Polycomb Group Protein Suppressor 2 of Zeste Is a Functional Homolog of Posterior Sex Combs

Abstract

The Drosophila melanogaster Polycomb group protein Posterior Sex Combs is a component of Polycomb repressive complex 1 and is central to Polycomb group-mediated silencing. A related Polycomb group gene, Suppressor 2 of zeste, is thought to be partially redundant in function. The two proteins share a small region of homology but also contain regions of unconserved sequences. Here we report a biochemical characterization of Suppressor 2 of zeste. Like Posterior Sex Combs, Suppressor 2 of zeste binds DNA, compacts chromatin, and inhibits chromatin remodeling. Interestingly, the regions of the two proteins responsible for these activities lack sequence homology. Suppressor 2 of zeste can also replace Posterior Sex Combs in a functional complex with other Polycomb group proteins, but unlike with their biochemical activities, complex formation is mediated by the region of Suppressor 2 of zeste that is homologous to that of Posterior Sex Combs. Our results establish Suppressor 2 of zeste as a functional homolog of Posterior Sex Combs and suggest that the two proteins operate via similar molecular mechanisms.

An Unexpected Regulatory Cascade Governs a Core Function of the Drosophila PRC1 Chromatin Protein Su(z)2

“…As expected, this allele is recessive lethal, which is in line with in vitro evidence suggesting that the CTR is essential, providing the compaction mechanism that is necessary for transcriptional silencing (Wu and Howe 1995;King et al 2005;Lo et al 2009). Despite lacking the CTR, however, Su(z)2 1 exhibits several gain of function phenotypes (Wu and Howe 1995;Emmons et al 2009), suggesting that some of the functions of Su(z)2 are compartmentalized within the HR alone.…”

“…Thus, it is believed that the HR serves as the hub of protein-protein interactions which, in part, help direct the CTR to genomic regions requiring regulation by PcG proteins. While characterization of the CTR has been driven by the genetics of both Su(z)2 (Wu and Howe 1995;Emmons et al 2009;Lo et al 2009) and Psc (Francis et al 2004;King et al 2005), the Su(z)2 locus has been more amenable to the genetic dissection of the HR (Wu and Howe 1995;Emmons et al 2009;Lo et al 2009). Thus, we have been continuing our study of Su(z)2 to characterize and analyze the functions of HR.…”

“…In particular, we have focused on the genetic dissection of a protein domain, called the homology region (HR), which was originally defined for its conservation among the PcG Drosophila Suppressor 2 of zeste [Su(z)2] gene, its paralogous Posterior sex combs (Psc) gene, and their mammalian homologs Bmi-1 and Mel-18 (Brunk et al 1991;van Lohuizen et al 1991). The HR, located at the N-terminal end of Su(z)2 and Psc, is believed to house the protein-protein interaction subdomains responsible for interactions with the other subunits of the PcG complexes PRC1 and dRAF, the H2AK118/9 ubiquitylation complex (Kyba and Brock 1998;King et al 2005;Lagarou et al 2008;Lo et al 2009;Schwartz and Pirrotta 2013;Simon and Kingston 2013;Kang et al 2015;Wani et al 2016). The C-terminal regions (CTRs) of Su(z)2 and Psc have been considered the core of these proteins as they are the main contributors of PRC1-mediated silencing; they can, on their own, recapitulate the inhibition of chromatin remodeling, compaction of chromatin, and transcriptional repression exhibited by PRC1 in vitro (King et al 2002(King et al , 2005Francis et al 2004;Lo et al 2009;Beh et al 2012).…”

Molecular Genetic Analysis of Suppressor 2 of zeste Identifies Key Functional Domains

“…In particular, genetic and molecular analyses indicate that the RF is required for Psc function in vivo, the HR is necessary for assembly of the PCC in vitro, and the CTR, which is functionally separable from the RF, is essential for wild-type Psc activity in vivo as well as for the inhibition of transcription and chromatin remodeling in vitro (King et al 2005). Importantly, Su(z)2 behaves similarly to Psc in in vitro assays; it can replace Psc in a complex with Pc, Ph, and Sce/dRing1, its HR is essential for formation of the complex, and its CTR inhibits chromatin remodeling (Lo et al 2009). This latter finding is consistent with studies in mammalian cells showing that Su(z)2, either full length or lacking the majority of its HR, can repress activator function (Bunker and Kingston 1994).…”

“…Similarly, somatic clones homozygous for Su(z)2 1.b8 in wing imaginal discs show derepression of homeotic genes and cellular overgrowth, whereas clones homozygous for loss-of-function (l-o-f) alleles of either Psc or Su(z)2 do not (Beuchle et al 2001). Su(z)2 also colocalizes with Psc and Pc at many sites on polytene chromosomes (Rastelli et al 1993;Platero et al 1996;Sharp et al 1997) and, very recently, co-immunoprecipitation experiments using Drosophila and cell-line extracts suggest that Su(z)2 exits in a complex that also contains Pc, Ph, and Sce/ dRing1, which are the three non-Psc members of PCC (Lo et al 2009). …”