A domain shared by the Polycomb group proteins Scm and ph mediates heterotypic and homotypic interactions

Mol Cell Biol. 1997 Nov;17(11):6683-92. doi: 10.1128/MCB.17.11.6683.


The Sex comb on midleg (Scm) and polyhomeotic (ph) proteins are members of the Polycomb group (PcG) of transcriptional repressors. PcG proteins maintain differential patterns of homeotic gene expression during development in Drosophila flies. The Scm and ph proteins share a homology domain with 38% identity over a length of 65 amino acids, termed the SPM domain, that is located at their respective C termini. Using the yeast two-hybrid system and in vitro protein-binding assays, we show that the SPM domain mediates direct interaction between Scm and ph. Binding studies with isolated SPM domains from Scm and ph show that the domain is sufficient for these protein interactions. These studies also show that the Scm-ph and Scm-Scm domain interactions are much stronger than the ph-ph domain interaction, indicating that the isolated domain has intrinsic binding specificity determinants. Analysis of site-directed point mutations identifies residues that are important for SPM domain function. These binding properties, predicted alpha-helical secondary structure, and conservation of hydrophobic residues prompt comparisons of the SPM domain to the helix-loop-helix and leucine zipper domains used for homotypic and heterotypic protein interactions in other transcriptional regulators. In addition to in vitro studies, we show colocalization of the Scm and ph proteins at polytene chromosome sites in vivo. We discuss the possible roles of the SPM domain in the assembly or function of molecular complexes of PcG proteins.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Binding Sites
  • Chromosome Mapping
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism*
  • Drosophila Proteins*
  • Drosophila melanogaster / genetics
  • Homeodomain Proteins / genetics
  • Homeodomain Proteins / metabolism*
  • Insect Proteins
  • Molecular Sequence Data
  • Mutagenesis, Site-Directed
  • Nucleoproteins / genetics
  • Nucleoproteins / metabolism*
  • Peptide Fragments / metabolism
  • Point Mutation
  • Polycomb Repressive Complex 1
  • Polycomb-Group Proteins
  • Protein Binding
  • Recombinant Fusion Proteins / metabolism
  • Repressor Proteins / genetics
  • Repressor Proteins / metabolism*
  • Saccharomyces cerevisiae / genetics


  • DNA-Binding Proteins
  • Drosophila Proteins
  • Homeodomain Proteins
  • Insect Proteins
  • Nucleoproteins
  • Pc protein, Drosophila
  • Peptide Fragments
  • Polycomb-Group Proteins
  • Recombinant Fusion Proteins
  • Repressor Proteins
  • SCM protein, Drosophila
  • ph-d protein, Drosophila
  • Polycomb Repressive Complex 1