Role of polycomb proteins Ring1A and Ring1B in the epigenetic regulation of gene expression

ABSTRACT Generation of cell diversity depends on epigenetic regulatory mechanisms. Polycomb group (PcG) proteins are central components of epigenetic regulation in metazoans. The system, initially associated with transcriptional program stability during development, is also involved in the regulation of other processes, such as maintenance of stem cell pluripotency and cell proliferation. PcG regulation involves chromatin modifications through covalent histone modifi-cations. One of these modifications, the monoubiquitylation of the H2A histone, depends on Ring1 proteins, which are essential for development in insects and mammals. In murine embryonic stem cells, Ring1A and Ring1B-dependent ubiquitylation of H2A is linked to repression of transcrip-tional initiation. Studies in mammalian cells have found a multiplicity of protein complexes containing Ring1A and Ring1B, suggesting an expanded regulatory role for Ring1A, Ring1B proteins in the epigenetic regulation of gene expression. KEY WORDS: Ring1A, Ring1B, polycomb, H2A ubiquitylation, epigenetic regulation Ring1A/Ring1 and its paralog Ring1B/Rnf2 are core components of the mammalian Polycomb system, which is one of the epigenetic regulators of cell diversity generation during embryoni