An E3-like factor that promotes SUMO conjugation to the yeast septins

Covalent attachment of the ubiquitin-related protein SUMO to other proteins participates in many processes including signal transduction, transcriptio nal regulation, and growth control. We report the characterization of Siz1 as an E3-like factor in the SUMO pathway. Siz1 is required for SUMO attachm ent to the S. cerevisiae septins in vivo and strongly stimulates septin sum oylation in vitro. Siz1 and the related protein Siz2 promote SUMO conjugati on to different substrates at different stages of the cell cycle and, toget her, are required for most SUMO conjugation in yeast. Siz1, Siz2, and the P IAS (protein inhibitor of activated STAT) proteins form a conserved family defined by an unusual RING-related motif. Our results suggest that this fam ily functions by promoting SUMO conjugation to specific substrates.