brahma
89B Helicase, a second Drosophia SNF2 homolog Many proteins of the SNF2 family, which share a similar DNA-dependent ATPase/putative helicase
domain, are involved in global transcriptional control and processing of DNA damage. Reported here is the partial cloning and characterization of 89B helicase, a gene encoding a new Drosophila
melanogaster member of the SNF2 family. 89B Helicase protein shows a high degree of homology in
its ATPase/helicase domain to the global transcriptional activators SNF2 and Brahma and to the DNA
repair proteins ERCC6 and RAD54. It is, however, most strikingly similar to the Saccharomyces
cerevisiae protein Mot1, a transcriptional repressor with many target genes for which no homolog
has yet been described. The region closely related to SNF2 is locate between nucleotides 1100 and 2522 of the partial 89B helicase cDNA. This segment contains the seven motifs common to DNA-dependent ATPases/presumptive helicases. The area of highest similarity at the DNA levels are motifs V and VI, which most characterizes the SNF2-related family of helicases. In the region containing these motifs 89B helicase is approximately 60% identical to the following: Drosophila brahma, its human homologs brg1, hbrm, ISWI and ERCC6. However, the greatest identity (65%) over the longest stretch encompassing these motifs (468bp) is to the yeast mot1 gene. While 89B Helicase protein exhibits 31% identity to Brahma in motifs V and VI, the greatest identity (also about 30%) to ERCC6, SNF2 and SNF2L is in the sequences encompassing domains I-IV, and 42% identity is shown with RAD26, but only over domains II-IV. In addition, the entire helicase domain is similarly placed in the C-termini of both 89 Helicase and Mot1, unlike several other members of the family, such as SNF2 itself and Brahma, in which an additional domain, termed the bromodomain, is present between the helicase domain and the C-terminus of the protein (Goldman-Levi, 1998).
89B helicase is expressed throughout fly development and its large transcript
encodes a >200 kDa protein. Staining with anti-89B Helicase antibodies reveals that the protein is
present uniformly in early embryos and then becomes localized to the ventral nerve cord and brain. The protein is expressed in unfertilized eggs and early embryos, and is ubiquitously distributed throughout the embryo (but not in pole cells) during the first part of embryogenesis, including the blastoderm stage, gastrulation and germband extension. At 8 hours of embryogenesis, during germband retraction, the protein becomes highly localized to the ventral nerve cord and brain. In the CNS, it is preferentially found in the longitudinal connectives rather than in the horizontal commissures of the scaffold. On
the polytene chromosomes, 89B Helicase is bound to several hundred specific sites that are randomly
distributed. It is not present in the puffed regions of the chromosomes. The homology of 89B Helicase to Mot1, its widespread developmental expression and its
large number of targets on the polytene chromosomes of larval salivary gland cells suggest that 89B
Helicase may play a role in chromosomal metabolism, particularly global transcriptional regulation (Goldman-Levi, 1998).
CHD genes, potential chromatin modifiers related to Brahma The murine gene CHD1 (MmCHD1; see Drosophila Chd1) was previously isolated in a search for proteins that would bind a
DNA promoter element. The presence of chromo (chromatin organization modifier) domains (such as those found in Drosophila Brahma and Imitation SWI) and an
SNF2-related helicase/ATPase domain (present also in Drosophila HP1 and Polycomb) led to speculation that this gene might regulate chromatin structure
or gene transcription. Three novel human genes are
related to MmCHD1. Examination of sequence databases produce several more related genes, most
of which are not known to be similar to MmCHD1, yielding a total of 12 highly conserved CHD
genes from organisms as diverse as yeast and mammals. A Drosophila homolog, DmCHD1, contains all the domains found in the human sequence; another homolog, DmCHD3, lacks the DNA-binding domain sequence. MmCHD1 preferentially binds via minor groove interactions to DNA that contains (A+T)-rich tracts including those in a matrix attachment region. The major region of sequence variation in CHD proteins is in
the C-terminal part of the protein, a region with DNA-binding activity in MmCHD1. Targeted deletion
of ScCHD1, the sole Saccharomyces cerevesiae CHD gene, was performed with deletion strains
being less sensitive than wild type to the cytotoxic effect of 6-azauracil. This finding suggests that
enhanced transcriptional arrest at RNA polymerase II pause sites (due to 6-azauracil-induced
nucleotide pool depletion) is reduced in the deletion strain and that ScCHD1 inhibits transcription.
This observation, along with the known roles of other proteins with chromo or SNF2-related
helicase/ATPase domains, suggests that alteration of gene expression by CHD genes might occur by
modifications of chromatin structure, with altered access of the transcriptional apparatus to its
chromosomal DNA template (Woodage, 1997).
A mammalian chromatin-associated
protein, CHD1 (chromo-ATPase/helicase-DNA-binding domain), might have an important role in the modification of chromatin structure. The Drosophila melanogaster CHD1 homolog (dCHD1) encodes an 1883-aa open reading frame that is
50% identical and 68% similar to the mouse CHD1 sequence, including conservation of the three
signature domains for which the protein was named. dCHD1 is related to both Drosophila Brahma and Imitation SWI as well as to Polycomb and HP1, based on the presence of both a helicase domain (found in Brahma and ISWI) and a chromo domain (found in Polycomb and HP1). When the chromo and ATPase/helicase domain
sequences in various CHD1 homologs are compared with the corresponding sequences in other
proteins, certain distinctive features of the CHD1 chromo and ATPase/helicase domains are
revealed. This suggests that CHD constitutes a distinct subgroup that diverged early in evolution from HP1 and PC subgroups as well as from ISWI type proteins. The dCHD1 gene maps to position 23C-24A on chromosome 2L. Western blot
analyses with antibodies raised against a dCHD1 fusion protein specifically recognize an
approximately 210-kDa protein in nuclear extracts from Drosophila embryos and cultured cells. Most
interestingly, these antibodies reveal that dCHD1 localizes to sites of extended chromatin
(interbands) and regions associated with high transcriptional activity (puffs) on polytene chromosomes
from salivary glands of third instar larvae. These observations strongly support the idea that CHD1
functions to alter chromatin structure in a way that facilitates gene expression (Stokes, 1996).
Arabidopsis thaliana BRAHMA (BRM, also called AtBRM) is a SNF2 family protein homolog of Brahma, the ATPase of the Drosophila SWI/SNF complex involved in chromatin remodeling during transcription. In contrast to its Drosophila counterpart, BRM is not an essential gene. Thus, homozygous BRM loss of function mutants are viable but exhibit numerous defects including dwarfism, altered leaf and root development and several reproduction defects. The analysis of the progeny of self-fertilized heterozygous brm plants and reciprocal crosses between heterozygous and wild type plants indicated that disruption of BRM reduced both male and female gametophyte transmission. This was consistent with the presence of aborted ovules in the self-fertilized heterozygous flowers that contained arrested embryos predominantly at the two terminal cells stage. Furthermore, brm homozygous mutants are completely sterile. Flowers of brm loss-of-function mutants have several developmental abnormalities, including homeotic transformations in the second and third floral whorls. In accordance with these results, brm mutants present reduced levels of APETALA2, APETALA3, PISTILLATA and NAC-LIKE, ACTIVATED BY AP3/PI. BRM strongly interacts with AtSWI3C. The interaction studies have been extended demonstrating that BRM interacts weakly with AtSWI3B but not with AtSWI3A or AtSWI3D. In agreement with these results, the phenotype described in this study for brm plants is very similar to that previously described for the AtSWI3C mutant plants, suggesting that both proteins participate in the same genetic pathway or form a molecular complex (Hurtado, 2006).
Yeast and Tetrahymena Brahma homologs brahma (brm), a member of the trithorax group, encodes a protein related to the yeast
SWI2/SNF2 protein, a subunit of a protein complex that assists sequence-specific activator
proteins by alleviating the repressive effects of chromatin (Dingwall, 1995).
The Saccharomyces cerevisiae SWI1, SWI2 (SNF2), SWI3, SNF5, and SNF6 gene products
play a crucial role in the regulation of transcription. Direct biochemical evidence has demonstrated
that all five SWI/SNF polypeptides are components of a large multisubunit complex. These five
polypeptides have a combined apparent molecular mass of approximately
2 MDa. Assembly of the SWI/SNF complex is not
disrupted by a mutation in the putative ATP-binding site of SWI2, although this mutation eliminates
SWI2 function (Peterson, 1994).
Considerable homology has recently been noted between the proteins encoded by the RAD5,
RAD16 and RAD54 genes of Saccharomyces cerevisiae. These genes are members of the RAD6,
RAD3 and RAD50 epistasis groups, respectively, which correspond to the three major DNA
repair pathways in yeast. These proteins also share homology with other eukaryotic proteins,
including those encoded by SNF2 and MOT1 of yeast, Brahma and lodestar of Drosophila and the
human ERCC6 gene. The homology shares features with known helicases, suggesting a newly
identified helicase subfamily (Glassner, 1994).
A 15-subunit complex with the capacity to remodel the structure of chromatin, termed RSC, has been isolated from S. cerevisiae on the basis of homology to the yeast SWI/SNF complex. RSC, a second yeast chromatin remodeler, is at least 10-fold more abundant than SWI/SNF and is essential for mitotic growth. At least three RSC subunits are related to SWI/SNF peptides. Like SWI/SNF, RSC exhibits a DNA-dependent ATPase activity stimulated by both free and nucleosomal DNA and a capacity to perturb nucleosome structure. No association of either RSC or SWI/SNF with RNA polymerase II holoenzyme was detected and no histone acetyltransferase activity was found. The functional distinction between SWI/SNF-related and NURF complexes corresponds with the classification of the ATPase components. The six chromatin-remodeling complexes so far described (yeast SWI/SNF, RSC, brahma complex, NURF, and the two mammalian SWI/SNF complexes) contain, respectively, the DNA-dependent ATPases Snf2/Swi2p, Sth1p, Brahma, ISWI, and mammalian Brg1p (or hBrm protein). All of these ATPases except ISWI are similar in their ATPase domains and in several additional regions, whereas their similarity to ISWI is limited to the ATPase domain alone. One such region, present in the carboxyl termini of all of the ATPases except ISWI, constitutes a bromodomain (Cairns, 1996).
Tetrahymena Histone Acetyltransferase A, p55, is a homolog of yeast Gcn5p which exists as a heterotrimeric complex in yeast cells with at least two other polypeptides. Current models suggest that this complex bridges enhancer-binding factors to the basal transcription machinery. Both the Tetrahymena protein and Gcn5p possess histone acetyltransferase activity and a highly conserved bromodomain. p55 preferentially acetylates Histone H3. The presence, in nuclear A-type histone acetyltransferases but not in cytoplasmic B-type HATs, of a bromodomain, known to function in protein-protein interaction, suggests that HAT A is directed to chromatin through protein interaction to facilitate transcriptional activation. It is also believed that there is a functional interaction between the Histone Acetyltransferase A type of complex and components of the SWI-SNF complex in yeast (Drosophila homologs: ISWI and
Brahma). Although the exact mechanism by which the SWI/SNF complex operates is unclear, an implication of this interaction is that one function of the SWI/SNF complex is to direct HAT A to specific sites in chromatin. These findings, combined with the recent demonstration that the SWI/SNF polypeptides are integral components of the RNA polymerase II holoenzyme in yeast, suggest a mechanism whereby HAT A is targeted to chromatin during transcriptional activation, establishing a direct link between histone acetylation and gene activation (Brownell, 1996).
In the yeast Saccharomyces cerevisiae, the SWI-SNF complex has been proposed to antagonize the repressive effects of chromatin by disrupting nucleosomes. The SIN genes were identified as suppressors of defects in the SWI-SNF complex, and the SIN1 gene encodes an HMG1-like protein that has been proposed to be a component of chromatin. Specific mutations (sin mutations) in both histone H3 and H4 genes produce the same phenotypic effects as do mutations in the SIN1 gene. Sin1 and the H3 and H4 histones interact genetically; the C terminus of Sin1 is shown to physically associates with components of the SWI-SNF complex. In addition, this interaction is blocked in the full-length Sin1 protein by the N-terminal half of the protein. Based on these and additional results, it is proposed that Sin1 acts as a regulatable bridge between the SWI-SNF complex and the nucleosome (Perez-Martin, 1998).
The genetic and biochemical results described in this study all point to Sin1 as a target of the SWI-SNF complex. One plausible scenario is that nucleosome disruption by the SWI-SNF complex involves not only the removal of H2A-H2B dimers but also the specific removal of other chromatin-associated proteins, such as Sin1. The experiments described in this study suggest a provisional model for Sin1 function. It is proposed that in solution, Sin1 is folded back on itself as the result of interactions between its Nt and Ct halves. This interaction would prevent Sin1 from interacting with the SWI-SNF complex in solution. The binding of Sin1 to the nucleosome (either to DNA or to histones) would, according to this model, release the inhibition. Since Sin1 is formally a repressor of transcription, while the SWI-SNF components are formally activators, the proposal that the two function together may seem paradoxical. However, it is possible that Sin1 functions both to stabilize chromatin, perhaps by interacting with the nucleosome core, and to destabilize it by recruiting the SWI-SNF complex. According to this view, Sin1 would function to maintain the balance between chromatin assembly and disassembly (Perez-Martin, 1998).
The HTA1-HTB1 locus is one of four cell cycle-regulated loci that encode the core histones in S. cerevisiae. The cis-acting regulatory sequences at this locus include three copies of a conserved activation (UAS) element, as well as a unique negative regulatory site that controls several key aspects of HTA1-HTB1 transcription. The negative site keeps the HTA1 and HTB1 genes repressed during the G1 and G2 periods of the cell cycle and shuts off HTA1 and HTB1 transcription when the intracellular levels of histones H2A and H2B rise. A number of trans-acting factors that act at the negative site to repress transcription have been identified through genetic screens. Two of these factors, the products of the evolutionarily conserved HIR1 and HIR2 genes, are transcriptional corepressors that do not possess intrinsic DNA binding activity. They are postulated to be recruited to the negative site by a sequence-specific binding factor, and once at this site they directly repress transcription. The current view is that the activity of these corepressors must be antagonized to allow HTA1 and HTB1 transcription to be activated to maximal levels (Dimova, 1999).
Several lines of evidence suggest that the chromatin structure of the HTA1-HTB1 locus might directly contribute to the repression of HTA1 and HTB1 transcription and that the Hir1 and Hir2 proteins are involved in this effect. As also noted for the Swi/Snf-regulated SUC2 gene, nucleosomes at the HTA1-HTB1 locus are organized into an extended array. Deletion of the HIR1 or HIR2 gene partially disrupts this array and concommitantly relieves transcriptional repression. In addition, Hir2p and its vertebrate ortholog, HIRA, bind to immobilized histones H2B, H3, and H4 in vitro, suggestive of a role for Hir proteins in chromatin assembly or structure. In support of this view, mutations in HIR1 and HIR2 have been found to act synergistically with mutations in Chromatin Assembly Factor I to decrease telomeric and HM silencing, a phenomenon strongly associated with repressive chromatin structure. It was therefore asked whether nucleosome remodeling factors were required for transcription of the HTA1-HTB1 locus by analyzing the effects of mutations in the yeast Swi/Snf complex on expression of an HTA1-lacZ reporter gene. Deletion of the SNF5 gene, which prevents complex assembly, or of the SNF2 gene, which removes the catalytic subunit, reduces lacZ mRNA levels approximately 7-fold over those found in wild-type cells. In addition, transcript levels from the HTB1 gene present at the endogenous HTA1–HTB1 locus are ~3-fold lower in each snf mutant compared to a wild-type strain. Together, the data identify the HTA1–HTB1 locus as a novel Swi/Snf-regulated locus (Dimova, 1999).
To determine whether Swi/Snf is required for HTA1-HTB1 transcription because of Hir-mediated repression at the negative site, the negative site from the promoter of an HTA1-lacZ gene was deleted, leaving the three UAS elements intact. In the absence of the negative site, HTA1-lacZ mRNA levels rose by ~2.5-fold in the wild-type strain. Identical levels of transcript were also present in the snf5delta mutant, indicating that the transcriptional defect had been bypassed. Next, it was asked whether the Hir proteins themselves were responsible for the Snf2p and Snf5p dependence of the locus. When either HIR gene was deleted in a snf5delta or snf2delta mutant, both HTA1-lacZ and HTB1 mRNAs were made at levels equivalent to those produced in hir1delta or hir2delta single mutants, where transcription was derepressed. Thus, when Hir repression is abolished by either cis- or trans-acting mutations, the Swi/Snf constituents Snf2p and Snf5p are no longer required for transcription of the HTA1-HTB1 locus. This indicates a role for the Swi/Snf complex in relieving Hir-mediated repression at the histone gene locus (Dimova, 1999).
The finding that Hir repression could be directly responsible for the Swi/Snf dependence of the HTA1-HTB1 locus suggested that components of the Swi/Snf complex might physically interact with the two Hir corepressors. To test for this association, HA-tagged Hir1 or Hir2 proteins were immunoprecipitated from whole-cell extracts using a monoclonal antibody against the HA epitope, and the coprecipitation of the Swi/Snf subunits Snf5p, Snf2p/Swi2p, and Swi3p was analyzed by Western blot analysis with polyclonal antibodies specific for each protein. All three Swi/Snf proteins coimmunoprecipitate with each Hir-HA protein. These results provide the first evidence for a physical interaction between components of the yeast Swi/Snf complex and locus-specific transcriptional repressors. Moreover, since three separate Swi/Snf subunits could be coimmunoprecipitated with Hir1p and Hir2p, this provides strong evidence that the Swi/Snf complex, or a Swi/Snf subcomplex, is associated with the two corepressors. It is also likely that this association involves only a subset of Swi/Snf complexes because a very small fraction of total cellular Snf5, Snf2, and Swi3 protein was found to interact with Hir1p and Hir2p (Dimova, 1999).
Hir1p and Hir2p play a direct role in keeping HTA1 and HTB1 transcription repressed throughout most of the cell cycle; only when Hir repression is relieved at the
G1/S boundary are the two histone genes transcriptionally activated. Swi/Snf is likely to function at the
HTA1-HTB1 promoter during the period of transcriptional activation, a point when Hir repression is itself inactive. This raises the question
of how the Hir corepressors could be involved in the recruitment of Swi/Snf. One possibility is that Swi/Snf associates with the HTA1-HTB1 promoter as a
consequence of cell cycle regulatory signals that act on Hir1p and Hir2p at the G1/S boundary. These signals might alter the
ability of the Hir proteins to act as corepressors and simultaneously convert them into coactivators that recruit Swi/Snf. This predicts that Swi/Snf-Hir protein
interactions or Swi/Snf association with the HTA1-HTB1 promoter might be regulated as a function of the cell cycle. The notion that transcriptional repressors could function as coactivators to target Swi/Snf to specific genes has also been suggested from other studies. RB, for
example, has been postulated to recruit hSwi/Snf to genes activated by the glucocorticoid receptor and E2F1. In the latter case, however, the RB-hSwi/Snf associations result in
the inhibition of E2F-mediated transactivation, and in this context RB functions as a corepressor rather than a coactivator. Genetic studies in yeast indicate that the in vivo role of Swi/Snf is to antagonize chromatin-mediated transcriptional repression. It is
therefore possible that the presence of Swi/Snf at the HTA1-HTB1 promoter and its physical interactions with Hir1p and Hir2p represent a role for the remodeling
complex in counteracting a repressive chromatin structure mediated by the Hir proteins. Thus, Hir1p and Hir2p could both target Swi/Snf and be antagonized by the
remodeling factor. The idea that Swi/Snf might relieve repression by acting on nonhistone regulatory proteins is also indicated from other studies in yeast. Both
genetic and biochemical data suggest that yeast Swi/Snf antagonizes repression mediated by the Tup1p/Ssn6p corepressors at the SUC2 locus and that it counteracts the repressive effects of the general
transcriptional repressor, Sin1p. Since Tup1p/Ssn6p and Sin1p are also postulated to
inhibit transcription partly through their interactions with chromatin constituents, Swi/Snf might selectively target certain classes of chromatin-associated repressors for inactivation.
Whether this is part of a general mechanism by which Swi/Snf functions in vivo remains to be determined (Dimova, 1999 and references).
The yeast SWI/SNF complex stimulates in vitro transcription from chromatin templates in an ATP-dependent manner. SWI/SNF function in this regard requires the presence of an activator with which it can interact directly, linking activator recruitment of SWI/SNF to transcriptional stimulation. In this study, it was determined the SWI/SNF subunits that mediate its interaction with activators. Using a photo-cross-linking label transfer strategy, it was shown that the Snf5, Swi1, and Swi2/Snf2 subunits are contacted by the yeast acidic activators, Gcn4 and Hap4, in the context of the intact native SWI/SNF complex. In addition, the same three subunits can interact individually with acidic activation domains, indicating that each subunit contributes to binding activators. Furthermore, mutations that reduce the activation potential of these activators also diminish its interaction with each of these SWI/SNF subunits. Thus, three distinct subunits of the SWI/SNF complex contribute to its interactions with activation domains (Neely, 2002).
The role that individual SWI/SNF subunits play in the function of the complex is
not clearly understood. A functional dissection of the yeast
complex has begun by determining the subunits that are involved in promoter recruitment.
Snf5, Swi1, and Swi2/Snf2 are all conserved in humans, which may indicate a
common mode for the recruitment of chromatin remodeling activity among species.
Several human SWI/SNF subunits have been implicated in interactions with
different domains of transcription factors. The BAF155 and BAF170 subunits (two
yeast Swi3 homologs), along with the BRG1 subunit (a yeast Swi2/Snf2 homolog),
directly interact with the zinc finger DNA-binding domain of EKLF. A
second yeast Swi2/Snf2 homolog, hBrm, directly interacts with the
transactivation domain of C/EBPbeta. BRG1 and hBrm are mutually exclusive
within mammalian SWI/SNF complexes; however, a number of proteins are common to
both hBrm- and BRG1-associated complexes. A recent report has demonstrated
that BRG1-containing SWI/SNF functionally and physically interacts with hHSF1,
and acidic and hydrophobic residues in two C-terminal activation domains of
hHSF1 are important for this interaction. However, it still remains to be
seen if hHSF1 directly interacts with BRG1 or associates with the complex via
another hSWI/SNF subunit. The importance of hydrophobic residues in the
interaction of hSWI/SNF with hHSF1 is consistent with the finding that
hydrophobic patches in the Gcn4 activation domain are important for both
interaction with yeast SWI/SNF and its transcriptional potency (Neely, 2002 and references therein).
Another subunit of hSWI/SNF, hSnf5/Ini1, has recently been shown to interact
with the proto-oncogene c-Myc and the viral transcriptional activator, EBNA2,
via domains that are distinct from their transactivation domains.
This subunit was originally identified via a yeast two-hybrid assay as a protein
that binds to the human immunodeficiency virus type 1 integrase, hence the name
Ini1 (for integrase interactor 1), and due to its sequence
homology to yeast Snf5 (Neely, 2002 and references therein).
BAF250, a yeast Swi1 homolog and component of the human SWI/SNF-A complex,
directly interacts with the glucocorticoid receptor. Systematic study
supports a conserved function (from yeast to humans) for the Snf5, Swi1, and
Swi2/Snf2 subunits in SWI/SNF recruitment by transcription activators. The
finding of direct interactions of these yeast SWI/SNF subunits with activators
as part of native complexes demonstrates that the interacting surfaces of these
subunits are indeed available for activator interactions in their natural
context, which has not yet been demonstrated for their human counterparts (Neely, 2002 and references therein).
Data are emerging that link two subunits of human SWI/SNF complexes, hSnf5/Ini1
and BRG1, to cancer. Interestingly, these subunits were found in the current
study, as well as in other recent studies, as targets for activators in the
recruitment of SWI/SNF. This opens the possibility that improper SWI/SNF
recruitment to its target genes results in the onset of cancer. Studies have
found that the hSnf5/Ini1 gene is mutated in many pediatric cancers,
indicating that it is a tumor suppressor gene. Furthermore,
heterozygous Snf5/Ini1 knockout mice develop nervous system and soft tissue
sarcomas, whereas homozygous mutations result in embryonic lethality.
A second component of human SWI/SNF complexes, BRG1, has also been
implicated as a tumor suppressor and as a target for mutation in human cancer.
BRG1 is also an important regulator of cell growth through its
interactions with the tumor suppressor pRb. A BRG1-containing
SWI/SNF complex has been purified via association with the tumor suppressor
and transcriptional regulator BRCA1 through a direct interaction between BRG1
and BRCA1. These experiments have shown that the yeast homologs of hSnf5/Ini1
and BRG1 can facilitate interactions of the SWI/SNF chromatin remodeling complex
with DNA-binding transcription activators. Other studies have demonstrated the
interaction of these human SWI/SNF subunits with DNA-binding factors.
These studies suggest that hSnf5 and BRG1 are essential for the proper
regulation of genes encoding other tumor suppressors by facilitating the
recruitment of SWI/SNF. In addition, these SWI/SNF subunits might link
chromatin-remodeling activity to target genes of some tumor suppressor proteins,
namely, pRb and BRCA1, via direct interactions with these proteins (Neely, 2002 and references therein).
C. elegans SWI/SNF complex is required for asymmetric cell division Asymmetric cell division is a fundamental process that produces cellular diversity during development. Two mutants in C. elegans (psa-1 and
psa-4) have been identified in which the asymmetry of T cell division is disrupted. psa-1 and psa-4 encode homologs of yeast SWI3 and SWI2/SNF2, respectively; these are
components of the SWI/SNF complex. RNA interference assay shows that homologs of other components of SWI/SNF are also involved in T cell division.
psa-1 and psa-4 are likely to be required in the T cell during mitosis to cause asymmetric cell division. Because the SWI/SNF complex is required for asymmetric division in S. cerevisiae, these results demonstrate that at least some aspects of the mechanism of asymmetric cell division are conserved between yeast and a multicellular organism (Sawa, 2000).
The asymmetric division of the T cells in C. elegans is regulated by LIN-44/Wnt and LIN-17/Frizzled. In normal development, the anterior daughter of the T cells (T.a) produces hypodermal cells, while the posterior daughter (T.p) generates neural cells, including socket cells, for phasmid neurons. In lin-17 mutants, both T.a and T.p produce two hypodermal cells and hence do not give rise to socket cells. This phenotype is called Psa for phasmid socket absent. psa-1(os22) and psa-4(os13) mutants were identified in a screen for the Psa phenotype. Both mutants show egg-laying defectiveness (Egl), protrusive vulva (Pvul), and embryonic lethality in addition to the Psa phenotype, although the phenotypes are much more severe in the psa-1 than in the psa-4 mutants. All the phenotypes are temperature sensitive in both mutants. To confirm that the psa-1 and psa-4 mutations affect the asymmetry of T cell divisions, the temperature-sensitive property was exploited to analyze when these genes are required for normal asymmetric division. In all the temperature-shift experiments, except for the upshift experiments with psa-1, significant differences were observed in the penetrance of the Psa phenotype between animals that were subjected to the temperature shift before the T cell division (during or after Q cell division, which occurs about 1 hr before the T cell division at 22.5°C) and animals that were subjected to the temperature shift after the T cell division. This result suggests that these genes are required for asymmetry during mitosis in the T cells (Sawa, 2000).
In contrast to lin-17 mutants, in which T.a and T.p have nearly identical fates, producing two hypodermal cells, psa-1 or psa-4 mutations variably affect the fates of T.p. This may reflect that the SWI/SNF complex has multiple targets during T cell division. Some but not all of such targets may be affected by the partial loss-of-function mutations of psa-1 or psa-4. Each target or combination of targets might regulate the cell fates of T.pa and T.pp independently. In addition, SWI/SNF seems to regulate multiple cell fate choices, not only whether cells adopt neural or hypodermal fates but also whether cells divide or not. For example, a type III lineage may be produced when neural differentiation but not the cell division pattern of T.pa is affected. Because psa-4 is required mainly for the short period around the time of T cell division, the chromatin structure altered by SWI/SNF may serve as a cellular memory that influences transcription in the granddaughters of the T cells (Sawa, 2000).
SWI/SNF may not be the sole chromatin-remodeling complex involved in T cell division. egl-27, which encodes a protein homologous to MTA1, has been shown to be required for T cell division. In mammals, MTA1 is a component of another remodeling complex, NURD. Although EGL-27 has not been shown to act in the NURD complex during T cell division, and the timing for the EGL-27 requirement is not yet known, EGL-27 may be needed for chromatin remodeling during the period of T cell division, as is SWI/SNF. Although synergistic interactions between psa-1, psa-4, and egl-27 were observed, it is not clear whether SWI/SNF and NURD have the same or distinct targets (Sawa, 2000).
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