Structural transitions of the RING1B C-terminal region upon binding the polycomb cbox domain.

Department of Biochemistry, University of Texas Health Science Center at San Antonio, MSC 7760, 7703 Floyd Curl Drive, San Antonio, Texas 78229-3900, USA.
Polycomb group (PcG) proteins are required for maintaining cell identity and stem cell self-renewal. RING1B and Polycomb (Pc) are two components of a multiprotein complex called polycomb repression complex 1 (PRC1) that is essential for establishing and maintaining long-term repressed gene states. Here we characterize the interaction between the C-terminal region of RING1B (C-RING1B) and the Pc cbox domain. The C-RING1B-cbox interaction displays a 1:1 stoichiometry with dissociation constants ranging from 9.2 to 180 nM for the different Pc orthologues. NMR analysis of C-RING1B alone reveals line broadening. However, when it is in complex with the cbox domain, there is a striking change to the NMR spectrum indicative of conformational tightening. This conformational change may arise from the organization of the C-RING1B subdomains. The C-terminal regions of all PcG RING1 proteins are composed of two stretches of conserved sequences separated by a variable linker sequence. While the entire C-RING1B region is required for cbox binding, the N- and C-terminal halves of C-RING1B can be separated and are able to interact, suggesting the presence of an intramolecular interaction within C-RING1B. The flexibility within the C-RING1B structure allowing transitions between the intramolecular bound and unbound states may cause the broadened peaks of the C-RING1B NMR spectrum. Binding the cbox domain stabilizes C-RING1B, whereby broadening is eliminated. The presence of flexible regions could allow C-RING1B to bind a variety of different factors, ultimately recruiting RING1B and its associated PcG proteins to different genomic loci.
Mesh Terms:
Amino Acid Sequence, DNA-Binding Proteins, Electrophoresis, Polyacrylamide Gel, Humans, Magnetic Resonance Spectroscopy, Models, Biological, Molecular Sequence Data, Protein Binding, Protein Structure, Tertiary, Repressor Proteins, Sequence Homology, Amino Acid, Surface Plasmon Resonance, Ubiquitin-Protein Ligases, Ultracentrifugation
Biochemistry Aug. 05, 2008; 47(31);8007-15 [PUBMED:18616292]
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