Review
TRENDS in Biochemical Sciences Vol.26 No.1 January 2001
41
Transcriptional repression, apoptosis,
human disease and the functional
evolution of the nuclear lamina
Merav Cohen, Kenneth K. Lee, Katherine L.Wilson and Yosef Gruenbaum
The number and complexity of genes encoding nuclear lamina proteins has
increased during metazoan evolution. Emerging evidence reveals that
transcriptional repressors such as the retinoblastoma protein, and apoptotic
regulators such as CED-4, have functional and dynamic interactions with the
lamina.The discovery that mutations in nuclear lamina proteins cause
heritable tissue-specific diseases, including Emery–Dreifuss muscular
dystrophy, is prompting a fresh look at the nuclear lamina to devise models
that can account for its diverse functions and dynamics, and to understand its
enigmatic structure.
Merav Cohen
Yosef Gruenbaum*
Dept of Genetics,The
Institute of Life Sciences,
The Hebrew University of
Jerusalem, Jerusalem
91904, Israel.
*e-mail:
[email protected]
Kenneth K. Lee
Katherine L.Wilson
Dept of Cell Biology and
Anatomy,The Johns
Hopkins University
School of Medicine, 725
N. Wolfe St, Baltimore,
MD 21205, USA.
All authors contributed
equally to the article.
The main feature of eukaryotic cells is the nucleus,
which enwraps the chromosomes and is the site of
DNA replication, RNA transcription and processing,
and ribosome assembly. The nuclear envelope (NE) is
the boundary between the nucleus and cytoplasm.
The NE is composed of the inner and outer nuclear
membranes (INM and ONM, respectively), which are
separated by a lumenal space continuous with the
ER lumen. Communication between the nucleoplasm
and cytoplasm takes place through pores in the
nuclear envelope, where the inner and outer
membranes join. Within these pores are nuclear pore
complexes (NPCs), which mediate and regulate
nuclear transport1. Underneath the INM is a
meshwork of nuclear-specific intermediate
filaments, termed the nuclear lamina, which
includes lamin proteins plus a growing number of
lamin-associated proteins2,3. Near the INM is the
peripheral chromatin, a large proportion of which is
heterochromatin (Fig. 1).
Lamins are type V intermediate filament
proteins. They range in size from 60 to 70 kDa and
have a characteristic structure: a small N-terminal
‘head’, a 52-nm coiled-coil ‘rod’ and a globular Cterminal ‘tail’4. Lamins form α-helical coiled-coil
dimers, which are the building blocks for further
assembly. In vitro, lamin dimers associate to form
head-to-tail polymers. The assembly pathway and
final structure(s) of lamin filaments are poorly
understood. Indeed, lamin filaments are probably
unlike the 10-nm diameter cytoplasmic intermediate
filaments, because no 10-nm filaments have been
detected in electron micrograph cross-sections, and
the lamina isolated from Xenopus oocyte nuclei forms
an orthogonal network rather than linear filaments4.
There is strong evidence that lamins are not
restricted to the nuclear periphery but exist
throughout the nuclear interior5. It is not known if
the peripheral and interior lamins form similar or
different structures. However, given the growing
number and types of lamin-binding proteins, some of
these partners might influence the assembly or
structural properties of lamins. The nuclear lamina
provides structural support for chromosomes, and is
required to maintain nuclear shape, space NPCs,
replicate DNA and efficiently segregate
chromosomes2,3. New functions for the lamina are
discussed below.
Metazoan evolution: a gradual increase in the
complexity of nuclear lamina proteins
Investigators are identifying a growing number of
integral and peripheral membrane proteins that
associate with lamins (Fig. 2). Hence, the term
‘nuclear lamina’ is general, and includes both lamins
and lamin-binding proteins. Vertebrates have three
lamin genes. The LMNA gene encodes four
alternatively spliced A-type lamins named A, C, A∆10
and C2. Two genes encode B-type lamins, LMNB1
(encoding lamin B1) and LMNB2 (encoding lamins
B2 and B3)4. Several of these proteins are
differentially expressed during development and
differentiation, suggesting tissue-specific functions.
In addition, vertebrates express many integral
membrane proteins in the INM, including three
isoforms of lamina-associated protein 1 (LAP1), at
least five isoforms of LAP2, as well as emerin
(mutations in which cause Emery–Dreifuss
muscular dystrophy; EDMD), MAN1, lamin B
receptor (LBR), nurim and probably UNC-84. A sixth
isoform of LAP2, named LAP2α, lacks a
transmembrane domain and is found throughout the
nuclear interior during interphase. The LAP2
isoforms plus emerin and MAN1 are members of a
family defined by a 43-residue ‘LEM’ domain near
their N terminus6.
There are two lamin genes in Drosophila (lamin
Dm0 and lamin C), and one in Caenorhabditis
elegans (Ref. 4). The number of lamins expressed in
each organism fits a pattern in which more complex
eukaryotes have greater lamin diversity.
Eukaryotes are defined as more complex if they have
more cells and more distinct cell types, tissues and
organs. Thus, adult hermaphrodite C. elegans
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42
Review
TRENDS in Biochemical Sciences Vol.26 No.1 January 2001
absent in C. elegans, including LBR, nurim and Atype lamins. Not surprisingly, some lamina proteins
might be unique to vertebrates. For example,
neither Drosophila nor C. elegans encode obvious
orthologs for LAP1 or LAP2.
Metazoan nuclear lamina proteins are absent from
yeast, and probably plants
Fig. 1. Schematic view of the nuclear envelope, lamina and chromatin. Shown are the inner and
outer membranes of the nuclear envelope, with their enclosed lumen. Also shown are lamin
filaments and selected nuclear envelope proteins including lamina-associated protein 1 (LAP1),
emerin, LAP2β, MAN1, UNC-84, lamin B receptor (LBR), nurim and otefin. Otefin is a peripheral
membrane protein that binds lamins; the other depicted proteins are anchored in the inner
membrane via one or more transmembrane domains. Most nuclear envelope proteins bind
specifically to either A-type or B-type lamins, which are depicted hypothetically as heteropolymers
by different shades of orange. Question marks indicate proteins whose binding properties are
unknown. The distinctly shaped knobs on A- and B-type lamins represent specific binding sites for
histones and other proteins, such as transcriptional regulators (not shown; see Fig. 5). The
peripheral lamina is proposed to tether proteins that regulate gene expression or modify
chromatin structure. Chromatin located near the nuclear envelope is depicted as structurally
condensed, to indicate that it is mostly transcriptionally inactive. Lamins are also found in the
interior of the nucleus, along with the α isoform of LAP2. The structure of the ‘internal lamina’ is
not understood, and is therefore hypothetically depicted as filaments that are thin, disconnected
and crossbridged. We have used artistic license to depict direct connections between the
peripheral and interior lamin filaments, and to suggest alternative structures for lamin filaments.
nematodes, which consist of 959 body cells, are
defined as less complex than adult Drosophila fruit
flies, which have ten times as many cells and can
form wings, legs, eyes, sensory bristles and other
specialized structures. The nearly complete genome
sequences for C. elegans and Drosophila
melanogaster allowed a search for homologs of
vertebrate INM proteins in ‘lower’ eukaryotes
(Fig. 2). C. elegans has three LEM-domain genes,
including homologs of vertebrate emerin and MAN1
(Ref. 7). A homolog for lem-3 has not yet been
identified in vertebrates. UNC-84, an abundant
INM protein in C. elegans, is required for nuclear
migration during development8. Homologs of unc-84
are present in Drosophila and humans, but nothing
is known about their localization or function. The
Drosophila genome has at least six LEM-domain
genes, three of which are probably homologs of
MAN1, emerin and lem-3. The other Drosophila
LEM-domain proteins are otefin, an otefin-like
protein and a novel protein with four putative
transmembrane domains (Fig. 2). Several INM
proteins present in vertebrates and Drosophila are
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The yeast Saccharomyces cerevisiae contains no
lamin genes9, and lacks all other INM proteins
known in multicellular eukaryotes. This complete
lack of nuclear lamina proteins in S. cerevisiae
contrasts with the conservation of NPC proteins in
yeast10, and suggests that metazoan nuclear lamina
proteins provide functions unique to multicellular
animals.
What about the other multicellular eukaryotes,
plants? DNA sequence information is available in
GenBank for large fractions of the Arabidopsis
genome and many plant cDNAs. Despite previous
reports, lamina proteins appear to be absent from
plants. Conversely, plant proteins such as MAF1,
which localizes to the plant NE, lack homologs in
animals11. These results suggest that NE proteins
and functions evolved separately in plants and
animals.
Thus, several unique INM proteins first appeared
in metazoans and increased in number and
complexity during evolution. Metazoan nuclear
lamina proteins first appeared probably around the
transition between closed and open mitosis (see
below).
Increase in the efficiency and extent of nuclear envelope
disassembly during animal evolution
S. cerevisiae has a closed mitosis wherein the NE
remains intact12. During closed mitosis, tubulin
proteins are imported to allow mitotic spindles to
assemble inside the nucleus. By contrast, the
mammalian NE undergoes ‘open’ mitosis, in which
the nuclear lamina and NPCs reversibly
disassemble, the nuclear membranes merge into
the ER and nuclear proteins are released into the
cytoplasm. During late anaphase and telophase this
process is reversed; nuclear membranes reassociate with chromatin, NPCs assemble and
nuclear proteins including lamins are imported
back into the nucleus13.
In vertebrates, the disassembly of the NE defines
the transition between prophase and
prometaphase12. Based on their extent and timing
of NE breakdown, C. elegans and Drosophila
embryos are intermediate between yeast and
vertebrates (Fig. 3). For example, in C. elegans, the
nuclear membranes and lamina remain intact
except at spindle poles until after the
metaphase–anaphase transition, and completely
disassemble only during mid-late anaphase7. In
Drosophila early embryos, NPCs disassemble
during prophase, similar to vertebrates. However,
Review
TRENDS in Biochemical Sciences Vol.26 No.1 January 2001
Yeast
C. elegans
D. melanogaster
B-type lamin
0
1
1
2 (3)
Emerin
0
1
1
1
MAN1
0
1
1
1
UNC-84
0
1
2
>2
Other LEMs
0
1
3
?
Otefin
0
0
1
?
YA
0
0
1
?
LBR
0
0
1
1
Nurim
0
0
1
1
A-type lamin
0
0
1
1 (4)
LAP1
0
0
0
1 (3)
LAP2
0
0
0
1 (>6)
Vertebrates
Ti BS
Fig. 2. Proposed orthologs to lamina proteins in Saccharomyces cerevisiae,
Caenorhabditis elegans, Drosophila melanogaster and vertebrates. The first number shows the
number of genes; the number of splicing isoforms is in parentheses. Brown indicates that no
ortholog was found; light yellow indicates the presence of a single gene; yellow indicates multiple
genes or multiple isoforms. Question marks indicate genes not yet found in the still-incomplete
vertebrate databases. Data was obtained from the GenBank database. The S. cerevisiae protein
Sad1 contains a region of homology to UNC-84, but is positioned on the outer nuclear membrane
and ER, and was excluded from this figure15.
until mid–late anaphase the nuclear membranes
(and a fraction of the lamina) remain intact and are
supplemented by a temporary second layer of
membranes14. Thus metazoan evolution might have
been accompanied by an increase in the ability of
different NE components to disassemble early in
mitosis.
The correlation between NE complexity and the
enhanced extent and efficiency of NE disassembly at
mitosis is intriguing. Open mitosis creates new problems
because mechanisms are needed to (i) ensure that all
chromosomes end up in a single nucleus; (ii) reassemble
the NE and interior architecture; and (iii) re-establish
the interphase organization of chromatin and
subnuclear organelles. Furthermore, lamin filaments,
no matter how useful during interphase, might interfere
with chromosome segregation during mitosis13. Thus,
open mitosis might have obligatorily co-evolved with
nuclear lamina proteins. Having more lamina proteins
probably conferred a selective advantage to metazoan
creatures, perhaps related to improvements in
chromatin organization, or improved nuclear signaling
or gene expression. In addition, open mitosis exposes the
chromatin to cytosolic proteins, which might provide
new means to regulate the cell cycle through access to
cytosolic replication licensing factors15. Furthermore,
the process of nuclear assembly itself might provide new
mechanisms for regulating chromatin structure during
development and differentiation.
Roles for nuclear lamina in apoptosis
Apoptosis16, or programmed cell death, can regulate
cell number, sculpt tissues during development and
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43
eliminate damaged cells. During apoptosis, nuclei
undergo specific morphological changes, including
proteolytic cleavage of the nuclear lamina,
clustering of NPCs, detachment of chromatin from
the NE and DNA cleavage. In C. elegans, the caspase
CED-3 executes apoptosis by cleaving target
proteins. CED-3 is activated by the APAF1 homolog,
CED-4. However, CED-4 is normally prevented from
activating CED-3 by the BCL2 homolog, CED-9. Cell
death is triggered when the BH3-domain protein,
EGL-1, binds and inhibits CED-9 (Ref. 17). In
normal C. elegans cells, antibodies against CED-9
and CED-4 revealed that both proteins are localized
to the mitochondria. However, when apoptosis is
triggered, either by EGL-1 activation or CED-9
destruction, CED-4 rapidly translocates to the
envelope (Fig. 4). This translocation does not depend
on CED-3, suggesting that CED-4 translocates to
the NE before caspase activation18. Although the
exact location of CED-4 in the NE is unknown, it
appears to overlap the lamina18. This result
indicates that the lamina provides an attachment
site for apoptotic signaling machinery.
Both A- and B-type lamins, LAP2α and LAP2β
are early targets for caspase degradation, before
detectable DNA cleavage or chromatin
condensation occurs19,20. Other INM proteins,
including LBR, are targeted later21. Lamins are
cleaved in the α-helical rod domain, probably by
caspase 6 (Ref. 22). LAP2β is cleaved by caspase 3
(Ref. 21). Cleaved fragments of lamins and LAP2β
remain associated with the NE but are not known to
play further roles.
Apoptotic nuclei resemble nuclei in lamindeficient cells, which have clustered NPCs,
detached chromatin and odd shapes23–25. Thus,
lamin destruction probably causes these same
changes during apoptosis. Direct evidence for the
importance of lamins in apoptosis was found by
expressing an uncleavable mutant form of lamin in
cultured cells22; although caspases were activated,
chromatin failed to condense and DNA cleavage
was delayed. This result suggests that lamin
degradation facilitates nuclease activation during
apoptosis. It would be interesting to follow
apoptosis in C. elegans cells that express
uncleavable lamin, and to determine whether
CED-4 can translocate to a lamin-depleted NE.
Heterochromatin tends to associate with the INM
Constitutive heterochromatin is transcriptionally
repressed; it is also highly condensed, late
replicating, rich in repetitive sequences, does not
recombine during meiosis and has regular
nucleosome spacing. Repressive chromatin structure
can spread to genes near heterochromatin26. A
comparison between four model organisms –
S. cerevisiae, C. elegans, D. melanogaster and mouse
– shows that the amount of constitutive
heterochromatin increases from less to more complex
44
Fig. 3. Increase in the
extent and timing of
nuclear envelope (NE)
disassembly during
animal evolution.
Abbreviations: +,
indicates the component
is mostly intact; +/–,
partial NE disassembly
and release to the
cytoplasm; +/– – , almost
complete disassembly; –,
complete disassembly;
NPC, nuclear pore
complex. For details see
Refs 7,12,14,31.
Review
Interphase
Prophase
Prometaphase
Metaphase
Early anaphase
Late anaphase
Telophase
Early G1
TRENDS in Biochemical Sciences Vol.26 No.1 January 2001
Yeast
C. elegans
early
C. elegans
late
Drosophila
early
Drosophila
late
Vertebrates
NPCs
Lamina
Membranes
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
NPCs
Lamina
Membranes
+
+
+
+
+
+
+
+/–
+
+
+/–
+/–
+/–
+/–
+/–
+/–
NPCs
Lamina
Membranes
+
+
+
+
+
+
+
–
+
+
–
–
–
–
–
–
NPCs
Lamina
Membranes
+
+
+
+
–
+/–
+
–
+/–
+
–
–
–
–
–
–
NPCs
Lamina
Membranes
+
–
+
+
–
+/– –
+
–
+/– –
+
–
–
–
–
–
–
NPCs
Lamina
Membranes
+
–
–
–
–
–
–
–
+/– –
+/–
–
–
–
–
–
–
NPCs
Lamina
Membranes
+
+
+/–
+
+
+/–
+
+
+/–
+
+/–
+/–
+/–
+/–
+/–
+/–
NPCs
Lamina
Membranes
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Ti BS
organisms. Heterochromatin in S. cerevisiae is
limited to telomeres and silent mating type loci26.
C. elegans has condensed chromatin at its periphery
(e.g. see electron micrographs in Ref. 27) and tandem
repeats, a hallmark of constitutive heterochromatin,
accounting for ~2.7% of the genome28. The
Drosophila genome contains an estimated 30%
heterochromatin, including most of the Y
chromosome. About 40% of human chromosome 22 is
repetitive noncoding DNA (Ref. 29).
Cytological studies show that a large proportion
of condensed chromatin, including centromeres and
telomeres, borders the INM (Ref. 30). The
positioning of inactive DNA is not random, because
the inactive X chromosome in female mammalian
cells is located near the NE, whereas the active X
chromosome extends into the nuclear interior31.
Even in S. cerevisiae, INM proximity helps to
silence partially silenced genes: when the DNAbinding domain of the yeast transcription factor
Gal4 was fused to a transmembrane protein, Gal4
became localized to the nuclear periphery, as did
target genes containing the Gal4 operator.
Furthermore, these genes became completely
http://tibs.trends.com
silenced, in a Sir (silent information regulatory
protein)-dependent manner32.
Many nuclear lamina proteins interact directly
with chromosomal proteins and might thereby
affect chromatin structure at the nuclear
periphery. For example, LBR interacts with the
heterochromatin-specific protein HP1 (Ref. 33),
and LEM-domain protein LAP2β interacts with
barrier-to-autointegration factor (BAF)34, a DNAbinding protein that inhibits autointegration of
retroviral DNA (Ref. 35). Lamins themselves can
bind specific histones36. Young arrest (YA) is a
Drosophila protein with developmentally regulated
expression that is required for the transition from
meiosis to mitosis; YA binds to both lamin B and
chromatin37. A key question for the future is
whether interactions between NE proteins and
chromatin directly modulate the higher-order
structure of chromatin.
Nuclear lamina and transcription
A growing number of transcription factors, most of
which are repressors, localize at the nuclear
periphery. Oct-1, a transcription factor containing a
Review
Normal
TRENDS in Biochemical Sciences Vol.26 No.1 January 2001
Death signal
CED-4
translocation
Mitochondrion
CED-9
CED-4
EGL-1
Cytoplasm
X
Nucleus
Ti BS
Fig. 4. Proposed translocation of apoptotic regulatory protein CED-4 to the nuclear envelope in
Caenorhabditis elegans (Ref. 18). CED-9 (green), which is homologous to mammalian BCL2, is located
on mitochondrial membranes in normal cells. CED-9 binds CED-4 (red; homologous to mammalian
APAF1), and prevents CED-4 from activating apoptosis. EGL-1 (blue) initiates the death signal by
releasing CED-4. CED-4 then translocates to the nuclear lamina, where it is proposed to bind an
unidentified lamina protein6 (X, yellow).
LAP2β
Outer nuclear membrane
LAP2β
Lamina
GCL
Lumen
BAF
?
Inner nuclear
membrane
?
GCL
DP
Rb
DP Genes on
E2F
E2F
Chromatin
P P
P
P
Rb
Genes off
45
(GCL) protein, which in Drosophila is required to
establish the germ cell lineage during
development41. The mouse GCL is active in
Drosophila (Ref. 42), where it localizes primarily at
the NE. Mouse GCL protein interacts with DP
protein, which heterodimerizes with transcription
factor E2F and regulates the cell cycle43.
Intriguingly, mouse GCL was independently
identified in a two-hybrid screen by its interaction
with the β-specific region of the INM protein LAP2β;
in addition, GCL co-localizes with LAP2β at the
nuclear lamina (A. Simon, pers. commun.). Figure 5
shows a model in which these pair-wise interactions
(e.g. between Rb and lamin A) occur simultaneously
to repress a given target gene. However, it is equally
possible that Rb and LAP2β form separate
complexes. The nuclear periphery is also used for
tethering transcription activators. For example, at
low glucose levels, the insulin activator IPF-1/PDX-1
is localized to the nuclear periphery. Elevated
glucose causes a rapid translocation of IPF-1/PDX-1
to the nucleoplasm where it stimulates the
transcription of the insulin gene44.
Oct-1, Rb, GCL and IPF-1/PDX-1 provide strong
support for the hypothesis that nuclear lamina
proteins play active roles in transcriptional
repression and chromatin structure. Further support
for this idea comes from the Drosophila Su(HW) and
Mod(mdg4) proteins, which establish chromatin
boundaries that regulate homeotic gene expression45.
Immunostaining of Drosophila cells showed that
Su(HW)–Mod(mdg4) complexes are restricted to only
10–20 spots, most of which are located near the
nuclear lamina45.
Ti BS
Nuclear lamina and genetic diseases
Fig. 5. A speculative model for the involvement of the nuclear lamina in transcriptional repression.
Transcriptional repressor retinoblastoma (Rb) associates with the nuclear lamina when it is active as a
repressor, whereas hyperphosphorylated Rb is not associated with the lamina. Rb represses genes
required for entry into S-phase by binding to E2F–DP heterodimers, and recruiting histone
deacetylases (not depicted).The model is based on binding observed between pairs of proteins in
vitro: lamins A/C –Rb, Rb–E2F, E2F–DP, DP–GCL (germ cell-less), GCL–LAP2β , LAP2β–BAF, and
LAP2β–lamin B (see text for details). It is not yet known which, if any, of these putative interactions
occur in vivo in the context of gene regulation. Nuclear lamina filaments are depicted generically as a
orange rod, with round knobs to represent binding sites specific for A-type lamins, and triangular
knobs to represent specific binding sites on B-type lamins.
POU domain that represses the aging-associated
collagenase gene, co-localizes with lamin B (Ref. 38).
In aging cells, the departure of Oct-1 from the NE
coincides with increased collagenase activity; both
events are reverted in immortalized cells38. This
result suggests that Oct-1 is active as a repressor
only when it is located at the NE. The
retinoblastoma (Rb) protein binds transcription
factor E2F and represses transcription by recruiting
histone deacetylase39. The active
(hypophosphorylated) form of Rb co-localizes with
lamins A/C at the nuclear periphery in vivo and
binds lamins A/C in vitro40. Thus, transcriptional
repression by Rb correlates with its lamin-binding
activity (Fig. 5). A third example is the germ-cell-less
http://tibs.trends.com
Interactions between the nuclear lamina and
transcription factors might explain how mutations
in nuclear lamina proteins cause inherited
diseases. For example, the X-linked form of
Emery–Dreifuss muscular dystrophy (EDMD) is
caused by mutations in the emerin gene46, whereas
autosomal–dominant EDMD is caused by
mutations in LMNA (Ref. 47; Fig. 6). Other
mutations in LMNA cause cardiomyopathy and
lipodystrophy47. These diseases are proposed to
result from defects in gene expression, due to loss of
specific attachment sites on the nuclear lamina
needed to establish or maintain particular patterns
of gene expression48. In this model, a missense
mutation that disrupts the binding of just one
factor to lamin filaments would cause a limited
phenotype, whereas complete loss of LMNA would
disrupt all proteins that depend on A-type lamin
filaments. This prediction is supported by the
severe combined phenotype seen in LMNAknockout mice49, which have defects in muscle, fat
and possibly bone (B. Burke, pers. commun.),
consistent with defects in tissue mesenchymal
stem cells48.
46
Review
TRENDS in Biochemical Sciences Vol.26 No.1 January 2001
R582H
Fig. 6. A schematic map of mutations in human lamin A that cause
three distinct inherited diseases. Shown is the lamin A protein (Ref. 50).
The N-terminal head, coiled-coil rod and C-terminal tail domains are
shown in different shades of blue. Mutations clinically associated with
inherited dilated cardiomyopathy type 1A are shown in red.The
mutations associated with autosomal–recessive and
autosomal–dominant Emery–Dreifuss muscular dystrophy (EDMD) are
shown in green and blue, respectively. Mutations associated with
Dunnington-type partial lipodystrophy are shown in brown (mutation
data taken from Ref. 47). More than half of the mutated residues that are
associated with human diseases are identical in the lamin A gene in
Xenopus and lamin A genes in hydra9. Numbers indicate exons, which
are drawn approximately to scale according to the number of amino
acid residues encoded. Mutations associated with sporadic (noninherited) disease are omitted.The clustering of mutations for dilated
cardiomyopathy 1A and Dunnington-type lipodystrophy to distinct
regions of the lamin protein indicates that these regions are used as
attachment sites for specific binding partners relevant to each disease.
Conclusions
The nuclear lamina cannot be viewed merely as the
‘nuts and bolts’ of nuclear structure. Although the
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between the cell nucleus and the cytoplasm.
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2 Goldberg, M. et al. (1999) The nuclear lamina:
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285–293
3 Gotzmann, J. and Foisner, R. (1999) Lamins and
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4 Stuurman, N. et al. (1998) Nuclear lamins: their
structure, assembly, and interactions J. Struct.
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5 Moir, R.D. et al. (2000) Disruption on nuclear
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6 Lin, F. et al. (2000) MAN1, an inner nuclear
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http://tibs.trends.com
9
10
11
12 Lamin A
R527P
T528K
L530P
8
I469T
7
R453W
6
EDMD
Acknowledgements
We thank biomedical
illustrator M. Linkinhoker
for rendering Figs 1 and 5;
H. Cedar, C. Machamer,
A. Simon, D. Shumaker
and J. Liu for useful
discussions and
comments on the
manuscript; and A. Simon
for sharing results before
publication.This work was
supported by grants from
the USA–Israel Binational
Science Foundation
(BSF), the Israel Science
Foundation (ISF) #125/98
and the German–Israel
Foundation (GIF) #1-573036.13 (toY.G.), and by
grants from the W.W.
Smith CharitableTrust and
NIH-RO1GM48646 (to
K.L.W.).
R482Q
R482W
R482L
K486N
R321∆(FS)
5
R336Q
R343Q
E358K
M371K
R386K
4
Q294P
3
H222Y
∆E112
2
R249Q
∆K261
N195K
R60G
L85R
Y45C
R50P
I63S
Q6X
E203G
Dilated cardiomyopathy 1A
1
G465D
Lipodystrophy
Ti BS
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assembly of lamin-based structures. These findings
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Unwinding the ‘Gordian knot’ of
helicase action
Panos Soultanas and Dale B.Wigley
Helicases are enzymes involved in every aspect of nucleic acid metabolism.
Recent structural and biochemical evidence is beginning to provide details of
their molecular mechanism of action. Crystal structures of helicases have
revealed an underlying common structural fold. However, although there are
many similarities between the mechanisms of different classes of helicase, not all
aspects of the helicase activity are the same in all members of this enzyme family.
Panos Soultanas
School of Chemistry,
University of Nottingham,
University Park,
Nottingham, UK NG7 2RD.
Dale B.Wigley*
ICRF Clare Hall
Laboratories, Blanche
Lane, South Mimms,
Herts, UK EN6 3LD.
*e-mail:
[email protected]
The sequence of the bases in the DNA of each
organism contains the genetic information that
determines its characteristics. However, this
genetic information is locked within a double helix
formed by two interacting and antiparallel DNA
strands. Specific hydrogen-bonded pairs are formed
between the bases of the interacting strands.
Gaining access to the genetic information is of
crucial importance for the replication and repair of
this information as well as to translate it into
proteins. For RNA, there is a need to remove
unwanted secondary structures and to dissociate
RNA–DNA heteroduplexes. Therefore, a diverse
class of enzymes has evolved whose functions are to
allow access to the genetic information. These are
known as ‘helicases’.
Since the discovery of the first helicase from
Escherichia coli nearly a quarter of a century ago1,
a large number of these enzymes have been
identified and characterized in many organisms.
Primary-structure comparisons have identified
several conserved regions of amino acid sequence
homology thought to be characteristic of helicases
(so-called ‘helicase signature motifs’) and have
resulted in the classification of these proteins into
distinct superfamilies based upon the extent of
homology of their primary sequences2.
Superfamilies I and II represent the largest and
most closely related groups of helicases. They
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