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.