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J. Biol. Chem., Vol. 280, Issue 30, 27769-27775, July 29, 2005

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Structural Organization of a Sex-comb-on-midleg/Polyhomeotic Copolymer^*

Chongwoo A. Kim, Michael R. Sawaya, Duilio Cascio, Woojae Kim, and James U. Bowie, A Leukemia and Lymphoma Society Scholar

From the Department of Chemistry and Biochemistry, UCLA-Department of Energy Center for Genomics and Proteomics, Molecular Biology Institute, University of California, Los Angeles, California 90095

The polycomb group proteins are required for the stable maintenance of gene repression patterns established during development. They function as part of large multiprotein complexes created via a multitude of protein-protein interaction domains. Here we examine the interaction between the SAM domains of the polycomb group proteins polyhomeotic (Ph) and Sex-comb-on-midleg (Scm). Previously we showed that Ph-SAM polymerizes as a helical structure. We find that Scm-SAM also polymerizes, and a crystal structure reveals an architecture similar to the Ph-SAM polymer. These results suggest that Ph-SAM and Scm-SAM form a copolymer. Binding affinity measurements between Scm-SAM and Ph-SAM subunits in different orientations indicate a preference for the formation of a single junction copolymer. To provide a model of the copolymer, we determined the structure of the Ph-SAM/Scm-SAM junction. Similar binding modes are observed in both homo- and heterocomplex formation with minimal change in helix axis direction at the polymer joint. The copolymer model suggests that polymeric Scm complexes could extend beyond the local domains of polymeric Ph complexes on chromatin, possibly playing a role in long range repression.

Received for publication, March 21, 2005 , and in revised form, May 18, 2005.

The atomic coordinates and structure factors (codes 1PK1 and 1PK3) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* This work was supported by Grant RO1 CA81000 from the National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Present address: Dept. of Biochemistry, MSC 7760, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Dr., San Antonio, TX 78229-3900.

To whom correspondence should be addressed: Boyer Hall, UCLA, 611 Charles E. Young Dr. E., Los Angeles, CA 90095-1570. Tel.: 310-206-4747; Fax: 310-206-4749; E-mail: bowie{at}mbi.ucla.edu.

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