Expression and regulation of the mammalian SUMO-1 E1 enzyme

SUMO-1 is a small ubiquitin-related protein. SUMO-1 conjugation requires enzymes with sequence and biochemical similarity to ubiquitin E1 and E2 enzymes. We have examined the expression, localization, and biochemical behavior of Aos1 and Uba2, subunits of the mammalian SUMO-1 E1 enzyme. Both of these proteins are expressed in multiple tissues and localized to the nucleus. Aos1 protein levels vary through the cell cycle. These changes in Aos1 concentration 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 species. Biochemical analysis reveals that Aos1 and Uba2 associate with each other in a simple heterodimeric complex without other subunits, unlike the budding yeast Uba2 homologue, which apparently associates with several different proteins. Although it is possible to reconstitute SUMO-1 conjugation with purified Uba2, Aos1, and Ubc9, this reaction is significantly less efficient than conjugation observed in cellular extracts, suggesting the possibility that there may be activators of SUMO-1 conjugation in vivo that have not yet been characterized. Taken together, these observations reveal that the SUMO-1 pathway is controlled on multiple levels during the cell cycle.