Cell cycle-regulated attachment of the ubiquitin-related protein SUMO to the yeast septins

SUMO is a ubiquitin-related protein that functions as a posttranslational m odification on other proteins. SUMO conjugation is essential for viability in Saccharomyces cerevisiae and is required for entry into mitosis. We have found that SUMO is attached to the septins Cdc3, Cdc11, and Shs1/Sep7 spec ifically during mitosis, with conjugates appearing shortly before anaphase onset and disappearing abruptly at cytokinesis. Septins are components of a belt of 10-nm filaments encircling the yeast bud neck. Intriguingly, only septins on the mother cell side of the bud neck are sumoylated. We have ide ntified four major SUMO attachment-site lysine residues in Cdc3, one in Cdc 11, and two in Shs1, all within the consensus sequence (IVL)KX(ED). Mutatin g these sites eliminated the vast majority of bud neck-associated SUMO, as well as the bulk of total SUMO conjugates in G2/M-arrested cells, indicatin g that sumoylated septins are the most abundant SUMO conjugates at this poi nt in the cell cycle. This mutant has a striking defect in disassembly of s eptin rings, resulting in accumulation of septin rings marking previous div ision sites. Thus, SUMO conjugation plays a role in regulating septin ring dynamics during the cell cycle.