THE UBIQUITIN-LIKE PROTEINS SMT3 AND SUMO-1 ARE CONJUGATED BY THE UBC9 E2 ENZYME

The ubiquitin-like protein SMT3 from Saccharomyces cerevisiae and SUMO -1, its mammalian homolog, can be covalently attached to other protein s posttranslationally. Conjugation of ubiquitin requires the activitie s of ubiquitin-activating (E1) and -conjugating (E2) enzymes and proce eds via thioester-linked enzyme-ubiquitin intermediates, Herein we sho w that UBC9, one of the 13 different E2 enzymes from yeast, is require d for SMT3 conjugation in vivo. Moreover, recombinant yeast and mammal ian UBC9 enzymes were found to form thioester complexes with SMT3 and SUMO-1, respectively. This suggests that UBC9 functions as an E2 in a SMTS/SUMO-1 conjugation pathway analogous to ubiquitin-conjugating enz ymes. The role of yeast UBC9 in cell cycle progression may thus be med iated through its SMT3 conjugation activity.