446
Light perception and signalling in higher plants
Péter Gyulay, Eberhard Schäferz and Ferenc Nagy§
Plants monitor changes in the ambient light environment using
sensory photoreceptor families: the phototropins and
cryptochromes, which absorb UV-A or blue light; the
phytochromes, which sense red/far-red light; and the UV-B
photoreceptors, which have not yet been identified. Recent
advances suggest that photoreceptor-induced signalling
cascades regulate light-modulated gene expression. These
regulatory networks interact at the levels of transcription,
posttranslational modification and nucleo-cytoplasmic
compartmentalisation.
Addresses
Institute of Plant Biology, Biological Research Centre of the Hungarian
Academy of Sciences, Temesvári krt. 62, Szeged, H-6726, Hungary
y
e-mail: [email protected]
§
e-mail: [email protected]
z
Institute of Botany/Biologie II, University of Freiburg, Schanzlestrasse 1,
D-79102, Freiburg, Germany
e-mail: [email protected]
Correspondence: Ferenc Nagy
Current Opinion in Plant Biology 2003, 6:446–452
This review comes from a themed issue on
Cell signalling and gene regulation
Edited by Kazuo Shinozaki and Elizabeth Dennis
1369-5266/$ – see front matter
ß 2003 Elsevier Ltd. All rights reserved.
DOI 10.1016/S1369-5266(03)00082-7
Abbreviations
ARR4
ARABIDOPSIS RESPONSE REGULATOR 4
cop
constitutive photomorphogenesis
CRY
cryptochrome
det
de-etiolated
FR
far-red
fus
fusca
FyPP
PHYTOASSOCIATED PROTEIN PHOSPHATASE
HY5
ELONGATED HYPOCOTYL IN LIGHT 5
NPH3
NON-PHOTOTROPIC HYPOCOTYL 3
PHOT
phototropin
PHY
phytochrome
PIF3
PHYTOCHROME-INTERACTING FACTOR 3
PKS1
PHYTOCHROME KINASE SUBSTRATE 1
R
red
RPT2
ROOT PHOTOTROPISM 2
SPA1
SUPPRESSOR OF PHYTOCHROME A 1
UV-A/B ultraviolet-A/B
ZTL
ZEITLUPE
Introduction
The photosensory system that plants have developed to
monitor their light environment contains three known
classes of photoreceptors — the phytochromes (PHY), the
cryptochromes (CRY), and the phototropins (PHOT) —
Current Opinion in Plant Biology 2003, 6:446–452
and the as yet unidentified ultraviolet-B (UV-B)-absorbing receptor molecules (Figure 1). Light-induced signal
transduction starts with the perception of light by these
specialised photoreceptors and culminates in the regulation of the expression of about 2500 genes in Arabidopsis
thaliana, which eventually enables the plant to respond at
the physiological level to changes in different modalities
(directionality, intensity, colour, and diurnal and seasonal
duration) of illumination. Recent studies have identified
novel components of the signaling cascades and have
unravelled molecular mechanisms that are involved in
signal transduction and integration. This review gives a
brief summary of these results and emphasises the most
important features of the newly emerging concept of
light-signal transduction.
The photoreceptors
The phytochrome apoprotein is encoded by a small
multigene family: in the model plant Arabidopsis thaliana
this family consists of five genes (PHYA, PHYB, PHYC,
PHYD and PHYE). All phytochromes exist as dimers that
are composed of two 125-kDa polypeptides, each carrying
a covalently linked open-chain tetrapyrrol chromophore.
Phytochromes are synthesised in the dark in their physiologically inactive red (R)-light absorbing Pr form. After
the absorption of a photon, this inactive Pr form is
photoconverted into the physiologically active far-red
(FR)-absorbing Pfr form, which in turn is transformed
back into the Pr form upon absorption of FR. The Pfr form
of PHYA is light labile, whereas the stability of the Pfr
forms of PHYB–PHYE is not significantly affected by
light. The carboxy-terminal domain of PHY functions in
dimerisation and contains a region that resembles prokaryotic two-component histidine kinases. The aminoterminal region is thought to define the photosensory
activity of the PHY molecule. Beyond inducing conformational change of the PHY molecule, light causes the
autophosphorylation of PHYA [1] and the phosphorylation of other proteins by phytochrome [2]. Light also
modulates the nucleo-cytoplasmic distribution of PHYA–
PHYE by inducing their translocation into the nucleus in
a light quality- and quantity-dependent fashion [3].
Cryptochromes (CRY1 and CRY2 in Arabidopsis) are
flavoproteins that are found in various taxa and are
thought to have evolved from photolyases. Unlike photolyases, however, CRY have no DNA-repair activity. The
amino-terminal part of the CRY molecule binds two types
of chromophore: pterin at one site and flavin adenine
dinucleotide (FAD) at another. The carboxy-terminal
parts of CRY1 and CRY2 contain a variable extension,
which is not found in photolyases, and are essential for
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Light perception and signalling in higher plants Gyula, Schäfer and Nagy 447
Figure 1
UV-B
?
UV-A/Blue
PHOT1
PHOT2
CRY1
CRY2
FarRed
PHYA
EID1
SPA1
cp3
Red
?
SUB1
ULI3
NPH3
RPT2
ARF7
PP7
PHYB
PKS1
FyPP
PIF3
FAR1
FHY3
NDPK2
pef1
FHY1
psi2
FIN219
LAF1
LAF6
PAT1
HFR1
IAA1/3/4/9/17
fin2
COP/DET/FUS
ARR4
COG1
PHYC
PHYD
PHYE
PIF4
srl1
?
GI
ELF3
SRR1
APRR1
APRR5 ?
APRR9 ?
ZTL
pef2
pef3
red1
HY5
HYH
Effectors
Genotoxic
stress
response
Phototropism
Chloroplast movement
Stomatal opening
Photomorphogenesis
Current Opinion in Plant Biology
Photoreceptors and potential light-signalling intermediates. Cloned components are capitalised; those that have been genetically identified but not yet
cloned are in lower case and italics. UV-B light is absorbed by unidentified receptor(s). High-fluence UV-B light damages DNA and elicits a series of
protection responses. Low-fluence UV-B affects photomorphogenesis by enhancing PHYB-specific responses. UVB LIGHT INSENSITIVE 3 (ULI3) [41]
is one of the signalling components that mediates these responses. The UV-A/blue part of the spectrum is monitored by phototropins (PHOT),
cryptochromes (CRY) and PHYTOCHROME A (PHYA). Phototropins regulate the majority of phototropic responses and intracellular chloroplast
movements. Cryptochromes provide the light signal for most of the blue-light-induced responses of photomorphogenesis. The CRY-induced signalling
pathway is probably short; to date, only one positively acting intermediate has been identified [33]. Phytochromes are the most extensively studied
photoreceptors. PHYB–E are the receptors that sense continuous red light, whereas PHYA (and to a smaller extent PHYE) responds to continuous farred and very low fluences of red and blue light. Signals from different photoreceptors are integrated by a complex regulatory network. This network
inhibits the activity of the COP/DET/FUS class of proteins (which are negative regulators of photomorphogenesis) and induces the expression of
downstream transcription factors, such as HY5 and HY5 HOMOLOGUE (HYH). The circadian clock affects this regulatory network at multiple levels.
Components that are common to light-input pathways and the circadian clock are framed in a dashed-outline box. APRR1/5/9, ARABIDOPSIS
PSEUDO-RESPONSE REGULATOR1/5/9; ARF7, AUXIN RESPONSE FACTOR7; COG1, COGWHEEL1; cp3, compacta3; EID1, EMPFINDLICHER IM
DUNKELROTEN LICHT1; ELF3, EARLY FLOWERING3; FAR1, FAR-RED IMPAIRED RESPONSE; FHY1/2, LONG HYPOCOTYL IN FAR-RED LIGHT1/2;
FIN219, FAR-RED INSENSITIVE219; GI, GIGANTEA; HFR1, LONG HYPOCOTYL IN FAR-RED LIGHT1; IAA1/3/4/9/17, INDOLE-3-ACETIC ACID
RESPONSE FACTOR1/3/4/9/17; LAF1/6, LONG AFTER FAR-RED LIGHT1/6; NDPK2, NUCLEOSIDE DIPHOSPHATE KINASE2; PAT1, PHYA SIGNAL
TRANSDUCTION1; PSI2, PHYTOCHROME SIGNALLING2; red1, red light elongated; srl1, short hypocotyl in red light; SRR1, SENSITIVITY TO RED
LIGHT REDUCED1; SUB1, SHORT UNDER BLUE LIGHT1.
CRY function [4]. The photochemical mechanism of
signal capture and transfer by CRY is likely to involve
a redox reaction. Transcription of CRY1 and CRY2 is
regulated by the circadian clock. The intracellular location of both cryptochromes is mostly nuclear: CRY2 is
localised to the nucleus constitutively, whereas CRY1 is
www.current-opinion.com
primarily found in the nucleus in the dark. The stability
of CRY1 is not affected by light, whereas CRY2 is
degraded rapidly upon blue-light illumination.
Phototropins (PHOT1 and PHOT2 in Arabidopsis) are
also flavoproteins. They carry two flavin mononucleotide
Current Opinion in Plant Biology 2003, 6:446–452
448 Cell signalling and gene regulation
(FMN) chromophores that are associated with the LOV
(light, oxygen, voltage)/PAS (PER, ARNT, SIM) domain
in the amino-terminal part of the molecule. PHOT1 and
PHOT2 are blue-light-sensitive receptor kinases, whose
carboxy-terminal parts contain a classical Ser/Thr kinase
domain. They mediate similar blue-light responses, but
have different photosensitivities as determined by their
amino-terminal domains. Besides controlling the phototropism of stem and root, phototropins also affect other
physiological responses, namely chloroplast movement
[5–7] and stomatal opening [8]. Upon signal capture, both
PHOT1 and PHOT2 undergo autophosphorylation, and
signal transfer from the activated receptors to downstream
components likely involves a kinase reaction.
Signalling intermediates and molecular
mechanisms involved in light-signal
transduction
After activation by light, receptors initiate downstream
signal propagation that results in transient or sustained
physiological responses. Classical genetic screens have
yielded several possible light-signalling mutants (Figure 1)
that can be classified into two major groups. Members of
the first class belong to the constitutive photomorphogenesis
(cop)/de-etiolated (det)/fusca (fus) group, which show signs of
photomorphogenesis in complete darkness. The COP/
DET/FUS genes are therefore assumed to function as
negative regulators of photomorphogenesis [9]. In harmony with the pleiotropic nature of cop mutants, it has
become evident that the COP9 signalosome plays a
central role in mediating the degradation of several regulatory proteins, thereby affecting light- and hormoneinduced signalling. In this review, we do not discuss the
COP9 signalosome further_and refer to the COP/DET/
FUS pathway only in the context of light signalling.
Members of the second class of possible light-signalling
mutants develop normally in darkness but have disturbed
responsiveness to light signals that are received by specific photoreceptors. Biochemical approaches and yeast
two-hybrid screens coupled with reverse genetics have
led to the identification of approximately 2500 genes
in Arabidopsis.
Phytochrome signalling intermediates
Until 1998, most of the available evidence suggested that
phytochromes are localised and act primarily in the cytoplasm. Signal transduction from the cytosol to the nucleus
was thought to be mediated by second messengers such as
cyclic guanosine monophosphate (cGMP), Ca2þ and/or a
phytochrome-induced phosphorylation cascade of regulatory proteins [10]. Recent data have radically changed
this view. The emerging model postulates that phytochrome not only induces a signalling cascade mediated by
Ca2þ and cGMP in the cytoplasm but also functions as a
light-regulated kinase (Figure 2); its Pfr conformer can
rapidly translocate into the nucleus, where it interacts
with transcription factors and thus can directly regulate
Current Opinion in Plant Biology 2003, 6:446–452
light-induced gene transcription [11]. This complex signalling network is attractive and is supported by a wealth
of experimental data. PHYA is known to be autophosphorylated, and itself functions as a kinase that phosphorylates PHYTOCHROME KINASE SUBSTRATE 1
(PKS1) [2] and Aux/IAA (Auxin/Indoleacetic acid) proteins [12]. Moreover, a very recent report showed that
PHYA can interact with the catalytic subunit of the Ser/
Thr-specific protein phosphatase FyPP, and that recombinant FyPP efficiently dephosphorylates oat PHYA in a
conformation-dependent manner [13]. Casein kinase II
(CKII)-mediated light-dependent phosphorylation has
also been suggested to regulate the stability and transcriptional activity of ELONGATED HYPOCOTYL IN
LIGHT 5 (HY5), a key component of phytochrome- and
cryptochrome-controlled signalling cascades [14]. There
is no evidence to show that the phosphorylation status and
kinase activity of PHYB–E, in contrast to that of PHYA,
affects light signalling. Moreover, a truncated PHYB
molecule that lacks the entire postulated kinase domain
efficiently restored wildtype red-light-inhibited hypocotyl elongation to a PHYB null mutant [15].
Light-induced change in the nucleo-cytoplasmic distribution of PHYA–E, that is, in their light-driven translocation into the nucleus, is an essential step in signalling.
This process controls the availability of activated photoreceptors for interactions with other regulatory proteins in
the nucleus. Nuclear PHYA–E is not evenly distributed:
it is concentrated in subnuclear structures called nuclear
speckles. Speckle formation exhibits a diurnal pattern in
plants grown under Light/Dark cycles and is reversible
by R/FR treatment [3,16]. Moreover, it seems to be
restricted to biologically active photoreceptors: mutant
PHYA and PHYB molecules that are unable to interact
with other proteins are imported into the nucleus but fail
to form these structures [3]. A missense mutation of
PHYA impaired both the speckle formation of this photoreceptor and a subset of PHYA-controlled responses [17].
Co-localisation studies and fluorescence-resonance energy
transfer (FRET) microscopy using PHYB::green fluorescent protein (GFP) and CRY2::red fluorescent protein
(RFP) also demonstrated the formation of these subnuclear structures [18]. On the basis of these findings,
it is tempting to postulate that PHYA- and/or PHYBcontaining speckles represent active transcriptional complexes, which possibly contain numerous physically and
functionally interacting proteins (Figure 2). To corroborate this assumption, the molecular composition of these
structures needs to be elucidated.
Although it is well documented that all phytochrome
species are imported into the nucleus in a manner that
is affected by light quality and quantity [3], it should
be noted that, even under inductive conditions, the majority of phytochromes remains cytosolic. The biological
function of cytosolic phytochromes and the molecular
www.current-opinion.com
Light perception and signalling in higher plants Gyula, Schäfer and Nagy 449
Figure 2
Cytosol
PKS1
P
Nucleus
?
PHYAr
P
Speckle
PK
FRc
S1
P
P
PHYAfr
P
P
PHYAfr
PHYAfr
PHYBfr
PIF3
PIF3
HFR1
PIF3
FyPP
PHYBfr
PHYBfr
P
PIF4
mRNA
P
HY5
P
Blue
HY5
COP1
CKII
P
P
PIF4
CRY
HY5
1
KS
Rc
P
LRE
P
P
Bfr
PHY
?
?
P
P
P
CRY
PHYBr
PKS1
?
COP9
CSN
COP1
Current Opinion in Plant Biology
Proposed model for light-regulated gene expression. Phytochromes (PHYs) are located in the cytosol in the dark. Upon appropriate light irradiation,
they are converted to their active (Pfr) conformation and autophosphorylated. PHYs phosphorylate different proteins in the cytosol, including PKS1.
PKS1 may function as a cytoplasmic-retention factor, whose activity is regulated by reversible phosphorylation. The phosphorylated Pfr forms of PHYs
could have cytoplasmic functions. Dephosphorylation by a phosphatase (FyPP) may trigger the translocation of PHYs to the nucleus, where they
interact with transcription factors and promote the formation of subnuclear bodies called ‘speckles’. Speckles are assumed to be higher order
regulatory complexes that are involved in controlling the light-modulated transcription of genes. The level of transcription from promoters containing
the light-responsive element (LRE) is determined by the concentration of active transcription factors at the specific site of action. Binding-site selection
is achieved by extensive heterodimerization of transcription factors (e.g. PIF3, HFR1). PIF4, like PIF3, is probably bound to the promoter in the dark. In
contrast to PIF3, however, PIF4 is probably displaced by activated PHYB. PHYs upregulate the expression of several transcription factors, including
HY5. HY5 is a key regulator of photomorphogenesis, and its abundance is also regulated at the level of protein stability. In the dark, HY5 is targeted for
proteolytic degradation by COP1. In light, this process is inhibited, in part, by the action of CRYs, possibly by purging COP1 from the nucleus. This
allows active HY5 to accumulate and bind to light-responsive promoters that contain G-boxes. CKII, casein kinase II; CSN, COP9 signalosome; FRc,
continuous far-red light; Rc, continuous red light.
mechanism that mediates their import into the nucleus
and retention in the cytoplasm in the dark are unknown.
It is interesting to note that no mutants have been
described in which the nuclear import of PHYA–E is
abolished or constitutive. Such mutants, unless lethal,
would be expected to have a robust photomorphogenic
phenotype that would be easily identifiable in a genetic
screen. Molecular mechanisms that regulate the stability/
degradation of phytochromes in the nucleus and/or the
cytoplasm are also largely unknown. The active conforwww.current-opinion.com
mation of PHYB is stabilised by the overexpression of the
response regulator ARABIDOPSIS RESPONSE REGULATOR 4 (ARR4), a finding that opened the way to
studies of possible interactions between light and cytokinin signalling pathways [19].
It is assumed that ubiquitin-mediated degradation is
responsible for the light lability of PHYA. EID1 (EMPFINDLICHER IM DUNKELROTEN LICHT1), an
F-box containing protein [20], and SUPPRESSOR OF
Current Opinion in Plant Biology 2003, 6:446–452
450 Cell signalling and gene regulation
PHYTOCHROME A 1 (SPA1) are negatively acting
factors that are possibly involved in the degradation of
PHYA signalling intermediates. COP1, acting as an E3
ligase, induces the degradation of HY5 in the nucleus in
the dark. On the basis of this finding, it is expected that
COP1, a negative regulator of photomorphogenesis, acts
by targeting certain proteins to the degradation machinery in the dark. Indeed it has been reported that COP1
interacts with SPA1 [21], CRY1 and PHYB in yeast
[22,23]. There is no evidence, however, that the degradation of these proteins is mediated by COP1 in planta. In
addition, the role of COP1-induced proteolytic degradation in light or at later stages of development is much less
clear. Thus, the function of COP1 and the role of proteolytic degradation in phytochrome-initiated signal
transduction, in contrast to CRY-induced signalling, is
either minor or not yet understood.
The most challenging findings of the past few years of
research into phytochrome-induced signalling have provided evidence that PHYA and PHYB can interact
directly, in a conformation-dependent manner, with the
transcription factor PHYTOCHROME-INTERACTING FACTOR 3 (PIF3) bound to light-regulated promoters [24]. The significance of this type of regulation
was further extended when other proteins were shown to
heterodimerise with PIF3 [25,26]. More importantly,
misexpression or mutation of the genes that encode these
proteins was found to affect phytochrome-controlled
aspects of photomorphogenesis. In addition, microarray
studies have indicated that phytochrome-regulated signalling affects gene expression at the level of transcription [27–29]. Quail [30] proposes that phytochrome
directly enhances the transcription of a set of master
regulatory proteins in hierarchical order, which in turn
trigger the expression of other regulators. This model
proposes a relatively short, straightforward transcriptional
cascade, which culminates in the light-modulated transcription of about 2500 genes in Arabidopsis [30].
Cryptochrome signalling intermediates
According to our present interpretation, the signalling
cascade controlled by CRY1 and CRY2 is organised
differently from that controlled by PHY and can be
relatively short. Recent studies have demonstrated that
the overexpression of the carboxy-terminal parts of CRY1
and CRY2 induces a constitutive but still phytochromestimulatable photomorphogenesis. The cop1-like phenotype indicates that in the absence of an inhibitory aminoterminal domain, the carboxy-terminals of CRYs are
constitutively active [4]. Moreover, CRY1 and CRY2
interact with COP1 in the nucleus in the dark [22,23].
It is postulated, therefore, that blue-light perception by
CRY photoreceptors triggers the rapid deactivation/
degradation of COP1 by an unknown mechanism, allowing the accumulation of HY5 in the nucleus, which in turn
enhances the transcription of target genes (Figure 2).
Current Opinion in Plant Biology 2003, 6:446–452
This model emphasises the role of regulated proteolysis;
it is certain, however, that other molecular mechanisms
also play a significant role in CRY1- and CRY2-mediated
signalling for the following reasons. First, microarray
experiments have shown that blue light modulates the
transcription of nearly as many genes as does red light.
Thus, there must be key regulatory transcription factors
other than HY5, for example HY5 HOMOLOGUE
(HYH), that are involved in mediating this process
[31]. Second, CRY2 is phosphorylated in vivo in a
blue-light-dependent fashion [32], and furthermore,
Moller and colleagues [33] provided evidence that a
novel Ser/Thr protein phosphatase (AtPP7), which has
high sequence similarity to the Drosophila retinal degradation C protein phosphatase, acts as a positive regulator
in blue light signaling. Moreover, the interaction of CRY1
with proteins such as PHYA [34] and ZEITLUPE (ZTL;
an F-box containing protein) [35] was demonstrated in
yeast, and the interaction of CRY2 and PHYB in vivo in
protoplasts was documented by FRET microscopy [18].
Remarkably, all of these interactions occur between
cryptochromes and other photoreceptors, such as PHYA
and PHYB, or putative receptors, such as ZTL. Although
the molecular interpretation of these findings is still
lacking, it is tempting to speculate that these interactions
may mediate crosstalk between light-induced signalling
cascades, and thus play a role in fine-tuning the response
to the light environment. This hypothesis is supported by
the fact that the absence of the Ca2þ-binding protein
SUB1 in mutant Arabidopsis plants affected both cryptochrome- and phytochrome-mediated signalling [36].
Phototropin signalling intermediates
The phototropin class of photoreceptors was identified
only recently, yet in the past two years significant
advances have been made toward identifying elements
that mediate the phototropin-specific signalling cascade.
PHOT1 and PHOT2 are bona fide receptor kinases and
the BR-C, TTK, BAB (BTB)/pox virus, zinc-finger (POZ)domain proteins NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) [37] and ROOT PHOTOTROPISM 2
(RPT2) [38] have been identified as possible downstream
signalling intermediates. PHOT1 and NPH3 interact
physically and both are associated with the plasma membrane. Many BTB/POZ-domain proteins are known to
interact with transcription factors. Therefore it is possible
that RPT2 and NPH3 serve as relays between phototropin at the plasma membrane and transcription factors
in the nucleus. One transcription factor that has been
shown to act downstream from phototropins is an auxinresponse factor, NPH4/ARF7 [39]. The nph4 mutant
lacks an auxin-responsive factor and displays impaired
responsiveness to phototropic stimuli. These features
suggest a link between PHOT1 signalling and auxinregulated transcription. However, further research is required to determine how the regulation of ion currents
through the plasma membrane and the interaction of
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Light perception and signalling in higher plants Gyula, Schäfer and Nagy 451
these receptors with different partners — for example,
Hþ-ATPase in guard cells [8], ion transporters or channel
proteins in other leaf cells [40], and maybe auxin transporters in stem cells — result in different movement
responses in different parts of the plant.
7.
Sakai T, Kagawa T, Kasahara M, Swartz TE, Christie JM,
Briggs WR, Wada M, Okada K: Arabidopsis nph1 and npl1: blue
light receptors that mediate both phototropism and chloroplast
relocation. Proc Natl Acad Sci USA 2001, 98:6969-6974.
8.
Kinoshita T, Doi M, Suetsugu N, Kagawa T, Wada M, Shimazaki K:
phot1 and phot2 mediate blue light regulation of stomatal
opening. Nature 2001, 414:656-660.
Conclusions
9.
Ma L, Gao Y, Qu L, Chen Z, Li J, Zhao H, Deng XW: Genomic
evidence for COP1 as a repressor of light-regulated gene
expression and development in Arabidopsis. Plant Cell 2002,
14:2383-2398.
Light-regulated signal transduction is mediated by a
complex, integrated molecular network. Single or multiple photoreceptors can induce different signalling cascades that partly overlap. Several components of these
cascades and some of the molecular mechanisms that
mediate photoreceptor-controlled signal transduction
have been identified. The terminal step of signalling,
the regulation of target-gene expression, occurs predominantly at the level of transcription but signal relay is
significantly affected by regulated degradation and the
compartmentalisation of the signalling intermediates.
Genetic screens and the analysis of mutants will undoubtedly lead to the identification of novel components of
signalling cascades. Unravelling of the mode of action of
these molecules and the molecular mechanisms by which
they regulate signalling will, however, require advanced
cell biological, biochemical and structural studies.
Acknowledgements
The authors thank the members of our laboratory for stimulating discussions,
Éva Ádám and László Kozma-Bognár for helpful comments on the
manuscript, and Erzsébet Fejes for manuscript preparation and editing.
Research was supported by grants from Howard Hughes Medical Institute
(HHMI 55000325), the Hungarian Foundation for Basic Science (OTKA,
T032565) and the Wolfgang Paul Award to Ferenc Nagy.
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