A DOMAIN SHARED BY THE POLYCOMB GROUP PROTEINS SCM AND PH MEDIATES HETEROTYPIC AND HOMOTYPIC INTERACTIONS

The Sex comb on midleg (Scm) and polyhomeotic (ph) proteins are member s of the Polycomb group (PcG) of transcriptional repressors. PcG prote ins maintain differential patterns of homeotic gene expression during development in Drosophila flies. The Scm and ph proteins share a homol ogy domain with 38% identity over a length of 65 amino acids, termed t he SPM domain, that is located at their respective C termini. Using th e yeast two-hybrid system and in vitro protein-binding assays, we show that the SPM domain mediates direct interaction between Scm and ph. B inding studies with isolated SPM domains from Scm and ph show that the domain is sufficient for these protein interactions. These studies al so show that the Scm-ph and Scm-Scm domain interactions are much stron ger than the ph-ph domain interaction, indicating that the isolated do main has intrinsic binding specificity determinants. Analysis of site- directed point mutations identifies residues that are important for SP M domain function. These binding properties, predicted alpha-helical s econdary structure, and conservation of hydrophobic residues prompt co mparisons of the SPM domain to the helix-loop-helix and leucine zipper domains used for homotypic and heterotypic protein interactions in ot her transcriptional regulators, In addition to in vitro studies, we sh ow colocalization of the Scm and ph proteins at polytene chromosome si tes in vivo. We discuss the possible roles of the SPM domain in the as sembly or function of molecular complexes of PcG proteins.