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1 STRUCTURE OF A BMI-1-RING1B POLYCOMB GROUP UBIQUITIN LIGASE COMPLEX Zhizhong Li 1,4 , Ru Cao 2 , Ming Wang 1 , Michael P. Myers 1 , Yi Zhang 2 , and Rui-Ming Xu 1,3,4 1 Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724; 2 Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7295 Running Title: Structure of a Bmi-1-Ring1B Complex 3 To whom correspondence should be addressed: Rui-Ming Xu, Structural Biology Program, Skirball Institute of Biomolecular Medicine, New York University School of Medicine, 540 First Avenue, New York, NY 10016, Tel. (212) 263-0585; Fax. (212) 263-2150; E-mail: rmxu@saturn.med.nyu.edu 4 Present Address: Structural Biology Program, Skirball Institute of Biomolecular Medicine and Department of Pharmacology, New York University School of Medicine, New York, New York 10016 Polycomb group (PcG) proteins Bmi-1 and Ring1B are core subunits of the PRC1 complex which plays important roles in the regulation of Hox gene expression, X-chromosome inactivation, tumorigenesis and stem cell self-renewal. The RING finger protein Ring1B is an E3 ligase that participates in the ubiquitination of lysine 119 of histone H2A, and the binding of Bmi-1 stimulates the E3 ligase activity. We have mapped the regions of Bmi-1 and Ring1B required for efficient ubiquitin transfer and determined a 2.5 Å structure of the Bmi-1-Ring1B core domain complex. The structure reveals that Ring1B “hugs” Bmi-1 through extensive RING domain contacts and its N-terminal tail wraps around Bmi-1. The two regions of interaction have a synergistic effect on the E3 ligase activity. Our analyses suggest a model where the Bmi-1-Ring1B complex stabilizes the interaction between the E2 enzyme and the nucleosomal substrate to allow efficient ubiquitin transfer. Polycomb Group (PcG) proteins are a set of evolutionarily conserved transcriptional repressors controlling homeotic gene expression during development (1). Biochemical and genetic characterizations of PcG proteins have revealed that they exist in distinct complexes, of which the two best characterized are the PRC1 and PRC2 complexes (2,3). The Drosophila PRC1 core complex consists of Polycomb (Pc), Posterior Sex Combs (Psc), Polyhomeotic (Ph), and a Ring finger protein (4), and the mammalian complex contains homologous proteins (5). The PRC2 complex is a histone methyltransferase complex methylating lysine 27 of histone H3 (6-9). Both PcG complexes have been implicated in diverse biological processes such as epigenetic inheritance, stem cell development, senescence and tumorigenesis (10-12). The PRC1 complex has at least two biochemical functions. One of which is to bind chromatin and prevent it from being remodeled by ATP-dependent remodeling factors (13). Using an http://www.jbc.org/cgi/doi/10.1074/jbc.M602461200 The latest version is at JBC Papers in Press. Published on May 18, 2006 as Manuscript M602461200 Copyright 2006 by The American Society for Biochemistry and Molecular Biology, Inc. by guest on March 18, 2016 http://www.jbc.org/ Downloaded from