Specific aspects of the ubiquitin system in spermatogenesis

The ubiquitin system is involved in numerous cellular processes, regulating the amounts and/or activities of specific proteins through posttranslation al coupling with ubiquitin or ubiquitin-like proteins. In spermatogenesis, there appears to be a special requirement for certain components of the ubi quitin system, as exemplified in human and mouse by mutation of USP9Y and H R6B, respectively. Both genes encode proteins which take part in the ubiqui tin system and are ubiquitously expressed, but their mutation generates no apparent phenotype other than male infertility. Different phases of mammali an spermatogenesis probably require different specialized activities of the ubiquitin system. It is anticipated that ubiquitination activities similar to those required during mitotic cell cycle regulation will play some role in control of the meiotic divisions. In spermatocytes, there is an intrica te link among DNA repair, the ubiquitin system, and regulation of meiotic c hromatin structure, as indicated by the co-localization of proteins involve d in these processes on meiotic recombination complexes. HR6B and its nearl y identical homolog HR6A are multiple function proteins, with ubiquitin-con jugating activity and essential roles in post-replication DNA repair. HR6B, possibly together with the ubiquitin-ligating enzyme mRAD18Sc, is most lik ely involved in chromatin re-organization during the meiotic and postmeioti c phases of spermatogenesis. Biochemical data indicate that, in particular during spermiogenesis, the general activity of the ubiquitin system is high , which most likely is related to the high requirement for massive breakdow n of cytoplasmatic and nuclear proteins during this last phase of spermatog enesis. (J. Endocrinol. Invest. 23: 597-604, 2000) (C) 2000, Editrice Kurti s.