Polycomb group (PcG) proteins were first identified in Drosophila but mammalian homologs have been identified. They interact with each other to form multimeric, chromatin-associated protein complexes. The human orthologs of the Drosophila PcG proteins are CBX2, CBX4, CBX6, CBX7 and CBX8, which exhibit distinct nuclear localisations and they contribute differently to transcriptional repression. They have been described as regulators of homeotic gene expression. It has been demonstrated that these proteins maintain the repressed state of these genes, involved in development, signalling or cancer []. PcG complexes can be recruited to DNA by interactions with specific DNA binding proteins [].Sumoylation is a reversible conjugation process similar to ubiquitination, in which proteins involved in a wide range of processes are modified, such as DNA repair, chromosome segregation and gene expression. It implies an activating enzyme (E1), a conjugating enzyme (E2), and a ligase (E3) As well as in the ubiquitination process, the E3 ligase determines substrate specificity. Polycomb protein Pc2, also known as CBX4, is a SUMO (small ubiquitin-related modifier) ligase E3. DNA damage activates CBX4 which sumoylates the heterogeneous nuclear ribonucleoprotein K (hnRNP K), a p53 cofactor required for transcriptional regulation of p53 target genes [].This entry includes de E3 SUMO-protein ligase CBX4 (also known as Pc2).
