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-dependent reaction by thioester bond formation with E1 (activating enzyme), transferred to E2 (conjugating enzyme), and passed to a substrate lysine. It remained unknown, however, whether any SUMO1/Smt3 ligases (E3s) are involved in 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 conserved 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 this conjugation. Furthermore, Siz1 was localized at the mother-bud neck in the M-phase and physically bound to both E2 and the target proteins.