Structure of a Bmi-1-Ring1B Polycomb Group Ubiquitin Ligase Complex

Zhizhong Li,Ru Cao,Ming Wang,Michael P Myers,Yi Zhang,Rui-Ming Xu

doi:10.1074/jbc.m602461200

Abstract

Polycomb group 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-A 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.

Highlights

Methylating lysine 27 of histone H3 (6 –9)
It was shown that Bmi-1, a Drosophila Posterior Sex Combs homolog that was originally discovered through its ability to collaborate with Myc in lymphomagenesis (20 –22), plays a central role in the assembly of the PRC1 complex and, whereas Bmi-1 displays no detectable ubiquitin ligase activity, the binding of Bmi-1 greatly stimulates the E3 ligase activity of Ring1B [15, 19]
We have biochemically and structurally characterized the Bmi-1 and Ring1B domains required for both a stable interaction and stimulation of the E3 ligase activity

Summary

Introduction

Methylating lysine 27 of histone H3 (6 –9). Both Polycomb group complexes have been implicated in diverse biological processes such as epigenetic inheritance, stem cell development, senescence, and tumorigenesis (10 –12). A recent study revealed that a human PRC1 complex composed of Bmi-1, HPH2, PC3, and Ring proteins (Ring1A and Ring1B), which are homologs of Drosophila Posterior Sex Combs, Polyhomeotic, Polycomb, and dRing, respectively, is an E3 ubiquitin ligase complex that mono-ubiquitinates lysine 119 of nucleosomal histone H2A [15]. It was shown that Bmi-1, a Drosophila Posterior Sex Combs homolog that was originally discovered through its ability to collaborate with Myc in lymphomagenesis (20 –22), plays a central role in the assembly of the PRC1 complex and, whereas Bmi-1 displays no detectable ubiquitin ligase activity, the binding of Bmi-1 greatly stimulates the E3 ligase activity of Ring1B [15, 19]. To gain mechanistic insights into the assembly and enzymatic activity of the PRC1 complex, we have determined the structural requirements for the Bmi-1-Ring1B interaction and the basis for stimulation of the E3 ligase activity

Methods

Results

Conclusion