Sumoylation regulates a wide range of cellular processes. However, little is 
known about the regulation of the SUMO machinery. In this study, we demonstrate 
that two lysine residues (Lys-153 and Lys-157) in the C-terminal region of the 
yeast E2-conjugating enzyme Ubc9 are the major and minor autosumoylation sites, 
respectively. Surprisingly, mutation of Lys-157 (ubc9(K157R)) significantly 
stimulates the level of Ubc9 autosumoylation at Lys-153. The functional role of 
Ubc9 autosumoylation is exemplified in our findings that cell cycle-dependent 
sumoylation of cytoskeletal septin proteins is inversely correlated with the 
Ubc9 autosumoylation level and that mutation of the Ubc9 autosumoylation sites 
results in aberrant cell morphology. Our study elucidates a regulatory mechanism 
that utilizes automodification of the E2 enzyme of the sumoylation machinery to 
control substrate sumoylation.