MBLR, a new RING finger protein resembling mammalian Polycomb gene products, is regulated by cell cycle-dependent phosphorylation

Background: The RING finger proteins function in a variety of fundamental cellular processes. The products of some members of the Polycomb group (PcG) bear ring finger domains and are defined as a subclass of RING finger proteins. Among them are Drosophila posterior sex combs and suppressor 2 of zeste, whose RING fingers are conserved in vertebrate PcG proteins Mel18 and Bmi1. Results: We have identified a new mammalian RING finger protein, termed MBLR due to its structural similarity to Mel18 and Bmi1 (Mel18 and Bmi1-like RING finger protein). MBLR interacts with some PcG proteins: in vitro biochemical data support the idea of a direct interaction of MBLR's RING finger domain with Ring1B, which is highly homologous to one of the mammalian PcG genes, Ring1A. We also show that MBLR acts as a transcriptional repressor in transiently transfected cells, as is the case for other PcG proteins. Immunocytochemical analysis reveals that MBLR protein is localized in a fine-grained distribution throughout the nucleoplasm in interphase cultured cells and in a fainter diffuse cytoplasmic distribution in mitotic cells. In addition, we find that serine 32 of MBLR is specifically phosphorylated during mitosis, most likely by CDK7, a component of the basal transcriptional machinery. Conclusion: Similarities to previously defined PcG proteins suggest that MBLR should be included in the same subclass of RING finger proteins as Mel18 and Bmi1. Although the biological relevance of the cell cycle-related phosphorylation remains to be demonstrated, serine 32 phosphorylation could nevertheless be functionally important.