Meiosis and small ubiquitin-related modifier (SUMO)-conjugating enzyme, Ubc9

In this review, we describe the role of a small ubiquitin-like protein modifier (SUMO)-conjugating protein, Ubc9, in synaptonemal complex formation during meiosis in a basidiomycete, Coprinus cinereus. Because its meiotic cell cycle is long and naturally synchronous, it is suitable for molecular biological, biochemical and genetic studies of meiotic prophase events. In yeast two-hybrid screening using the meiotic-specific cDNA library of C. cinereus, we found that the meiotic RecA homolog CcLim15 interacted with CcUbc9, CcTopII and CcPCNA. Moreover, both TopII and PCNA homologs were known as Ubc9 interactors and the targets of sumoylation. Immunocytochemistry demonstrates that CcUbc9, CcTopII and CcPCNA localize with CcLim15 in meiotic nuclei during leptotene to zygotene when synaptonemal complex is formed and when homologous chromosomes pair. We discuss the relationships between Lim15/Dmc1 (CcLim15), TopII (CcTopII), PCNA (CcPCNA) and CcUbc9, and subsequently, the role of sumoylation in the stages. We speculate that CcLim15 and CcTopII work in cohesion between homologous chromatins initially and then, in the process of the zygotene events, CcUbc9 works with factors including CcLim15 and CcTopII as an inhibitor of ubiquitin-mediated degradation and as a metabolic switch in the meiotic prophase cell cycle. After CcLim15-CcTopII dissociation, CcLim15 remains on the zygotene DNA and recruits CcUbc9, Rad54B, CcUbc9, Swi5-Sfr1, CcUbc9 and then CcPCNA in rotation on the C-terminus. Finally during zygotene, CcPCNA replaces CcLim15 on the DNA and the free-CcLim15 is probably ubiquitinated and disappears. CcPCNA may recruit the polymerase. The idea that CcUbc9 intervenes in every step by protecting CcLim15 and by switching several factors at the C-terminus of CcLim15 is likely. At the boundary of the zygotene and pachytene stages, CcPCNA would be sumoylated. CcUbc9 may also be involved with CcPCNA in the switch from the replicative polymerase being recruited at zygotene to the repair-type DNA polymerases being recruited at pachytene.