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The Ring domain proteins Ring1B and Bmi1 are important members of chromatin modulating Polycomb group proteins, which form large multimeric protein complexes. These complexes covalently modify histones
leading to the transcriptional repression of specific genes. The maintenance PRC1 core complex is an E3 ligase that monoubiquitinates histone H2A. Ligase activity strongly depends on the Ring1b
protein but is enhanced by Bmi1 in vitro. We determined the crystal structure of the Ring-Ring heterodimeric complex of Ring1b and Bmi1 (Buchwald et al, EMBO J).
The arrangement of the Ring-domains is similar to the BRCA1/BARD1 complex, but complex formation depends on an N-terminal arm of Ring1b that embraces the Bmi1 Ring-domain. Our data provide a basis for understanding the critical enzymatic activity at the core of the PRC1 complex, which is implicated in stem cell maintenance and cancer.
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