BIOLOGICAL OVERVIEW

Polycomb Group proteins function as a molecular machine in gene silencing. Extra sex combs is unique among PcG proteins in that it is only expressed transiently and required only during the time when transition from transient to stable repression is occuring (Struhl, 1982). Other PcG proteins are available for a longer period during embryonic development. It is argued that PcG proteins cooperate in gene silencing. If so, what is there about the function of ESC that either allows or requires its shorter duration, compared with other PcG proteins?

Like other PcG proteins, ESC has no DNA binding motif. Instead, it holds five consensus WD motifs distributed throughout the protein. WD repeats function in protein interaction, and the presence of this motif implys a function for ESC in protein interaction and the assembly of a multi-protein complex involved in gene silencing. The yeast protein Tup1 carries seven copies of the WD repeat, and serves as a model for ESC function. Alpha 2 is a DNA-binding yeast protein that regulates specific genes of the cell mating type. Evidence suggests that Alpha 2 recruits the Tup 1 repressor via WD repeats. After recruitment, Tup 1 may act to provide at least partial repression of transcription (Komachi, 1994).

PcG proteins are required for stable long term transcriptional silencing of the homeotic genes. Among PcG genes, esc is unique in being critically required for establishment of PcG-mediated silencing during early embryogenesis, but not for its subsequent maintenance throughout development. Esc has been shown to be physically associated with the PcG protein Enhancer of Zeste [E(z)]. Esc, together with E(z), is present in a 600 kDa complex that is distinct from complexes containing other PcG proteins. This Esc complex has been purified and it also contains the histone deacetylase Rpd3 and the histone-binding protein p55 (Chromatin assembly factor 1 subunit), which is also a component of the chromatin remodeling complex NURF and the chromatin assembly complex CAF-1. The association of Esc and E(z) with p55 and Rpd3 is conserved in mammals. Rpd3 is required for silencing mediated by a Polycomb response element (PRE) in vivo and E(z) and Rpd3 are bound to the Ubx PRE in vivo, suggesting that they act directly at the PRE. It is proposed that histone deacetylation by this complex is a prerequisite for establishment of stable long-term silencing by other continuously required PcG complexes (Tie, 2001).

To test whether the association of Esc and E(z) with p55/Caf1 and Rpd3 has been conserved in mammals, the human complex containing the Esc homolog (EED) was examined for the presence of Rpd3 and p55 homologs. Database searches reveal that Drosophila Rpd3 is most closely related to two human histone deacetylases, HDAC1 and HDAC2 (77% and 75% identical to Rpd3). Similarly, there are two closely related p55 homologs in mammals, RbAp48 and RbAp46 (91% and 86% identical to p55). RbAp48 and RbAp46 have also been found together in the SIN3 and Mi-2 deacetylase complexes, as have HDAC1 and HDAC2. A test was performed to see whether all four proteins are associated with the human EED complex. A GST-ESC fusion protein encoding full-length Esc can pull down full-length in vitro translated Esc and a GST-ESC1-60 fusion protein encoding just the N-terminal 60 residues of Esc is sufficient to pull down full-length in vitro translated Esc. Similarly, GST-EED1-81, which contains the corresponding N-terminal region of EED, binds directly to in vitro translated EED. In addition to FLAG-Esc, GST-ESC1-60 also pulls down p55 and Rpd3 from Drosophila embryo nuclear extract. This strongly suggests that GST-ESC1- 60 specifically pulls down the Esc complex. GST-EED1-81 pulls down HDAC1, HDAC2 and RbAp48 from HeLa cell nuclear extract. RbAp46 has also been detected. Thus, the association of ESC with p55 and Rpd3 is mirrored in the conserved association of mammalian EED with RbAp48, RbAp46 and HDAC1 and HDAC2. These results confirm the previously reported association of EED with HDAC1 and HDAC2 (Tie, 2001).

The presence of p55 in the ESC complex provides a direct molecular link to chromatin. The highly conserved mammalian p55 homologs, RbAp48 and RbAp46, have been shown to bind directly to histone H4 and possibly H2A, but not H2B or H3. The N- and C-terminal regions of RbAp48 that mediate binding to histone H4 are virtually identical to the corresponding regions of Drosophila p55, strongly suggesting that p55 has the same histone-binding specificity (Tie, 2001).

What, then, is the role of p55 in the Esc complex? It is unlikely that p55 is responsible for the selective recruitment or targeting of Esc and E(Z) to the ~100 specific chromosomal sites at which they co-localize. The histone-binding activity of p55 does not, by itself, suggest a mechanism for such specificity and p55 binds to many more sites on the polytene chromosomes than Esc and E(Z), presumably reflecting its distribution in other complexes, such as CAF1 and NURF. It seems more likely that p55 acts after the Esc complex is recruited and serves to direct the deacetylase activity of Rpd3 to local histone substrates. This is analogous to the role proposed for RbAp46 in the heterodimeric HAT1 complex. RbAp46 greatly stimulates the acetyltransferase activity of the non-histone-binding HAT1 catalytic subunit, presumably by tethering it to its substrate via its histone-binding activity. Similarly, although recombinant Rpd3 can deacetylate histone H4 in vitro, free Rpd3 does not bind to H4 when the two are co-expressed in vivo and is unlikely to be able to deacetylate nucleosomal histones. This suggests that p55 may play a similar essential role in the Esc complex by targeting Rpd3 to histone substrates for deacetylation (Tie, 2001).

The presence of Rpd3 in the Esc complex suggests that histone deacetylation is an intrinsic activity of the Esc complex and that Rpd3 is required for PRE-mediated silencing. The related mammalian EED complex has been shown to contain the Rpd3 homologs HDAC1 and HDAC2, and immunoprecipitates containing this complex can deacetylate a histone H4 tail-peptide in vitro. In yeast, Rpd3-dependent repression in vivo has been shown to be associated with deacetylation of histones H4 and H3. Which nucleosomes would be deacetylated by the Esc complex? Histone deacetylation by yeast Rpd3 appears to be highly localized, extending only one or two nucleosomes from a site to which it is recruited. Since components of the Esc complex are physically associated with the Ubx PRE in vivo, Esc-mediated deacetylation may be restricted to nucleosomes comprising and immediately adjacent to PREs. Nucleosomes outside the PRE might also be targeted if the PRE has long-range interactions with the promoter or if the Esc complex itself also binds to the promoter or other regions outside the Ubx PRE, a possibility that the data presented here do not rule out. Although an effect is observed of several Rpd3 mutations on silencing of a PRE-mini-white reporter, which is an extremely sensitive assay, PcG phenotypes have not been reported for Rpd3 mutants. A hypomorphic Rpd3 allele associated with the insertion of a P-element transposon in the noncoding 5' untranslated region has been analyzed in the most detail. Homozygous mutant embryos derived from germline clones of this allele do not exhibit PcG phenotypes, but have a pair-rule phenotype similar to that of ftz mutants. Abundant ubiquitously distributed Rpd3 RNA and protein of maternal origin are detectable in early (0-2 hour) wild-type embryos, but are reduced no more than fivefold in these Rpd3 mutant embryos derived from germline clones. By stage 9-10, the level of maternally derived Rpd3 RNA and protein is greatly diminished. Localized zygotic expression of Rpd3 becomes detectable in the brain and ventral nervous system of wild-type embryos, but is not detectable in these mutant embryos, suggesting that this Rpd3 allele may have a stronger effect on zygotic expression than maternal expression. If Rpd3 protein derived from maternally synthesized RNA is sufficient to promote development of a normal cuticular phenotype, then it remains possible these mutant embryos may contain sufficient maternally derived protein to do so and that germline clones of a true null Rpd3 allele would display PcG phenotypes. Alternatively, it is possible that the function of Rpd3 in the Esc complex is not absolutely essential for Esc-dependent silencing or is redundant, i.e. when eliminated, it can be compensated by another histone deacetylase, either one normally associated with the Esc complex or a related one that can associate with the complex in the absence of Rpd3. A number of other histone deacetylases have been identified in Drosophila and at least two are reported to be ubiquitously distributed in the early embryo (Tie, 2001).

However, unlike mammals, which have two very closely related Rpd3 orthologs (HDAC1 and HDAC2), both of which are associated with mouse EED, the Drosophila genome contains no equally closely related homolog of Rpd3. The next most closely related Drosophila HDAC is an unequivocal ortholog of mammalian HDAC3, which is a class I HDAC like Rpd3. Interestingly, mouse HDAC3 has been reported to interact with the mouse Esc homolog EED in a yeast two-hybrid assay, consistent with the possibility that Rpd3 function in the Esc complex might be at least partially redundant. Further genetic analysis of Rpd3 should help to clarify its role in the Esc complex (Tie, 2001).

The 600 kDa Esc complex is distinct from complexes containing PC and other PcG proteins. This suggests that the Esc complex and other PcG complexes are likely to have separate functions. Furthermore, in embryos lacking any functional Esc protein, some weak residual Pc-dependent silencing activity is still detected, also supporting separate, if interdependent, functions. Similar conclusions have been drawn for the homologous mammalian PcG complexes, which have been reported to be expressed in temporally distinct stages of B cell differentiation, further suggesting they have distinct functions. In Drosophila, derepression of homeotic genes is detected slightly earlier in Esc mutants than in other PcG mutants, raising the possibility that Esc complex function might be required earlier than other PcG complexes. However, unlike the apparent temporal separation of the homologous complexes during mammalian B cell development, both Esc- and PC-containing complexes are present together throughout most of embryogenesis, before Esc disappears, and E(z), like other PcG proteins, is required continuously throughout development. The phenotypic similarities between Esc, E(z) and other PcG mutants, the genetic interactions among them and their common association with PREs, suggests that their functions, however distinct at the biochemical level, are interdependent (Tie, 2001).

What role might Esc-mediated histone deacetylation play in PcG silencing? Given the critical early requirement for Esc, Esc-mediated deacetylation of PRE-associated nucleosomes might be an essential prerequisite for the initial binding of one or more components of PRC1 or other PcG complexes to PREs. A schematic model is presented for such a function of the Esc complex in which Esc complex-mediated deacetylation of PRE associated histones is a critical step in establishing stable long-term PcG silencing. Alternatively, the Esc complex may be required for events subsequent to the initial binding of other PcG proteins to a PRE, perhaps for their assembly into active silencing complexes or for interaction of PRE-bound PcG complexes with the promoter. Indeed, repression of a reporter gene by a tethered GAL4-Pc fusion protein remains dependent on endogenous Esc and E(z) as well as other PcG proteins. This indicates that, at least for PC, constitutive binding to DNA does not bypass the requirement for Esc and E(z). This also suggests that while the biochemical evidence reveals no stable direct association of the Esc complex with other PcG complexes, it remains possible that there is a transient or less stable association in vivo that is essential for establishing PcG silencing (Tie, 2001).

The association of mammalian EED with the two closely related HDACs and two histone-binding proteins could reflect the existence of two separate EED complexes or some different functionality of the EED complex compared with the Esc complex. Consistent with this latter possibility, EED has recently been shown to be required after embryogenesis for aspects of adult hematopoietic development. Interestingly, analysis of the complete Drosophila genome sequence using the BLASTP and TBLASTN algorithms reveals that p55 has no other closely related Drosophila homologs, strongly suggesting that it is the functional counterpart of both RbAp48 and RbAp46 in Drosophila. Likewise, Rpd3 is the only Drosophila counterpart of mammalian HDAC1 and HDAC2. Given the remarkably high degree of similarity between RbAp48 and RbAp46 and HDAC1 and HDAC2, it is not yet clear whether each of these proteins has a distinct or redundant role in the EED complex. Perhaps this situation reflects a greater degree of functional specialization or versatility within the mammalian EED complexes. Since HDAC1 and HDAC2 have also been found together with RbAp48 and RbAp46 in other co-repressor complexes, it is also possible that the EED and Esc complexes represent specialized relatives of these complexes, perhaps more dedicated to a specific subset of genes (Tie, 2001).

Drosophila ESC-like can substitute for ESC and becomes required for Polycomb silencing if ESC is absent

The Drosophila esc-like gene (escl) encodes a protein very similar to ESC. Like ESC, ESCL binds directly to the E(Z) histone methyltransferase via its WD region. In contrast to ESC, which is present at highest levels during embryogenesis and low levels thereafter, ESCL is continuously present throughout development and in adults. ESC/E(Z) complexes are present at high levels mainly during embryogenesis but ESCL/E(Z) complexes are found throughout development. While depletion of either ESCL or ESC by RNAi in S2 and Kc cells has little effect on E(Z)-mediated methylation of histone H3 lysine 27 (H3K27), simultaneous depletion of ESCL and ESC results in loss of di- and trimethyl-H3K27, indicating that either ESC or ESCL is necessary and sufficient for di- and tri-methylation of H3K27 in vivo. While E(Z) complexes in S2 cells contain predominantly ESC, in ESC-depleted S2 cells, ESCL levels rise dramatically and ESCL replaces ESC in E(Z) complexes. A mutation in escl that produces very little protein is viable and exhibits no phenotypes but strongly enhances esc mutant phenotypes, suggesting they have similar functions. esc escl double homozygotes die at the end of the larval period, indicating that the well-known 'maternal rescue' of esc homozygotes requires ESCL. Furthermore, maternal and zygotic over-expression of escl fully rescues the lethality of esc null mutant embryos that contain no ESC protein, indicating that ESCL can substitute fully for ESC in vivo. These data thus indicate that ESC and ESCL play similar if not identical functions in E(Z) complexes in vivo. Despite this, when esc is expressed normally, escl appears to be entirely dispensable, at least for development into morphologically normal fertile adults. Furthermore, the larval lethality of esc escl double mutants, together with the lack of phenotypes in the escl mutant, further suggests that in wild-type (esc⁺) animals it is the post-embryonic expression of esc, not escl, that is important for development of normal adults. Thus escl appears to function in a backup capacity during development that becomes important only when normal esc expression is compromised (Kurzhals, 2008).

The rescue of embryos containing no ESC protein by ESCL over-expression did produce an occasional fly with a mild PcG phenotype, a hint that ESCL may not be quite as effective as ESC either in forming complexes with other PRC2 components or in promoting H3K27 methylation by PRC2 complexes. Some of the RNAi results also hint that ESCL-containing complexes may not be quite as effective at H3K27 methylation as ESC-containing complexes. In particular, depletion of ESC reduced the 3mH3K27 level more than depletion of ESCL did, in both S2 and Kc cells. This occurred despite the fact that the ESCL level increases quite substantially when ESC alone is depleted. While differences in efficiency of knockdown might contribute to this, it may also indicate that more ESCL is required to produce the same level of 3mH3K27 achieved with less ESC, i.e., ESC-containing complexes may be more effective at H3 methylation than ESCL-containing complexes. A possible basis for such a difference is suggested in previous work demonstrating that ESC and ESCL both bind to histone H3 (Tie, 2007), in which it was noticed that ESCL binding to H3 appears to be somewhat weaker (Kurzhals, 2008).

The results demonstrate that while ESCL is expressed throughout development and during adulthood, it is apparently dispensable, under normal culture conditions, at least for development of morphologically normal fertile adults. This is somewhat surprising in light of the complete lethality of esc escl double mutants, which clearly indicates that the well-known 'maternal rescue' of the embryonic lethality of esc mutants is absolutely dependent on ESCL. esc escl double homozygotes die at the end of an extended larval period and have markedly smaller brain hemispheres, wing and eye-antennal discs, while the other discs appear to be similar in size to wild-type discs. The normally large wing disc is likely to be smaller due to transformation into a haltere disc due to derepression of Ubx in the wing disc, a phenotype typically seen only as a partial transformation in adults of very strong Polycomb mutant genotypes. Transformation of the leg discs to other segmental identities, which would not be reflected in changes in disc size, is also likely in the esc escl mutants, since the signature 'extra sex combs' phenotype seen in maternally rescued adult esc homozygotes is already enhanced when these adults are also heterozygous for escl. Similarly, the decrease in the size of the eye antennal disc is consistent with the partial antenna to leg transformations observed in the esc adults that are also heterozygous for the escl mutation. Mutations in E(z) and Su(z)12 have a similar late lethal phase and also exhibit small imaginal discs and larval brains, consistent with their functional collaboration in the same complexes. Why then is ESCL largely dispensable, given the impact of the escl mutation on development to adulthood in esc escl double mutants (Kurzhals, 2008).

The demonstration that ESCL can substitute fully for ESC in vivo, provided it is expressed at adequate levels at all times, further suggests that the two proteins are qualitatively equivalent and that in the absence of the other, either one is necessary and sufficient for normal development to adulthood. Nonetheless, it appears that normally ESCL does not come into play in a substantial way unless ESC levels are compromised. A reason for this is suggested by the discovery that in S2 cells most ESCL protein is in free form when ESC is relatively abundant, and that the level of ESCL, along with the proportion of it found in the 600-kDa complex, increases dramatically in cells that have been depleted of ESC by RNAi. The amount of ESCL that is bound to the Ubx PRE also increases after ESC depletion. These observations suggest that ESCL levels are regulated in part by ESC levels through a mechanism involving competition for participation in E(Z) complexes, which serves to stabilize ESCL. Since the actual abundance of ESC and ESCL is not known, it cannot be determined to what extent this may be due to intrinsic differences in the affinity of the two proteins for these complexes or is simply a reflection of mass action favoring the more abundant of the two proteins. Abundance almost certainly plays a key role in early embryos, when the ESC level is highest and the ESCL level is at its lowest. It is harder to explain why ESCL does not become essential in postembryonic stages, when the ESC level is much lower and the ESCL level is at its highest. This might hint that even at low levels ESC has an intrinsic advantage over ESCL in competition for complex assembly. Alternatively, it is possible that the levels of ESC in postembryonic stages have been underestimated by measurements in bulk extracts and that in imaginal discs, critical tissues for development of the adult, ESC levels remain high enough to continue to out-compete ESCL for complex formation in those tissues. However, in bulk extracts from late larvae, a substantial fraction of ESCL is detected in the 600-kDa complex and ESCL is readily co-immunoprecipitated with E(Z) from these extracts. Whichever the case, it appears that ESCL has little or no role when ESC is present at normal levels. It remains possible that ESCL becomes more important in adults or under specific environmental conditions (Kurzhals, 2008).

The results of RNAi-mediated knockdown in S2 and Kc cells clearly demonstrate that depletion of either ESC or ESCL has no appreciable effect on the levels of di-methyl and tri-methyl H3K27, while simultaneous depletion of both proteins greatly reduces the tri- and di-methyl isoforms. This strongly suggests that ESC and ESCL are functionally interchangeable for this function of PRC2 and indicates that each protein is produced at sufficient levels in S2 cells to promote normal levels of H3K27 methylation in the absence of the other. This also suggests that the common role of both proteins in PRC2 complexes, i.e., binding to histone H3 and stimulating E(Z)-mediated H3K27 methylation (Tie, 2007), is the basis of their interchangeability in vivo (Kurzhals, 2008).

In summary, while the complementary temporal expression patterns of ESC and ESCL during development initially suggested that ESCL might become essential during postembryonic development, the data presented here suggest otherwise, beginning with the lack of any obvious phenotypes in an strong escl mutant that makes little residual protein. The reason for the apparent dispensability of ESCL would appear to be due simply to the sufficiency of normal ESC levels at all times to promote normal development. Ironically, this is now made even clearer by the observed late larval lethality of the esc escl double mutants, since together with the viability and normal morphology of escl mutants, it demonstrates very clearly that, while ESCL is absolutely required for survival to adulthood in the absence of zygotic ESC expression, ESC alone is sufficient to promote survival to adulthood. Even the variable extra sex combs phenotype seen in maternally rescued esc mutant adults is unlikely to reflect a postembryonic requirement for ESCL, since the escl mutants themselves do not exhibit this phenotype even at low frequency. Unless a true protein null escl allele does, this means that esc by itself is sufficient for development into normal adults (Kurzhals, 2008).

On the other hand, the data presented here do help to solve the puzzle posed by previous genetic evidence indicating that esc is unique among Polycomb Group proteins in being required only during early embryogenesis, and more recent biochemical evidence that ESC is essential for H3K27 methylation by E(Z), which is required continuously. While the maternal rescue of esc null embryos to adulthood by a single esc⁺ allele in the mother is remarkable and indicates that there is a critical early requirement for ESC, this genetic result does not reflect a requirement for ESC only in the early embryo. It requires the backup function provided by ESCL, which becomes required after the maternally deposited esc⁺ gene products are depleted in esc⁻ embryos. The elevated level of ESCL and ESCL/E(Z) complexes caused by ESC depletion in S2 cells also suggests that this may involve compensatory elevation of ESCL/E(Z) complexes as maternally derived ESC disappears in esc⁻ embryos. Similar considerations pertain to the experiments demonstrating rescue of esc null mutants by a brief pulse of zygotic esc⁺ expression from a heat-inducible hsp70-esc transgene during the first 4 h of embryogenesis, which almost certainly also requires the backup function of ESCL. Genetic experiments have been interpreted to suggest that ESC was required only in the early embryo to ensure normal development to adulthood, except for the variable partial transformation of T2 and T3 legs to T1 identity. However, while it is now evident that the existence of ESCL and its continuous expression explains the apparent requirement for ESC only during embryogenesis, inferred from maternal rescue, these genetic experiments do not necessarily reveal a normal requirement for ESCL. On the contrary, the lack of obvious phenotypes in the escl mutant, together with the inability of maternally derived ESC alone to promote normal development to adults in the absence of ESCL, leads to the conclusion that under normal circumstances, not only is ESC sufficient to promote normal development in the absence of ESCL, but also that esc expression during postembryonic stages is required for development of normal adult morphology. This conclusion was also arrived at almost 50 years ago based on genetic mosaic experiments in which esc mutant phenotypes were observed in clones of homozygous esc mutant cells induced at various times during the larval period (Hannah-Alava, 1958). The absence of phenotypes in the escl mutant, particularly the variable T2 and T3 extra sex combs phenotype seen in maternally rescued esc mutants, is consistent with the conclusion that postembryonic expression of ESC itself is required for development of normal adult morphology (Kurzhals, 2008).

Given the apparent redundancy of ESCL, at least for development of morphologically normal, fertile adults, why then has it persisted? All vertebrates for which complete genome sequences are available contain a single ESC/ESCL ortholog encoded by the EED gene, which is expressed continuously in all tissues. While there are several different EED isoforms arising from use of alternative translation initiation sites, it is not known whether their different N-termini make them functionally different. The C. elegans genome also contains a single ESC ortholog, mes-6 (Korf, 1998). Structural similarities between the esc and escl genes, particularly their intron-exon arrangements and the ESC and ESCL protein sequences, as well as the close proximity of the two genes on chromosome 2L, indicate that they arose by a gene duplication event. To date, outside of plants, the presence of two ESC/ESCL genes appears to be restricted to the Drosophilidae lineage. A single ortholog is present in the next most closely related lineages for which genome sequences are complete, including Culicidae dipterans (three mosquitoes), lepidopterans (Bombyx mori), coleopterans (Tribolium casteneum) and hymenopterans (Apis mellifera). All twelve Drosophila species for which complete genome sequence is available have two genes that are clearly recognizable ESC and ESCL orthologs. The most distant of these species diverged over 40 million years ago, sufficient time for one member of a duplicate gene pair to degenerate in the absence of selection for its function. A careful analysis of the changes that the esc and escl coding sequences have undergone in all these species should reveal whether or not the escl gene shows any signs of having begun to degenerate. However, the qualitative functional equivalence of ESC and ESCL revealed in this study suggests that the persistence of ESCL and the conservation its function in H3K27 methylation by PRC2 complexes may be due to a fitness advantage conferred by ESCL for a function that has not yet been identified, perhaps a function it performs during adulthood (Kurzhals, 2008).

GENE STRUCTURE

Exons - four

Bases in 3' UTR - 281

PROTEIN STRUCTURE

Amino Acids - 425

Structural Domains

The predicted ESC protein contains five copies of the WD motif and three other WD related sequences. The WD motif is found in G-protein beta subunits as well as non-G proteins involved in diverse cellular functions, including transcriptional repression. The sequence alterations of a number of esc mutations cause amino acid substitutions within the WD repeats, identifying them as essential for the function of the ESC protein as a repressor of homeotic gene expression. The WD motif is implicated in protein-protein interaction (Frei, 1985a and b, and Sathe, 1995a).

The Drosophila Extra sex combs (Esc) protein, a member of the Polycomb group (PcG), is a transcriptional repressor of homeotic genes. Genetic studies have shown that Esc protein is required in early embryos at about the time that other PcG proteins become engaged in homeotic gene repression. The Esc protein consists primarily of multiple copies of the WD repeat, a motif that has been implicated in protein-protein interaction. To further investigate the domain organization of Esc protein, esc homologs have been isolated and characterized from divergent insect species. Esc protein is highly conserved in housefly (72% identical to Drosophila Esc), butterfly (55% identical), and grasshopper (56% identical). The butterfly homolog provides Esc function in Drosophila, indicating that the sequence similarities reflect functional conservation. Homology modeling using the crystal structure of another WD repeat protein, the G-protein beta-subunit, predicts that Esc protein adopts a beta-propeller structure. The sequence comparisons and modeling suggest that there are seven WD repeats in Esc protein which together form a seven-bladed beta-propeller. Conserved regions in Esc protein have been located with respect to this predicted structure. Site-directed mutagenesis of specific loops, predicted to extend from the propeller surface, identifies conserved parts of Esc protein required for function in vivo. It is suggested that these regions might mediate physical interaction with Esc partner proteins (Ng, 1997).

date revised: 17 January 2001

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