Regulation of the Polycomb protein RING1B ubiquitination by USP7

The E3 ubiquitin ligase RING1B plays an important role in Polycomb-mediated gene silencing by monoubiquitinating histone H2A. Both the activity and stability of RING1B are controlled by ubiquitination in two distinct manners. Self ubiquitination of RING1B generates K6, K27 and K48-based mixed polyubiquitin chain, and is required for its activity as a ligase. On the other hand, its proteasomal degradation is mediated by another ligase; E6-AP catalyzes the formation of K48-based chains. Since these two modes of ubiquitination target the same lysine residues and are therefore mutually exclusive, an important mode of regulation of RING1B should be at the level of deubiquitination. Here we identify USP7 as a deubiquitinating enzyme that regulates the ubiquitination state of RING1B. RING1B interacts with USP7, which is mediated in part by its RING domain. In addition, USP7 was found in a complex with other Polycomb proteins, suggesting a broad role in regulating these complexes. Although, USP7 directly and specifically deubiquitinates RING1B in vitro and in vivo, it does not discriminate between the activating and proteolysis-targeting modes of ubiquitination, and therefore has a stabilizing effect on RING1B.