Expression and regulation of the mammalian SUMO-1 E1 enzyme

SUMO-1 is a small ubiquitin-related protein. SUMO-1 conjugation requires en zymes with sequence and biochemical similarity to ubiquitin E1 and E2 enzym es. We have examined the expression, localization, and biochemical behavior of Aos1 and Uba2, subunits of the mammalian SUMO-1 E1 enzyme. Both of thes e proteins are expressed in multiple tissues and localized to the nucleus. Aos1 protein levels vary through the cell cycle. These changes in Aos1 conc entration may play a role in the regulation of the SUMO-1 pathway, because they correlate with changes in the abundance of some SUMO-1-conjugated spec ies. Biochemical analysis reveals that Aos1 and Uba2 associate with each ot her in a simple heterodimeric complex without other subunits, unlike the bu dding yeast Uba2 homologue, which apparently associates with several differ ent proteins. Although it is possible to reconstitute SUMO-1 conjugation wi th purified Uba2, Aos1, and Ubc9, this reaction is significantly less effic ient than conjugation observed in cellular extracts, suggesting the possibi lity that there may be activators of SUMO-1 conjugation in vivo that have n ot yet been characterized. Taken together, these observations reveal that t he SUMO-1 pathway is controlled on multiple levels during the cell cycle.