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Mol. Cell. Biol., 11 1997, 6683-6692, Vol 17, No. 11
AJ Peterson, M Kyba, D Bornemann, K Morgan, HW Brock and J Simon
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.
Copyright © 1997, American Society for Microbiology
A domain shared by the Polycomb group proteins Scm and ph mediates heterotypic and homotypic interactions
Department of Biochemistry, University of Minnesota, St. Paul 55108, USA.
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