1 Sinsheimer Laboratories, Department of Molecular, Cellular, and Developmental
Biology, University of California, Santa Cruz, CA 95064, USA
2 Howard Hughes Medical Institute, Department of Molecular, Cellular, and
Developmental Biology, University of California, Santa Cruz, CA 95064,
USA
* Author for correspondence (e-mail: chisholm{at}biology.ucsc.edu)
Accepted 11 August 2003
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SUMMARY |
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Key words: C. elegans, Epidermis, Morphogenesis, Ankyrin repeat, Spectrin
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Introduction |
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During early epidermal elongation, body muscle cells migrate to positions
underlying dorsal and ventral epidermis. At the same time, attachment
structures begin to form in the epidermis adjacent to the muscle cells.
Attachment structures were first observed using electron microscopy, and were
termed fibrous organelles (Francis and
Waterston, 1991; Hresko et
al., 1994
). Attachment structures are confined to the regions of
the epidermal cells overlying muscle and some other cells, suggesting that
muscle cells regulate attachment structure localization within the epidermal
cytoskeleton (Hresko et al.,
1994
). Epidermal attachment structures form part of a molecular
link between the contractile apparatus of the muscle and the cuticular
exoskeleton.
Epidermal elongation is not dependent solely on cell autonomous processes
in the epidermis but also requires nonautonomous function of muscle cells.
Mutations that severely disrupt muscle function also block epidermal
elongation soon after muscle contractions normally start, leading to the Pat
(paralysed, arrested at twofold) phenotype
(Barstead and Waterston, 1989;
Barstead and Waterston, 1991
;
Gettner et al., 1995
;
Williams and Waterston, 1994
).
The requirement for muscle function in elongation remains unexplained;
defective muscles might hinder epidermal cell shape changes, or muscle
contraction might provide additional stretching forces on the epidermis
allowing elongation beyond the twofold stage.
An alternative explanation comes from the observation that muscle cells
influence the organization of attachment structures in the epidermis. Loss of
embryonic attachment structure components results in elongation defects. The
spectraplakin VAB-10A is the C. elegans ortholog of Plectin, a
conserved component of hemidesmosomes; VAB-10A is localized to attachment
structures and is essential for elongation
(Bosher et al., 2003). The
cytoplasmic intermediate filament IFB-1A is a component of attachment
structures (W.-M. Woo and A.D.C., unpublished) and is also essential for
elongation beyond the twofold stage
(Karabinos et al., 2001
). The
transmembrane protein Myotactin localizes to attachment structures and is
essential for elongation; in the absence of Myotactin, attachment structures
do not remain localized to muscle-adjacent regions of the epidermis,
suggesting that Myotactin receives a muscle-dependent signal that anchors
attachment structures (Hresko et al.,
1999
).
Here, we report that vab-19 is essential for epidermal elongation
and that it encodes the C. elegans ortholog of a tumor suppressor
locus, Kank (Sarkar et al.,
2002). A functional VAB-19 fusion with green fluorescent protein
(VAB-19::GFP) localizes to epidermal attachment structures. The function of
vab-19 is not required for the assembly of attachment structures but
is required for their localization to muscle-adjacent regions of epidermis.
VAB-19 is also required for normal epidermal actin organization, and
vab-19 mutant phenotypes are specifically suppressed by loss of
function in the ßH spectrin SMA-1. VAB-19 might antagonize the
function of ßH spectrin in the apical actin cytoskeleton,
permitting attachment structures to develop beyond a certain stage and
allowing epidermal cells to change shape.
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Materials and methods |
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Four-dimensional microscopy
We made four-dimensional movies as previously described
(Chin-Sang et al., 1999). We
recorded movies from ten vab-19(e1036cs) embryos (progeny of
parents shifted to 15°C as L4s), eight vab-19(ju406) and five
vab-19(ju406)/nDf2 embryos. All vab-19 mutant embryos
arrested at the twofold stage.
Molecular cloning of vab-19
The vab-19(e1036) mutation was genetically mapped to the
lin-42 sup-9 interval on the left arm of chromosome II (map data are
available from Wormbase). The chromosomal deficiencies nDf2 and
nDf3 fail to complement vab-19. Cosmids were obtained from
the Sanger Institute (Hinxton, UK). DNA preparation and subcloning followed
standard procedures (Sambrook and Russell,
2001).
In rescue experiments, we assayed rescue of the vab-19 lethal phenotypes and of the Vab phenotypes of e1036 animals raised at a semipermissive temperature. Rescue of lethal phenotypes was scored qualitatively (growth or no growth of the transgenic strain at 15°C). Rescue of Vab phenotypes was scored in Rol animals; over 1000 animals were scored for each array tested. At 20°C, 20% of vab-19(e1036) animals are Vab; rescue was defined as a statistically significant reduction in the proportion of Vab in Rol animals.
Initial transformation experiments showed that cosmid F42G2 contained
partial vab-19 rescuing activity; the vab-19 exons contained
in F42G2 encode the N-terminal 484 amino acids of VAB-19. The minimal genomic
fragment that fully rescues vab-19 phenotypes is contained in clone
pCZ440, made by ligating a 6.6 kb MluI-XbaI fragment from
cosmid F42G2 with a 6.5 kb MluI-SalI fragment of cosmid
K02G6. vab-19 corresponds to Wormbase gene model T22D2.1. cDNAs for
vab-19 had been isolated in a genome-wide expressed sequence tag
project (Maeda et al., 2001).
The longest cDNA, yk481c2, was sequenced completely and found to lack the
first seven bases of the predicted coding sequence. In the process of allele
sequencing, we also identified one error in the published genomic sequence,
which changes one predicted intron to an exon. The predicted vab-19
gene structure between residues 470 and 504 was confirmed by sequencing
yk481c2 and another cDNA, yk656c2. The 6.5 kb MluI-SalI
genomic DNA fragment in pCZ440 was replaced with the corresponding cDNA
fragment from yk481c2 to make a vab-19 minigene (pCZ437). To make the
VAB-19::GFP fusion construct pCZ441, GFP coding sequence from pPD113.37 (Fire
lab vector kit) was inserted in frame at the BamHI site of
pCZ437.
To identify lesions in vab-19 alleles, vab-19 genomic DNA (including all exons and exon-intron junctions) was amplified from vab-19 mutant and wild-type animals. DNA sequences were determined using 33P-labeled primers and the fmol sequencing kit (Promega). All lesions were confirmed on both strands and from DNAs prepared in independent polymerase chain reactions (PCRs). Sequences of primers used are available on request.
To determine the effect of the ju406 mutation on vab-19 splicing, reverse-transcription PCR (RT-PCR) was performed on total RNA isolated from vab-19(ju406) animals (primer sequences available on request). We sequenced 13 RT-PCR products and found two kinds of mutant transcript containing frame shifts and premature stop codons in exons 3 or 4.
Germ-line transformation
Germ-line transformation followed standard procedures
(Mello et al., 1991) using
5-10 ng µl1 of vab-19 DNA and 50 ng
µl1 pRF4 coinjection marker. In all transgenic analysis,
at least five independent lines were scored. Integrants of functional
VAB-19::GFP arrays were obtained by TMP-UV mutagenesis. Five independent
integrants of VAB-19::GFP (juIs167, juIs168, juIs169, juIs170 and
juIs171) were identified and backcrossed multiple times before data
collection. The VAB-19::GFP transgenes displayed identical expression patterns
and rescue of vab-19 phenotypes. juIs167 was used in Figs
3,
4,
5, and juIs169 was
used in Fig. 6.
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Electron microscopy
L1 animals were processed for electron microscopy as previously described
(Hallam et al., 2002). Serial
thin sections were taken from three wild-type L1s and two
vab-19(e1036) L1s; the vab-19 mutants were progeny of
parents that had been shifted to 15°C at the L4 stage. The wild-type
sections are longitudinal sections from the head region.
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Results |
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Although our analysis did not reveal defects earlier than elongation in vab-19 mutants, we could not rule out the possibility that lack of VAB-19 causes defects in early development and that these defects indirectly affect elongation. To address this question, we performed temperature shift experiments using the cold-sensitive allele e1036. When vab-19(e1036) embryos grown at 22.5°C were shifted to 15°C before elongation, almost 100% of these embryos arrested at the twofold stage. When vab-19(e1036) embryos grown at 15°C were shifted to 22.5°C before elongation, about 80% of these animals survived to adulthood and appeared wild type in morphology (Fig. 1O). Thus, these experiments identify a vab-19 temperature-sensitive period at the twofold stage of elongation, suggesting that VAB-19 function is directly involved in epidermal elongation.
VAB-19 is a member of a conserved ankyrin repeat-containing protein
family
We mapped vab-19 between lin-42 and sup-9 on the
left arm of chromosome II, and tested genomic DNA clones from this region for
their ability to rescue vab-19(e1036) phenotypes in transgenic lines
(Fig. 2A). vab-19
phenotypes were partly rescued by cosmid F42G2. Further rescue analysis showed
that vab-19 corresponds to the gene T22D2.1 (see Materials and
methods).
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We determined the molecular lesions of vab-19 alleles.
ju406 alters the exon 3 splice acceptor (tccag|AAA to
tccaa|AAA); vab-19(ju406) animals express aberrantly spliced
transcripts that encode truncated proteins (see Materials and methods). The
cold-sensitive allele e1036 results in a premature stop codon
(R732opal) before the ankyrin repeat domain. e1036 behaves
genetically as a null at 15°C and as a hypomorph at higher temperatures.
At the restrictive temperature, e1036 appears to make the mRNA
unstable and subject to nonsense-mediated decay by the SMG system
(Mango, 2001), because the
lethal and Vab phenotypes of e1036 were almost completely suppressed
by a mutation in smg-3 (Table
1); smg-3 did not suppress ju406 (data not
shown). vab-19(e1036) might retain partial function owing to
read-through of the stop codon at the permissive temperature or in a
smg background; alternatively, truncated proteins made in
e1036 might themselves be cold sensitive.
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To determine whether VAB-19 is a component of epidermal attachment
structures, we asked whether VAB-19::GFP localized with known components of
attachment structures. Attachment structures contain several intermediate
filament (IF) proteins, some of which are recognized by monoclonal antibody
MH4 (Francis and Waterston,
1991). During early elongation, MH4 staining accumulated in a
single patch in dorsal epidermal cells
(Fig. 4B), whereas VAB-19::GFP
remained diffuse (Fig. 4A).
During the intermediate elongation stage, VAB-19::GFP and MH4 staining
displayed more extensive colocalization
(Fig. 4D-F). During later
elongation and afterwards, VAB-19::GFP and MH4 staining fully co-localized to
circumferential bands (Fig.
4G-L). Because MH4-positive IFs become localized before
VAB-19::GFP, VAB-19 probably is not required for the initial recruitment of
IFs to attachment structures.
The transmembrane protein Myotactin maintains localization of attachment
structures to muscle-adjacent parts of the epidermis
(Francis and Waterston, 1991;
Hresko et al., 1999
).
Myotactin itself initially localizes to muscle-adjacent regions of the
epidermis and later becomes organized into attachment structures. VAB-19::GFP
colocalized with Myotactin before the twofold stage
(Fig. 4M-O). Both Myotactin and
VAB-19::GFP localized to circumferential bands
(Fig. 4P-R). These bands are
interrupted by gaps where neuronal processes intervene between muscle and
epidermis (Fig. 4J,P,Q,
arrows). In contrast to Myotactin or VAB-19::GFP, MH4 staining is present or
enhanced at such positions (Fig.
4K, arrow). These differences suggest that VAB-19 does not
colocalize with all epidermal IFs but only those associated with muscle
adjacent attachment structures.
VAB-19::GFP was also expressed in pharyngeal marginal cells, which contain
prominent tonofilament-like attachment structures
(Albertson and Thomson, 1976).
Pharyngeal attachment structures resemble epidermal attachment structures in
both ultrastructure and molecular composition, because they contain
MH4-positive IFs, VAB-10A and Myotactin
(Francis and Waterston, 1991
).
In contrast to Myotactin, which is localized to the outer basal surface of
marginal cells (Fig. 4T),
VAB-19::GFP was localized throughout the apical-basal axis, although it was
concentrated at the apical and basal surfaces
(Fig. 4S, arrow; compare with
Fig. 4U). Thus, in pharyngeal
marginal cells, VAB-19 might be a component of the ends of IF-containing
attachment structures.
To examine how VAB-19 becomes localized to epidermal attachment structures, we fused GFP to different portions of VAB-19 and expressed these truncated proteins under the control of the vab-19 promoter. Truncated proteins lacking the ankyrin repeats were localized normally (Fig. 3G). By contrast, a truncated protein lacking the N-terminal domain was diffusely localized in dorsal and ventral epidermal cells (Fig. 3H). Thus, the VAB-19 N-terminal domain, containing the conserved motifs A and B, might be both necessary and sufficient for localization of VAB-19. GFP-tagged fusion proteins containing only motif A or only motif B were diffusely expressed (not shown), suggesting that both motifs are required together for normal VAB-19 localization.
Initial development of epidermal attachments is normal in
vab-19 mutants
Because, during later stages of embryogenesis, VAB-19 localizes in or close
to attachment structures, we examined the effect of vab-19 mutations
on the localization of other attachment structure components. One of the first
proteins to become localized to attachment structures is VAB-10A, a C.
elegans spectraplakin orthologous to Plectin and Drosophila Shot
(Bosher et al., 2003). During
early and intermediate (twofold) stages of epidermal elongation, the
localization of intermediate filaments (MH4) and VAB-10A appeared normal in
vab-19 mutants (Fig.
5). However, during later elongation, IFs or VAB-10A became
delocalized, expanding into regions not adjacent to body wall muscle, yet
remaining in regular circumferential bands (compare
Fig. 5C and 5F for MH4
staining). Thus, VAB-19 is a component of attachment structures but is not
essential for their formation.
Because of the similarities between VAB-19 and Myotactin distribution, we also analysed the localization of Myotactin in vab-19 mutants. Myotactin staining appeared normal in 1.5-fold and twofold vab-19 embryos (Fig. 5). However, in vab-19 mutants, Myotactin never became localized into circumferential stripes and instead remained localized to longitudinal bands in muscle-adjacent regions of the epidermis (compare Fig. 5I and 5M). Thus, VAB-19 is required to recruit Myotactin to attachment structures but not for its localization to muscle-adjacent regions.
To learn whether VAB-19 is required for aspects of attachment structure morphology not discernible by immunostaining, we used electron microscopy to compare attachment structures in wild-type and vab-19 mutant L1s. Attachment structures were present in vab-19 mutants and were correctly distributed with respect to actin bundles, as judged by the registration of attachment structures and cuticle annuli (which form directly above actin CFBs) in longitudinal sections (Fig. 5O and inset). The epidermis appears thicker in vab-19 mutants and attachment structures are extended, possibly because of the reduced length of the animal. Thus, VAB-19 is not essential for the correct distribution of attachment structures with respect to actin CFBs.
We asked whether attachment structure components were required for VAB-19
localization by introducing VAB-19::GFP into animals lacking Myotactin or
VAB-10A. In Myotactin (let-805) mutants, VAB-19::GFP was normally
localized to muscle-adjacent regions of the epidermis but never became
distributed into circumferential stripes. Within 15-20 minutes of muscle
contraction starting (430 minutes), VAB-19::GFP became dispersed into
puncta within epidermal cells (Fig.
6B). Thus, VAB-19 requires Myotactin for its subcellular
localization in epidermal cells. By contrast, in
vab-10A(ju281ts) mutants, VAB-19::GFP remained confined to
the regions of epidermal cells overlying muscle
(Fig. 6D) but never became
localized to attachment structures. Thus, neither Myotactin nor VAB-10A is
required for VAB-19 to become localized adjacent to muscles but both are
required for VAB-19 to be recruited to attachment structures.
Disorganization of the actin cytoskeleton in vab-19 mutants
can be suppressed by reduction in spectrin function
The driving force for epidermal elongation is provided by contraction of
circumferential actin bundles (CFBs)
(Priess and Hirsh, 1986).
Because vab-19 mutants have defects in elongation, we asked whether
VAB-19 is required for normal actin distribution. During elongation, actin
CFBs are circumferentially oriented in the epidermis
(Fig. 7A-D). Actin distribution
in vab-19 mutants appeared normal until the 1.5-fold stage
(Fig. 7E). As the mutant
embryos developed to the twofold stage, the epidermal actin cytoskeleton
became disorganized. For example, actin bundles were absent or reduced from
apical epidermis (Fig. 7F,
arrow). Actin CFBs were often misoriented or splayed
(Fig. 7G,H). Although some of
these actin bundles might represent misplaced actin from other parts of the
epidermis, the apical CFBs constitute the most prominent actin structures in
the epidermis at this stage. The disorganization of actin bundles was usually
more obvious where body muscles had detached from epidermal cells. This result
suggests that muscle detachment might result in the disorganization of actin
bundles, which then blocks epidermal elongation.
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Discussion |
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VAB-19 is a member of a conserved protein family and is a component
of C. elegans epidermal attachment structures
Analysis of VAB-19 homologs in Drosophila and vertebrates has
shown that the VAB-19 family is defined by a highly conserved C-terminal
domain containing four ankyrin (ANK) repeats, and a less well conserved
N-terminal domain containing two novel motifs. ANK repeats are found in many
proteins with diverse functions; according to the protein families database
Pfam (Bateman et al., 2002),
over 112 C. elegans proteins are predicted to contain ANK repeats.
The ANK repeat domains of VAB-19 family members are 58-60% identical to each
other; outside the VAB-19 family, the closest ANK repeats (37% identity) are
from zebrafish Diversin (Schwarz-Romond et
al., 2002
), which shares no other similarities with VAB-19. ANK
repeats function as protein-protein interaction interfaces
(Sedgwick and Smerdon, 1999
).
The high degree of conservation in the ANK repeats suggests that VAB-19 family
members bind one or more conserved proteins via this domain.
The ANK-repeat domain of VAB-19 is neither necessary nor sufficient for the normal subcellular localization of the protein, as assayed using GFP fusion proteins. By contrast, two conserved motifs at the N-terminus are necessary and sufficient for VAB-19::GFP subcellular localization to attachment structures. The N-terminus of VAB-19 shows no similarity to IF-associated proteins, suggesting that VAB-19 is localized indirectly to IF-containing structures. Three non-IF proteins are found in embryonic attachment structures the spectraplakin VAB-10A and the transmembrane receptors Myotactin and MUP-4. Our molecular epistasis experiments suggest that the final localization of VAB-19 requires both VAB-10A and Myotactin. VAB-10A is required for VAB-19 localization to attachment structures but not to muscle-adjacent regions; by contrast, Myotactin is required for both aspects of VAB-19 localization. These proteins might direct VAB-19 localization via the VAB-19 N-terminal motifs.
Role of VAB-19 in development of attachment structures
Using molecular epistasis, we have explored the functional relationships
between VAB-19 and other components of attachment structures. In
vab-19 mutants, the localization of known attachment structure
components (IFs/MH4 and VAB-10A/Plectin) appears normal until late in
embryogenesis. At this point, attachment structures redistribute into
non-muscle-adjacent regions in the epidermis, although they remain in regular
circumferential bands (Fig.
5F), suggesting that trans-epidermal attachments initially
assemble normally but, in the absence of VAB-19, they lose their normal
association with muscle cells. Consistent with this interpretation, our
ultrastructural analysis has shown that attachment structures are present and
correctly placed with respect to cuticular structures in vab-19
mutants.
Like VAB-19, Myotactin anchors attachment structures to muscle-adjacent
regions (Hresko et al., 1999).
Myotactin and VAB-19 are mutually required for their correct final
localization. Myotactin's early localization to muscle-adjacent regions does
not require VAB-19, but the assembly of Myotactin into circumferential bands
is dependent on VAB-19 (Fig.
5M). Myotactin is a large transmembrane protein with an
extracellular domain consisting of
32 fibronectin type-III repeats,
suggesting that it could bind a ligand on muscle cells or muscle-associated
extracellular matrix and thereby anchor attachment structures in
muscle-adjacent regions. Unlike VAB-10A or intermediate filaments, the final
localization of VAB-19 to attachment structures is completely dependent on
Myotactin. VAB-19 might function in the Myotactin-dependent stabilization of
attachment structures in muscle-adjacent regions; in the absence of Myotactin,
VAB-19 would lose its association with attachment structures, and attachment
structures lose their association with muscle.
The refinement of attachment structure localization into circumferential
bands must involve interaction with CFBs. VAB-19 is not required for the
segregation of attachment structures from actin CFBs, because attachment
structures retain their striped appearance in vab-19 mutants.
However, VAB-19 could play a role in cross-talk between attachment structures
and the cortical actin cytoskeleton. At present, the evidence for a specific
interaction between VAB-19 and the actin cytoskeleton comes from the
suppression of vab-19 by sma-1. By analogy to other
spectrins, SMA-1/ßH-spectrin probably cross-links apical actin
and might stabilize integral membrane proteins within the apical epidermis
(Dubreuil and Grushko, 1998).
Is VAB-19 localized to the apical ends of attachment structures, consistent
with a role in regulating the cortical actin cytoskeleton or SMA-1? Although
epidermal attachment structures are too short to resolve apical versus basal
localization, we can partly address this question by examining pharyngeal
marginal cells, which contain longer attachment structures. In these cells,
VAB-19::GFP appeared to be concentrated at the apical and basal ends of
attachment structures. Although we do not know the role of vab-19 in
pharyngeal marginal cells, the similarities between pharyngeal and epidermal
attachment structures suggest that, by analogy, VAB-19 might be present at the
apical surface of the epidermis, consistent with a role in regulation of the
cortical actin cytoskeleton. We infer from our genetic analysis that VAB-19
might locally inhibit or antagonize SMA-1 function, perhaps to allow
attachment structures to move during the cell shape changes of epidermal
elongation. Thus, in sma-1 mutants, apical membrane proteins might be
less highly cross-linked to cortical actin, partly bypassing the function of
vab-19. Further tests of these models will require identification of
proteins that interact with VAB-19.
Role of attachment structures in epidermal cell shape changes
Why are epidermal attachment structure components required for epidermal
elongation? One model suggests that the function of attachment structures in
elongation is indirect and reflects their role in muscle contraction. In
mutants such as vab-19, muscles detach from the epidermis soon after
muscle contraction begins, presumably because of a failure in the mechanical
strength of epidermal muscle attachments. Muscle function is, for unknown
reasons, required for epidermal elongation and so muscle detachment leads to
elongation arrest. Alternatively, attachment structures might directly
influence the organization of epidermal actin bundles, which are themselves
required for elongation. Epidermal attachments and circumferential actin
bundles are regularly interspersed in the muscle-adjacent parts of epidermal
cells; during later stages, attachment structures are in register with the
cuticular ridges known as annuli, whereas actin bundles coincide with the
furrows separating annuli (Costa et al.,
1997). The complementary distribution of actin CFBs and attachment
structures suggests that they are mutually required for each other's
organization. This is exemplified by VAB-10A/Plectin (an attachment-structure
component) and VAB-10B/MACF, an actin-associated protein, which are mutually
required for their organization into complementary circumferential bands
(Bosher et al., 2003
). The
complementary pattern of actin and attachment structures arises gradually:
actin and attachment structures might partly co-localize at early stages and
only later segregate into alternating bands. Thus, the elongation defects of
mutants defective in muscle might arise because muscles induce the development
of epidermal attachment structures, which themselves influence the
organization of epidermal actin.
What is the conserved cellular function of the VAB-19 family?
There are VAB-19 orthologs in the Drosophila, mouse, rat and human
genomes. At present, however, little is known of the function of these VAB-19
family members. Based on the conservation of sequence in both the N-terminal
motifs and the C-terminal ankyrin repeats, VAB-19 family members probably
interact with conserved protein partners and function in regulating the
cytoskeleton.
The Drosophila genome contains a single vab-19 ortholog,
whose function has not yet been determined. However, unlike C. elegans,
Drosophila lacks cytoplasmic IFs
(Fyrberg and Goldstein, 1990;
Goldstein and Gunawardena,
2000
). Instead, Drosophila muscle attachment structures
contain microtubule (MT) bundles (Mogensen
and Tucker, 1988
). The Drosophila spectraplakin Shot
(previously known as Kakapo) is a component of attachment structures and
contains a MT-binding domain but not an IF-binding domain
(Gregory and Brown, 1998
;
Röper et al., 2002
). It
will be important to determine whether the Drosophila VAB-19 is a
component of MT-based muscle-attachment structures. The existence of a
conserved VAB-19 family member in a species lacking cytoplasmic IFs further
supports the hypothesis that VAB-19 does not interact directly with IFs.
Nevertheless, VAB-19 and VAB-10-like spectraplakins could represent an
evolutionarily conserved core of cell-matrix attachment structures.
The only vertebrate VAB-19 homolog for which functional data exist is the
human gene Kank, which might function as a tumor suppressor for renal
cell carcinoma, the most common malignancy of the adult kidney
(Sarkar et al., 2002).
Expression of Kank in HEK293 cells suppressed cell growth and
morphological changes, and expression of Kank in human kidney tumor
(G-402) cells resulted in disorganization of actin
(Sarkar et al., 2002
). In
light of the possible interaction of VAB-19 with actin in C. elegans,
the VAB-19 family might play a conserved role in the actin cytoskeleton. The
ability of human Kank partly to rescue vab-19 mutant phenotypes
supports the contention that the VAB-19 family has a conserved cellular
function. It will clearly be informative to determine the subcellular
localization and function of VAB-19 family members in vertebrate cells.
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ACKNOWLEDGMENTS |
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