Polycomb group protein RING1B is a direct substrate of Caspases-3 and -9

Both Caspase-3 and Caspase-9 play critical roles in the execution of mitochondria-mediated apoptosis. Caspase-9 binds to Apaf-1 in the presence of cytochrome c and dATP/ATP, and is activated by self-cleavage. Caspase-3 is activated by cleavage of caspase-8 and caspase-9. Over hundred direct caspase-3 substrates are identified whereas only few direct caspase-9 substrates are known. Here, we demonstrate that Ring1B, a component of polycomb protein complex that plays important roles in modulating chromatin structures, is a direct substrate of active caspase-3 and caspase-9 both in vitro and in vivo. The specific cleavage sites for caspase-3 and caspase-9 were mapped to Asp¹⁷⁵ and Asp²⁰⁸, respectively. Importantly, cleavage of Ring1B by active caspases-3 and caspase-9 triggers the redistribution of Ring1B, from exclusive nuclear localization to even distribution throughout the entire cell. The transcriptional repression activity of Ring1B was also disrupted by caspase cleavage. Our data suggest that caspases-3 and caspase-9 play novel roles in transcription by regulating polycomb protein function through direct cleaving of Ring1B.