UBA2 peptide

The SUMO-1 has been identified as a protein that is highly similar to ubiquitin and shown to conjugate to RanGAP1, PML, Sp200 and I kappa B alpha. The conjugation steps are thought to be similar to those of ubiquitination; and human Ubc9, which is homologous to the E2 enzyme for the ubiquitin conjugation step, was identified and shown to be necessary for the conjugation of SUMO-1 to its target protein. Other essential enzymes involved in this modification, however, remain to be clarified. Here we cloned human Sua1 (SUMO-1 activating enzyme) and hUba2, which are human homologs of yeast Saccharomyces cerevisiae Aos1 and Uba2, respectively. The recombinant proteins, Sua1p and hUba2p, formed a complex. In this complex, hUba2 bound SUMO-1 and this complex had the activity of the SUMO-1 activating enzyme, Furthermore, in an in vitro system, RanGAP1 was modified by SUMO-1 in the presence of Sua1p/Uba2p and hUbc9p, showing that the modification of SUMO-1 could be catalyzed by two enzyme steps, although ubiquitination usually requires three enzyme steps. (C) 1999 Academic Press.

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.