Destruction of mitotic cyclins by ubiquitindependent proteolysis is required for cells to complete mitosis and enter interphase of the next cell cycle. In clam eggs, this process is catalyzed by a cyclin-selective ubiquitin carrier protein, E2-C, and the cyclosome͞anaphase promoting complex (APC), a 20S particle containing cyclin-selective ubiquitin ligase activity. Here we report cloning a human homolog of E2-C, UbcH10, which shares 61% amino acid identity with clam E2-C and can substitute for clam E2-C in vitro. Dominant-negative clam E2-C and human UbcH10 proteins, created by altering the catalytic cysteine to serine, inhibit the in vitro ubiquitination and destruction of cyclin B in clam oocyte extracts. When transfected into mammalian cells, mutant UbcH10 inhibits the destruction of both cyclin A and B, arrests cells in M phase, and inhibits the onset of anaphase, presumably by blocking the ubiquitin-dependent proteolysis of proteins responsible for sister chromatid separation. Thus, E2-C͞UbcH10-mediated ubiquitination is involved in both cdc2 inactivation and sister chromatid separation, processes that are normally coordinated during exit from mitosis.

A large fraction of the mammalian genome is transcribed into long noncoding RNAs. The RNAs remain largely uncharacterized as the field awaits new technologies to aid functional analysis. Here, we describe a unique use of locked nucleic acids (LNAs) for studying nuclear long noncoding RNA, an RNA subclass that has been less amenable to traditional knockdown techniques. We target LNAs at Xist RNA and show displacement from the X chromosome with fast kinetics. Xist transcript stability is not affected. By targeting different Xist regions, we identify a localization domain and show that polycomb repressive complex 2 (PRC2) is displaced together with Xist. Thus, PRC2 depends on RNA for both initial targeting to and stable association with chromatin. H3K27-trimethyl marks and gene silencing remain stable. Time-course analysis of RNA relocalization suggests that Xist and PRC2 bind to different regions of the X at the same time but do not reach saturating levels immediately. Thus, LNAs provide a tool for studying an emerging class of regulatory RNA and offer a window of opportunity to target epigenetic modifications with possible therapeutic applications.dosage compensation | X inactivation