosomes
Bovine
Bovine
Bovine
Human sperm
Bovine
Bovine
Osteoclasts
Bovine
Rat
Bovine
Bovine/potato
Human sk mus/bovine
Potato
Potato
Rat
Bovine/potato
Potato
Human sperm
Bovine
Bovine
Bovine
Various cell lines
Function
Acetyl-CoA formation
Acetyl-CoA formation
Regulation of the PDC
TCA
TCA
TCA
TCA
TCA
TCA
TCA
TCA
TCA
AA metabolism
AA Metabolism
Chaperone
Chaperone
Chaperone
Chaperone
Chaperone
Detoxification
Detoxification
Detoxification
mRNA turnover
Oxidative stress defense
Protein synthesis
Synthesis of phosphocreatine
Transcription termination
mtDNA maintenance
mtDNA maintenance
Nucleoside triphosphate balance
Steroid hormone synthesis
Regulation of PLAs?
Acetylation of spermidine
SH3 domain-binding protein
b-OX
Glycerolipid biosynthesis
Apoptosis
Apoptosis
Apoptosis
mtDNA transcription?
transport
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
OXPHOS
HC pump
Transport
Transport
Osmoregulation
Refs
[11]
[8]
[8]
[27]
[22]
[22]
[22]
[22]
[22,47]
[22,47]
[22,23]
[8]
[31]
[54]
[39]
[22]
[22]
[29]
[8]
[43]
[18]
[44]
[38]
[22]
[32]
[45]
[42]
[26]
[50]
[49]
[19]
[53]
[21]
[28]
[35]
[41]
[30]
[17,52]
[36]
[24]
[8,45]
[25]
[25]
[45,55]
[8]
[45]
[37]
[40]
[34]
[46]
[34]
[22]
[33]
[48]
[48]
[20]
[22]
[22]
[8]
[45]
[45]
[45]
[51]
Abbreviations: AA, amino acid (S, serine; T, threonine; Y, tyrosine); ANT, adenine nucleotide transporter; Axmito, mitochondrial annexin; b-OX, b-oxidation; BCKAD,
branched chain ketoacid dehydrogenase; CI–CV, respiratory chain complexes 1–5; CPT, carnitine palmitoyltransferase; CREB, cAMP-responsive element (Cre)-binding
protein; CYP, cytochrome P450; DBP, dodecamer-binding protein; EF, elongation factor; GST, glutathione S-transferase; HSP, heat shock protein; IM, inner membrane; IMS,
intermembrane space; M, matrix; MnSOD, manganese super oxide dismutase; mTERF, mitochondrial transcription termination factor; mtGAT, mitochondrial glycerol-3phosphatase acetyltransferase; mthsp, mitochondrial HSP; mtTBP, mitochondrial telomere-binding protein; NDK, nucleoside diphosphate kinase; OM, outer membrane;
OXPHOS, oxidative phosphorylation; P site; site of phosphorylation; PDC E1/3, pyruvate dehydrogenase complex E1/3 subunit; PDK, pyruvate dehydrogenase kinase; PLA,
phospholipase A; ScIRP, subunit c-immunoreactive peptide; SSAT, spermidine/spermine acetyltransferase; StAR, steroidogenic acute regulatory protein; TCA, tricarboxylic
acid cycle; TRAP-1, tumor-necrosis factor type 1 receptor-associated protein; VDAC, voltage-dependent anion channel.
b
Phosphorylation is observed only in vitro on recombinant protein.
c
Protein translocates to mitochondria (i.e. protein is not a resident mitochondrial protein).
www.sciencedirect.com
TRENDS in Biochemical Sciences
Kinase or
Group or family
phosphatase
Protein kinases
TK
Src
Lyn
Fyn
Csk
Fgr
Abl
EGFR
(a) Cytosol
IMS–IM
IMS–IM
IMS–IM
IMS–IM
IMS–IM
?
IM
[8]
[8]
[8]
[8]
[8]
[8]
[8]
----
TKL
RAF1
RIP3
OM
?
[7]
[68]
STE
MEK
PAK5
OM
OM
[7]
[65]
CKI
OM
[64]
GSK3β
ERK
CKII
JNK
SAPK3
CDK11
PCTAIRE2
[7,61]
Inside or ?
[7]
OM, IMS or M
[63]
Inside or ?
[7]
OM
[7]
OM?
[70]
OM?
?
[72]
PKA
Akt
PKC
DMPK
[7]
OM or inside?
[7,62]
IM or Matrix
[7]
OM or IM?
[72]
?
PINK1
Unknown
[69]
mTOR
PDK
BCKDK
OM
Matrix
Matrix
[66]
[67]
[58]
CMGC
AGC
CAMK
Atypical
A
K
A
P
Inhibited
Active
C
C
RII
Intermembrane
space
Classic
MKP-1
PTP1D
PTPMT1
SHP-2
CDC
----
Eya
----
LMW
CTD
OM
OM
IM
IM or IMS
Apoptosis
(b) Matrix
Pyruvate
P
Acetyl CoA
E2
P
E2
E3
E3
PDPs
Active
Inhibited
(c) Intermembrane
space
2H+
COX COX
1/2O2
+
2H+
2e–
P
COX
COX
?
H2O
Active
Inhibited
Matrix
----
Ti BS
TIM50
IM
[8,74]
PPP
PP1
PP2A
OM
OM
[87]
[7]
PPM
PP2Cγ?
PDPs
BCKDP
?
Matrix
Matrix
[60]
[67]
[58]
PPs
P
E1
PDKs
E1
[8]
[8]
[73]
[8]
P
BAD BCL
↑ cAMP or ?
DSP
Ti BS
Figure 2. Mitochondrial kinases and phosphatases. A highly evolutionarily diverse
group of more than 30 kinases and phosphatases has been reported to localize to
mitochondria. The kinases and phosphatases of each principal group or family that
have been reported at least once to have a mitochondrial localization are listed,
along with the submitochondrial localization where they were found. For further
information on kinase and phosphatase family structure and nomenclature, see
Manning et al. [57], Alonso et al. [56] and Cohen [93]. Note that the cladogram is not
to scale. Abbreviations; IM, inner membrane; IMS, intermembrane space; OM,
outer membrane.
mitochondria is poorly understood for most proteins. What
is clear, however, is that kinases and phosphatases, like
the phosphoproteins listed earlier, are present in all
compartments of the mitochondrion (Figure 3) and that
their activities impinge on diverse mitochondrial functions.
A few signaling molecules, however, seem to localize
primarily to mitochondria. In addition to the PDC kinases
and phosphatases, this group includes the PTEN-induced
kinase PINK1 [69], the dual-specific PTP targeted to the
mitochondrion PTPMT1 [73] and the aspartic-acid-based
www.sciencedirect.com
BAD
cAMP
PKA
Protein phosphatases
PTPs
29
Location Refs
GYC
CKI
Vol.31 No.1 January 2006
14-3-3
Opinion
Figure 3. Examples of reversible phosphorylation in the regulation of mitochondrial
function. (a) The phosphorylation of BAD by protein kinase A (PKA) in the outer
membrane. The A-kinase anchoring protein (AKAP) AKAP121 recruits PKA to the
outer membrane. cAMP-mediated activation of PKA leads to dissociation of the
catalytic subunits (C) from the regulatory subunits (RII), enabling the catalytic
subunit to phosphorylate BAD. Phosphorylated BAD dissociates from the outer
membrane and becomes bound by 14–3-3 in the cytosol [30]. (b) The PDC converts
pyruvate into acetyl-CoA in the matrix through the action of three subunits, E1, E2
and E3, of differing biochemical function. Pyruvate dehydrogenase kinases (PDKs)
associated with the E2 subunit can inactivate the complex by phosphorylating the
E1 subunit on three separate serine residues. Pyruvate dehydrogenase phosphatases (PDPs) associated with the same subunit reactivate the PDC by dephosphorylating the E1 subunit [3]. (c) COX, complex IV of the electron transport chain,
concurrently reduces oxygen to water and pumps protons across the mitochondrial
membrane, aiding in creation of the electron-motive force needed to drive ATP
production. Increases in cAMP concentrations lead to tyrosine phosphorylation of
COX, which exists as a dimer, by an unidentified kinase. This phosphorylation
inhibits the activity of COX, potentially by disrupting dimer formation [80].
phosphatase/ATPase Tim50 [74]. Although the substrates
for each of these proteins are currently unknown, it is
clear from biological and genetic data that they possess
crucial functions in mitochondria. PINK1 is targeted to
mitochondria by an amino (N)-terminal signal sequence,
although its submitochondrial localization remains to be
determined. This kinase, which shares high sequence
homology with the Ca2C/calmodulin-regulated kinase
family, seems to be involved in pro-survival activities
and, most importantly, mutations in conserved residues of
PINK1 lead to the early onset of a rare form of Parkinson’s
disease [69].
30
Opinion
TRENDS in Biochemical Sciences
Recently, PTPMT1 was identified as the first PTP that
is localized primarily inside the mitochondrion [73].
PTPMT1, like PINK1, is targeted to the mitochondrion
by an N-terminal signal peptide and is found tightly
associated with the matrix face of the inner mitochondrial
membrane. PTPMT1 is highly expressed in pancreatic b
cells, whose mitochondria have the important function of
coupling glucose metabolism to the secretion of insulin.
Knockdown of PTPMT1 expression in the b cell line INS-1
832/13 results in enhanced ATP production, leading to a
marked increase in insulin secretion [73].
Tim50, a key component of the TIM (translocase of the
inner membrane) complex, has sequence homology to the
CTD family of aspartic-acid-based phosphatases/ATPases.
Tim50, like other members of this family, might function
as an ATPase, but it has been also shown to possess
phosphatase activity against the phosphotyrosine analog
para-nitrophenyl phosphate in vitro [74]. Knockdown of
Tim50 expression by RNA-mediated interference causes
both increased sensitivity to death stimuli by accelerating
the release of cytochrome c in 293T cells and major
disruptions in the neuronal development of zebrafish [74].
Thus, not only are kinases and phosphatases recruited to
mitochondria from elsewhere in the cell, but the
mitochondrion seems to possess a contingent of resident
signaling molecules. This modest group of three is likely to
grow, but even now it is evident that the proper
functioning of resident kinases and phosphatases is
significant to cellular, tissue and organismal physiology.
Established regulation of mitochondrial events by
reversible phosphorylation
Although the effect of phosphorylation on most known
mitochondrial phosphoproteins is unclear and/or the
kinases and phosphatases responsible are unidentified, a
few phosphorylation events have been well characterized.
These examples, like the phosphoproteins and signaling
molecules discussed earlier, are not restricted to one area
of the mitochondrion.
Phosphorylation on the mitochondrial outer membrane
has a crucial role in regulating apoptosis. A particularly
well-defined event is the phosphorylation of BAD, a
proapoptotic member of the BCL-2 family. Harada et al.
[30] have shown that PKA, after treatment with the prosurvival cytokine interleukin-3, translocates to the outer
membrane. Once anchored to an A-kinase anchoring
protein (AKAP) on the outer membrane, PKA phosphorylates BAD on Ser 112, contributing to the inactivation
and disassociation of BAD from mitochondria (Figure 3a).
Phosphorylation of BAD on Ser 136 by p70S6 kinase
and on Ser 155 by an unidentified kinase have also been
implicated in the inactivation of BAD [75,76].
The most well-established example of reversible phosphorylation acting as a regulatory mechanism in mitochondria is that of the PDC in the matrix (Figure 3b). The
PDC is a large multimeric complex consisting of three core
subunits: pyruvate dehydrogenase (E1), dihydrolipoamide
acetyltransferase (E2) and dihydrolipoamide dehydrogenase (E3). Assembled, this complex catalyzes the conversion of glycolysis-derived pyruvate to acetyl-coenzyme A
(CoA), the main precursor to the tricarboxylic acid cycle
www.sciencedirect.com
Vol.31 No.1 January 2006
[3]. As the link between these two major energy-producing
pathways, the PDC must be properly regulated for the
maintenance of cellular glucose homeostasis.
Since its identification as the first mitochondrial
phosphoprotein in 1969, the PDC and its regulation by
phosphorylation have been studied exhaustively. Phosphorylation and dephosphorylation of the PDC are carried
out by PDKs and PDPs, respectively. At least four PDK
isoforms and two PDP isoforms are known, all of which are
associated with the E2 subunit of the PDC. The
phosphorylation events occur on three separate serine
residues of the E1 subunit, each leading to significant
inactivation of this complex [67]. Although at least one
other study has concluded that another kinase, GSK3b,
can regulate the PDC [77], little has been done to follow up
this initial observation. Notably, there is now at least one
report of a PDK itself being phosphorylated. This
phosphorylation, carried out by PKC, has been shown to
inactivate PDK, potentially demonstrating an additional
level of PDC regulation by reversible phosphorylation
[27]. Interestingly, these phosphorylation events are not
seen in the bacterial PDC, which, despite showing high
overall homology to the eukaryotic PDC, is regulated only
by allosteric mechanisms and product inhibition [78].
Similarly, the plant plastid PDC is not regulated by
phosphorylation, but the plant mitochondrial PDC is [79].
Thus, the PDC is a prime example of mitochondria using
phosphorylation to add a level of regulation to an
otherwise evolutionarily conserved process.
As indicated by the mitochondrial tyrosine kinases and
phosphatases listed earlier, phosphorylation within this
organelle is not limited to serine and threonine residues.
An example of tyrosine phosphorylation that affects
mitochondrial energetics is seen in the regulation of
cytochrome c oxidase (COX) in the inner membrane
(Figure 3c). COX, as the terminal enzyme in the
respiratory chain, coordinately reduces oxygen to water
while pumping protons across the inner membrane.
Similar to the PDC, COX is allosterically regulated by
ATP and ADP, as well as the thyroid hormone T2 and
possibly Ca2C ions. In addition to these forms of
regulation, Lee, et al. [80] have now shown that COX
becomes phosphorylated in a cAMP-dependent manner
both in vitro and in HepG2 cells in vivo. COX comprises 13
subunits and has been crystallized as a dimer. The
phosphorylation site has been identified as Tyr 304 of
subunit I, which is located at the dimer interface on the
intermembrane. The phosphorylation event markedly
inhibits COX activity, perhaps by disrupting dimer
formation [80].
A second example of tyrosine phosphorylation of COX
has been shown in osteoclasts by Miyazaki et al. [40].
These authors demonstrated that a portion of the nonreceptor tyrosine kinase c-Src, similar to the Lyn tyrosine
kinase (Ref. [8] and see earlier), localizes inside mitochondria and leads to tyrosine phosphorylation of COX on an
unidentified site of subunit II. The result of the
phosphorylation event is opposite to that seen for subunit
I, leading to enhanced COX activity. These examples
demonstrate the importance of further characterizing the
phosphorylation events of the mitochondrial proteins
Opinion
TRENDS in Biochemical Sciences
listed in Table 1, reminding us that we cannot fully
appreciate the functions of these proteins until we do so in
the context of their various regulatory states.
Supporting cast
Having established that there is a strong cast of
phosphoproteins and signaling molecules in the mitochondrion, it seems appropriate to discuss the possible
signaling mechanisms of these molecules. The most wellstudied signal transduction events involve the communication of a signal outside the cell, such as a drug or
hormone, across the plasma membrane to effect changes
in the cytoplasm and often gene transcription in the
nucleus. To achieve such signaling, cells have evolved
specialized ways by which to communicate across
membranes. Well-known examples include the receptor
protein tyrosine kinases and G-protein-coupled receptors
that traverse the plasma membrane, and the transcription
factor NFkB that translocates from the cytoplasm to the
nucleus after exposure of its masked nuclear localization
sequence upon degradation of IkB [81].
Given the membrane structure of these organelles,
signaling to mitochondria might be better compared to
cell-to-cell, rather than intracellular, signaling. To transmit signals to mitochondria, clever mechanisms similar to
those used in cell communication must have evolved. For
example, how are signals that need to penetrate into the
matrix transduced across the outer and highly impermeable inner membrane? How are kinases and phosphatases
that take up primary residence elsewhere in the cell
triggered to translocate to mitochondria? How are the
activities of resident signaling molecules such as PTPMT1
regulated? Once a phosphorylation event occurs within
the mitochondrion, how does it affect change and how is
the signal turned over?
Intriguing recent studies suggest that many of the
same strategies and mechanisms that are used at the
plasma membrane and in the cytosol might be important
in mitochondrial signaling. For instance, after epidermal
growth factor (EGF) treatment, the EGF receptor (EGFR)
receptor kinase has been reported to translocate to the
inner mitochondrial membrane, where it subsequently
interacts with COX subunit II [82]. This report is
interesting given that Grb10 and Raf-1, two signaling
proteins known to be activated by EGFR, have been found
in mitochondrial fractions [82]. Previous studies also
suggest that steroid receptors can translocate to mitochondria [5]. Although more work is needed to validate
these observations, if supported they demonstrate a direct
and unexpected means by which signals can be transduced from outside mitochondria into the matrix and/or
inner membrane.
Phosphorylation of a target protein can modify its
function in several ways. A common mechanism is the
creation of a new binding site for adaptor proteins that can
activate, inhibit or change the location of a target protein.
The identification of one such class of adaptor proteins in
mitochondria, the 14–3-3 family, is a second significant
finding relating to the potential mechanisms of signaling
by phosphorylation within this organelle. 14–3-3 proteins
constitute a highly conserved family of phosphowww.sciencedirect.com
Vol.31 No.1 January 2006
31
serine/threonine binding proteins involved in the regulation of various enzymes. Bunney et al. [83] have
demonstrated the existence of 14–3-3 proteins in barley
mitochondria, where they bind the b-subunit of the F1 ATP
synthase in a phosphorylation-dependent manner. Furthermore, the authors demonstrated that this binding
drastically reduced ATP synthase activity, thereby introducing a potential new control point in ATP generation.
It is now well established that many signaling
processes occur on molecular scaffolds that orientate
kinases and phosphatases towards their selected substrates [84]. Many such scaffolding proteins have now
been localized to mitochondria, including the AKAPs
WAVE-1, AKAP121 and Rab32, a receptor for inactive/
active C kinase (RICK/RACK) named PICK1, the Jun
N-terminal kinase scaffold protein Sab, and the adaptor
protein Grb10 [7,85]. AKAP121, in addition to its role in
targeting PKA to BAD (Figure 2), is also responsible for
targeting the actin-associated phosphatase PTP1D and
possibly Src to the mitochondrial outer membrane [86].
The identification of mitochondrial 14–3-3 proteins and
AKAPs adds another dimension to mitochondrial signaling and further demonstrates that some well-established
cellular
signaling
strategies
are
preserved
in mitochondria.
A final puzzling aspect of mitochondrial signaling is
how kinases and phosphatases are themselves regulated.
Numerous kinases that have their primary residence
elsewhere in the cell but become targeted to the
mitochondrion, such as Abl, Akt, GSK3b and PKCd,
seem to do so only in their active state [7,8,61,62]. Thus,
the extent of some kinase activities within mitochondria
might simply be dictated by the number of enzymes that
are allowed to enter the organelle; in other words, at the
level of mitochondrial import.
For resident signaling molecules, however, different
means of regulation must be in place. Although these
processes remain to be determined, it is likely that second
messengers will have a key role. The activities of the PDK
and PDP isoforms are known to be controlled by ions and
small molecules such as Mg2C, Ca2C, KC and ADP [3].
The characterization of mitochondrial nitric oxide
synthases and the recent discovery of a soluble adenylate
cyclase in mitochondria provide further opportunities for
second messengers to contribute to regulating mitochondrial signaling molecules [87,88]. Finally, reactive oxygen
species, which have been established as a means of
regulating signaling molecules elsewhere in the cell
[89,90], will almost certainly be involved in regulating
kinases and phosphatases in mitochondria, where the
bulk of reactive oxygen species is produced.
Concluding remarks
The discovery that mitochondria have crucial functions
beyond mere energy production has revolutionized mitochondrial biology, and all indications are that the complexity and versatility of this organelle are yet to be fully
appreciated. Even after several thorough proteomic
surveys, it is estimated that only two-thirds of the
mammalian mitochondrial proteome is known [91,92].
Much of the remaining third is likely to be comprised of
32
Opinion
TRENDS in Biochemical Sciences
low-abundance proteins, such as signaling proteins, that
were below the detection level of these mass spectrometric
analyses. What is also clear from these studies is the high
variability in protein content among mitochondria from
different tissues. Mootha et al. [91], for example, found
that only w50% of the proteins in their proteomics effort
were conserved across the four tissues (brain, heart, liver
and skeletal muscle) that they examined. It is likely that
different mitochondrial signaling pathways not only will
vary from tissue to tissue in the same way, but might very
well contribute to this observed mitochondrial diversity.
Nonetheless, as we have shown here, there is more than
sufficient evidence to conclude that reversible phosphorylation is involved in the regulation of mitochondrial
processes. With over 60 reported phosphoproteins, 30
kinases and phosphatases and various auxiliary signaling
proteins, the mitochondrion is certainly an underappreciated site for signaling by reversible phosphorylation. But
to what extent? To answer this question, further exhaustive techniques, such as tandem mass spectrometry of
mitochondrial phosphoproteins, and more directed individual studies describing the specific effects of phosphorylation events must be performed. Such studies have the
ability to reveal the true breadth of mitochondrial
functions regulated by phosphorylation, which is likely
to expand well beyond what has been covered here. With
mitochondria garnering more and more attention, not only
for their expanding repertoire of functions but also for
their inextricable role in human disease, it is imperative
that we come to a fuller understanding of the means by
which their functions are regulated. We expect that
reversible phosphorylation, as it is elsewhere in the cell,
will be found at center stage.
Acknowledgements
We thank members of the Dixon laboratory for critically reading the
manuscript. This work was supported by funds from the National
Institutes of Health and the Walther Cancer Institute (to J.E.D.), and
by National Institutes of Health Pharmacology Training Grant NIH 2 T32
GM07752–25 (to D.J.P.)
References
1 DiMauro, S. and Schon, E.A. (2003) Mitochondrial respiratory-chain
diseases. N. Engl. J. Med. 348, 2656–2668
2 Newmeyer, D.D. and Ferguson-Miller, S. (2003) Mitochondria:
releasing power for life and unleashing the machineries of death.
Cell 112, 481–490
3 Voet, D. and Voet, J.G. (2004) Electron Transport and Oxidative
Phosphorylation. In Biochemistry (3rd edn), pp. 797–842, John Wiley
& Sons
4 Butow, R.A. and Avadhani, N.G. (2004) Mitochondrial signaling: the
retrograde response. Mol. Cell 14, 1–15
5 Goldenthal, M.J. and Marin-Garcia, J. (2004) Mitochondrial signaling
pathways: a receiver/integrator organelle. Mol. Cell. Biochem. 262,
1–16
6 MacDonald, M.J. et al. (2005) Perspective: emerging evidence for
signaling roles of mitochondrial anaplerotic products in insulin
secretion. Am. J. Physiol. Endocrinol. Metab. 288, E1–E15
7 Horbinski, C. and Chu, C.T. (2005) Kinase signaling cascades in the
mitochondrion: a matter of life or death. Free Radic. Biol. Med. 38,
2–11
8 Salvi, M. et al. (2005) Tyrosine phosphorylation in mitochondria: a
new frontier in mitochondrial signaling. Free Radic. Biol. Med. 38,
1267–1277
www.sciencedirect.com
Vol.31 No.1 January 2006
9 Thomson, M. (2002) Evidence of undiscovered cell regulatory
mechanisms: phosphoproteins and protein kinases in mitochondria.
Cell. Mol. Life Sci. 59, 213–219
10 Burnett, G. and Kennedy, E.P. (1954) The enzymatic phosphorylation
of proteins. J. Biol. Chem. 211, 969–980
11 Linn, T.C. et al. (1969) a-Keto acid dehydrogenase complexes. X.
Regulation of the activity of the pyruvate dehydrogenase complex
from beef kidney mitochondria by phosphorylation and dephosphorylation. Proc. Natl. Acad. Sci. U. S. A. 62, 234–241
12 Wong, T.W. and Goldberg, A.R. (1983) Tyrosyl protein kinases in
normal rat liver: identification and partial characterization. Proc.
Natl. Acad. Sci. U. S. A. 80, 2529–2533
13 Tomaska, L. (2000) Mitochondrial protein phosphorylation: lessons
from yeasts. Gene 255, 59–64
14 Andersson, S.G. et al. (1998) The genome sequence of Rickettsia
prowazekii and the origin of mitochondria. Nature 396, 133–140
15 Hunter, T. (1995) Protein kinases and phosphatases: the yin and yang
of protein phosphorylation and signaling. Cell 80, 225–236
16 Hughes, W.A. and Halestrap, A.P. (1981) The regulation of branchedchain 2-oxo acid dehydrogenase of liver, kidney and heart by
phosphorylation. Biochem. J. 196, 459–469
17 Alnemri, E.S. et al. (1992) Overexpressed full-length human BCL2
extends the survival of baculovirus-infected Sf9 insect cells. Proc.
Natl. Acad. Sci. U. S. A. 89, 7295–7299
18 Anandatheerthavarada, H.K. et al. (1999) Dual targeting of cytochrome P4502B1 to endoplasmic reticulum and mitochondria involves
a novel signal activation by cyclic AMP-dependent phosphorylation at
Ser128. EMBO J. 18, 5494–5504
19 Arakane, F. et al. (1997) Phosphorylation of steroidogenic acute
regulatory protein (StAR) modulates its steroidogenic activity. J. Biol.
Chem. 272, 32656–32662
20 Azarashvili, T.S. et al. (2002) Phosphorylation of a peptide related to
subunit c of the F0F1-ATPase/ATP synthase and relationship to
permeability transition pore opening in mitochondria. J. Bioenerg.
Biomembr. 34, 279–284
21 Bordin, L. et al. (2002) Phosphorylation of recombinant human
spermidine/spermine N1-acetyltransferase by CK1 and modulation
of its binding to mitochondria: a comparison with CK2. Biochem.
Biophys. Res. Commun. 290, 463–468
22 Bykova, N.V. et al. (2003) Identification of 14 new phosphoproteins
involved in important plant mitochondrial processes. FEBS Lett. 540,
141–146
23 Bykova, N.V. et al. (2003) Phosphorylation of formate dehydrogenase
in potato tuber mitochondria. J. Biol. Chem. 278, 26021–26030
24 Cammarota, M. et al. (1999) Cyclic AMP-responsive element binding
protein in brain mitochondria. J. Neurochem. 72, 2272–2277
25 Chen, R. et al. (2004) The phosphorylation of subunits of complex I
from bovine heart mitochondria. J. Biol. Chem. 279, 26036–26045
26 Cho, J.H. et al. (2001) The modulation of the biological activities of
mitochondrial histone Abf2p by yeast PKA and its possible role in the
regulation of mitochondrial DNA content during glucose repression.
Biochim. Biophys. Acta 1522, 175–186
27 Churchill, E.N. et al. (2005) Reperfusion-induced translocation of
dPKC to cardiac mitochondria prevents pyruvate dehydrogenase
reactivation. Circ. Res. 97, 78–85
28 Court, N.W. et al. (2004) Phosphorylation of the mitochondrial protein
Sab by stress-activated protein kinase 3. Biochem. Biophys. Res.
Commun. 319, 130–137
29 Hadari, Y.R. et al. (1997) p75, a member of the heat shock protein
family, undergoes tyrosine phosphorylation in response to oxidative
stress. J. Biol. Chem. 272, 657–662
30 Harada, H. et al. (1999) Phosphorylation and inactivation of BAD by
mitochondria-anchored protein kinase A. Mol. Cell 3, 413–422
31 Harris, R.A. et al. (1986) Regulation of branched-chain a-ketoacid
dehydrogenase complex by covalent modification. Adv. Enzyme Regul.
25, 219–237
32 He, H. et al. (2001) Phosphorylation of mitochondrial elongation factor
Tu in ischemic myocardium: basis for chloramphenicol-mediated
cardioprotection. Circ. Res. 89, 461–467
33 Hojlund, K. et al. (2003) Proteome analysis reveals phosphorylation of
ATP synthase b-subunit in human skeletal muscle and proteins with
potential roles in type 2 diabetes. J. Biol. Chem. 278, 10436–10442
Opinion
TRENDS in Biochemical Sciences
34 Kadenbach, B. et al. (2000) Mitochondrial energy metabolism is
regulated via nuclear-coded subunits of cytochrome c oxidase. Free
Radic. Biol. Med. 29, 211–221
35 Kerner, J. et al. (2004) Phosphorylation of rat liver mitochondrial
carnitine palmitoyltransferase-I: effect on the kinetic properties of the
enzyme. J. Biol. Chem. 279, 41104–41113
36 Kharbanda, S. et al. (2000) Translocation of SAPK/JNK to mitochondria and interaction with Bcl-xL in response to DNA damage. J. Biol.
Chem. 275, 322–327
37 Lee, I. et al. (2005) cAMP-dependent tyrosine phosphorylation of
subunit I inhibits cytochrome c oxidase activity. J. Biol. Chem. 280,
6094–6100
38 Li, H. and Zassenhaus, H.P. (2000) Phosphorylation is required for
high-affinity binding of DBP, a yeast mitochondrial site-specific RNA
binding protein. Curr. Genet. 37, 356–363
39 Lund, A.A. et al. (2001) In vivo modifications of the maize
mitochondrial small heat stress protein, HSP22. J. Biol. Chem. 276,
29924–29929
40 Miyazaki, T. et al. (2003) Regulation of cytochrome c oxidase activity
by c-Src in osteoclasts. J. Cell Biol. 160, 709–718
41 Onorato, T.M. et al. (2005) Phosphorylation of rat liver mitochondrial
glycerol-3-phosphate acyltransferase by casein kinase 2. J. Biol.
Chem. 280, 19527–19534
42 Prieto-Martin, A. et al. (2004) Phosphorylation of rat mitochondrial
transcription termination factor (mTERF) is required for transcription termination but not for binding to DNA. Nucleic Acids Res. 32,
2059–2068
43 Robin, M.A. et al. (2002) Bimodal targeting of microsomal CYP2E1
to mitochondria through activation of an N-terminal chimeric
signal by cAMP-mediated phosphorylation. J. Biol. Chem. 277,
40583–40593
44 Robin, M.A. et al. (2003) Phosphorylation enhances mitochondrial
targeting of GSTA4-4 through increased affinity for binding to
cytoplasmic Hsp70. J. Biol. Chem. 278, 18960–18970
45 Schulenberg, B. et al. (2003) Analysis of steady-state protein
phosphorylation in mitochondria using a novel fluorescent phosphosensor dye. J. Biol. Chem. 278, 27251–27255
46 Steenaart, N.A. and Shore, G.C. (1997) Mitochondrial cytochrome c
oxidase subunit IV is phosphorylated by an endogenous kinase. FEBS
Lett. 415, 294–298
47 Steiner, A.W. and Smith, R.A. (1981) Endogenous protein phosphorylation in rat brain mitochondria: occurrence of a novel ATP-dependent
form of the autophosphorylated enzyme succinyl-CoA synthetase.
J. Neurochem. 37, 582–593
48 Struglics, A. et al. (1998) Two subunits of the F0F1-ATPase are
phosphorylated in the inner mitochondrial membrane. Biochem.
Biophys. Res. Commun. 243, 664–668
49 Struglics, A. and Hakansson, G. (1999) Purification of a serine and
histidine phosphorylated mitochondrial nucleoside diphosphate
kinase from Pisum sativum. Eur. J. Biochem. 262, 765–773
50 Tomaska, L. (1998) Phosphorylation of mitochondrial telomere
binding protein of Candida parapsilosis by camp-dependent protein
kinase. Biochem. Biophys. Res. Commun. 242, 457–460
51 Varma, T. et al. (2003) Protein kinase C-dependent phosphorylation
and mitochondrial translocation of aldose reductase. FEBS Lett. 534,
175–179
52 Yamamoto, K. et al. (1999) BCL-2 is phosphorylated and inactivated
by an ASK1/Jun N-terminal protein kinase pathway normally
activated at G2/M. Mol. Cell. Biol. 19, 8469–8478
53 Yoshii, K. et al. (2000) Purification, identification and phosphorylation
of annexin I from rat liver mitochondria. Acta Med. Okayama 54,
57–65
54 Davie, J.R. et al. (1995) Expression and characterization of branchedchain a-ketoacid dehydrogenase kinase from the rat. Is it a histidineprotein kinase? J. Biol. Chem. 270, 19861–19867
55 Schilling, B. et al. (2005) Mass spectrometric identification of a novel
phosphorylation site in subunit NDUFA10 of bovine mitochondrial
complex I. FEBS Lett. 579, 2485–2490
56 Alonso, A. et al. (2004) Protein tyrosine phosphatases in the human
genome. Cell 117, 699–711
57 Manning, G. et al. (2002) The protein kinase complement of the
human genome. Science 298, 1912–1934
www.sciencedirect.com
Vol.31 No.1 January 2006
33
58 Wynn, R.M. et al. (2004) Molecular mechanism for regulation of the
human mitochondrial branched-chain a-ketoacid dehydrogenase
complex by phosphorylation. Structure (Camb.) 12, 2185–2196
59 Feliciello, A. et al. (2005) cAMP–PKA signaling to the mitochondria:
protein scaffolds, mRNA and phosphatases. Cell. Signal. 17, 279–287
60 Signorile, A. et al. (2002) Serine (threonine) phosphatase(s) acting on
cAMP-dependent phosphoproteins in mammalian mitochondria.
FEBS Lett. 512, 91–94
61 Bijur, G.N. and Jope, R.S. (2003) Glycogen synthase kinase-3b is
highly activated in nuclei and mitochondria. NeuroReport 14,
2415–2419
62 Bijur, G.N. and Jope, R.S. (2003) Rapid accumulation of Akt in
mitochondria following phosphatidylinositol 3-kinase activation.
J. Neurochem. 87, 1427–1435
63 Bordin, L. et al. (1994) Spermine-mediated casein kinase II-uptake by
rat liver mitochondria. Biochim. Biophys. Acta 1199, 266–270
64 Clari, G. et al. (1994) Spermine effect on the binding of casein kinase I
to the rat liver mitochondrial structures. Biochem. Biophys. Res.
Commun. 205, 389–395
65 Cotteret, S. et al. (2003) p21-activated kinase 5 (Pak5) localizes to
mitochondria and inhibits apoptosis by phosphorylating BAD. Mol.
Cell. Biol. 23, 5526–5539
66 Desai, B.N. et al. (2002) FKBP12-rapamycin-associated protein
associates with mitochondria and senses osmotic stress via mitochondrial dysfunction. Proc. Natl. Acad. Sci. U. S. A. 99, 4319–4324
67 Holness, M.J. and Sugden, M.C. (2003) Regulation of pyruvate
dehydrogenase complex activity by reversible phosphorylation.
Biochem. Soc. Trans. 31, 1143–1151
68 Kasof, G.M. et al. (2000) The RIP-like kinase, RIP3, induces apoptosis
and NF-kB nuclear translocation and localizes to mitochondria. FEBS
Lett. 473, 285–291
69 Valente, E.M. et al. (2004) Hereditary early-onset Parkinson’s disease
caused by mutations in PINK1. Science 304, 1158–1160
70 Feng, Y. et al. (2005) Death signal induced relocalization of cyclin
dependent kinase 11 to mitochondria. Biochem J. 392, 65–73
71 Hirose, T. et al. (2000) Identification of tudor repeat associator with
PCTAIRE 2 (Trap). A novel protein that interacts with the N-terminal
domain of PCTAIRE 2 in rat brain. Eur. J. Biochem. 267, 2113–2121
72 Wansink, D.G. et al. (2003) Alternative splicing controls myotonic
dystrophy protein kinase structure, enzymatic activity, and subcellular localization. Mol. Cell. Biol. 23, 5489–5501
73 Pagliarini, D.J. et al. (2005) Involvement of a mitochondrial
phosphatase in the regulation of ATP production and insulin secretion
in pancreatic b cells. Mol. Cell 19, 197–207
74 Guo, Y. et al. (2004) Tim50, a component of the mitochondrial
translocator, regulates mitochondrial integrity and cell death.
J. Biol. Chem. 279, 24813–24825
75 Datta, S.R. et al. (2000) 14–3-3 proteins and survival kinases
cooperate to inactivate BAD by BH3 domain phosphorylation. Mol.
Cell 6, 41–51
76 Harada, H. et al. (2001) p70S6 kinase signals cell survival as well as
growth, inactivating the pro-apoptotic molecule BAD. Proc. Natl.
Acad. Sci. U. S. A. 98, 9666–9670
77 Hoshi, M. et al. (1996) Regulation of mitochondrial pyruvate
dehydrogenase activity by tau protein kinase I/glycogen synthase
kinase 3b in brain. Proc. Natl. Acad. Sci. U. S. A. 93, 2719–2723
78 de Kok, A. et al. (1998) The pyruvate dehydrogenase multi-enzyme
complex from Gram-negative bacteria. Biochim. Biophys. Acta 1385,
353–366
79 Mooney, B.P. et al. (2002) The complex fate of a-ketoacids. Annu. Rev.
Plant Biol. 53, 357–375
80 Lee, I. et al. (2005) cAMP-dependent tyrosine phosphorylation of
subunit I inhibits cytochrome c oxidase activity. J. Biol. Chem. 280,
6094–6100
81 Hunter, T. (2000) Signaling – 2000 and beyond. Cell 100, 113–127
82 Boerner, J.L. et al. (2004) Phosphorylation of Y845 on the epidermal
growth factor receptor mediates binding to the mitochondrial
protein cytochrome c oxidase subunit II. Mol. Cell. Biol. 24,
7059–7071
83 Bunney, T.D. et al. (2001) 14-3-3 protein is a regulator of the
mitochondrial and chloroplast ATP synthase. Proc. Natl. Acad. Sci.
U. S. A. 98, 4249–4254
34
Opinion
TRENDS in Biochemical Sciences
84 Wong, W. and Scott, J.D. (2004) AKAP signalling complexes: focal
points in space and time. Nat. Rev. Mol. Cell Biol. 5, 959–970
85 Danial, N.N. et al. (2003) BAD and glucokinase reside in a
mitochondrial complex that integrates glycolysis and apoptosis.
Nature 424, 952–956
86 Cardone, L. et al. (2004) Mitochondrial AKAP121 binds and targets
protein tyrosine phosphatase D1, a novel positive regulator of src
signaling. Mol. Cell. Biol. 24, 4613–4626
87 Ghafourifar, P. and Cadenas, E. (2005) Mitochondrial nitric oxide
synthase. Trends Pharmacol. Sci. 26, 190–195
88 Zippin, J.H. et al. (2003) Compartmentalization of bicarbonatesensitive adenylyl cyclase in distinct signaling microdomains.
FASEB J. 17, 82–84
Vol.31 No.1 January 2006
89 Chiarugi, P. and Cirri, P. (2003) Redox regulation of protein tyrosine
phosphatases during receptor tyrosine kinase signal transduction.
Trends Biochem. Sci. 28, 509–514
90 Tonks, N.K. (2005) Redox redux: revisiting PTPs and the control of cell
signaling. Cell 121, 667–670
91 Mootha, V.K. et al. (2003) Integrated analysis of protein composition,
tissue diversity, and gene regulation in mouse mitochondria. Cell 115,
629–640
92 Taylor, S.W. et al. (2003) Characterization of the human heart
mitochondrial proteome. Nat. Biotechnol. 21, 281–286
93 Cohen, P.T. (1997) Novel protein serine/threonine phosphatases: variety is the spice of life. Trends Biochem. Sci. 22,
245–251
Articles of interest in other Trends and Current Opinion journals
Adding amino acids to the genetic repertoire
Jianming Xie and Peter G Schultz
Current Opinion in Chemical Biology (2005) Vol. 9, pp. 548–554
Coincidence detection in phosphoinositide signaling
Jez G. Carlton and Peter J. Cullen
Trends in Cell Biology (2005) Vol. 15, pp. 540–547
What does structure tell us about virus evolution?
Dennis H Bamford, Jonathan M Grimes and David I Stuart
Current Opinion in Structural Biology (2005) Vol. 15, pp. 655–663
On the origin of genomes and cells within inorganic compartments
Eugene V. Koonin and William Martin
Trends in Genetics (2005) Vol. 21, pp. 647–654
AGS proteins: receptor-independent activators of G-protein signaling
Joe B. Blumer, Mary J. Cismowski, Motohiko Sato and Stephen M. Lanier
Trends in Pharmacological Sciences (2005) Vol. 26, pp. 470–476
Dendrimers as artificial enzymes
Jacob Kofoed and Jean-Louis Reymond
Current Opinion in Chemical Biology (2005) Vol. 9, pp. 656–664
Membrane recruitment of effector proteins by Arf and Rab GTPases
Masato Kawasaki, Kazuhisa Nakayama and Soichi Wakatsuki
Current Opinion in Structural Biology (2005) Vol. 15, pp. 681–689
Cell toxicity and conformational disease
Robin W. Carrell
Trends in Cell Biology (2005) Vol. 15, pp. 574–580
www.sciencedirect.com