A novel factor required for the SUMO1/Sm3 conjugation of yeast septins

SUMO1/Smt3, a ubiquitin-like protein modifier, is known to be conjugated to other proteins and modulate their functions in various important processes . Similar to the ubiquitin system, SUMO1/Smt3 is activated in an ATP-depend ent reaction by thioester bond formation with El (activating enzyme), trans ferred to E2 (conjugating enzyme), and passed to a substrate lysine. It rem ained unknown, however, whether any SUMO1/Smt3 ligases (E3s) are involved i n the final transfer of this modifier. Here we report a novel factor Siz1 ( YDR409w) required for septin-sumoylation of budding yeast, possibly acting as E3. Siz1 is a member of a new family (Miz1, PIAS3, etc.) containing a co nserved domain with a similarity to a zinc-binding RING-domain, often found in ubiquitin ligases. In the siz1 mutant septin-sumoylation was completely abolished. A conserved cysteine residue in the domain was essential for th is conjugation. Furthermore, Siz1 was localized at the mother-bud neck in t he M-phase and physically bound to both E2 and the target proteins. (C) 200 1 Elsevier Science B.V. All rights reserved.