Like ubiquitin, the small ubiquitin-like modifier-1 (SUMO-1) protein can be covalently conjugated to proteins; however, unlike ubiquitylation, which often leads to proteasome-dependent protein degradation, sumoylation affects protein-protein interactions and can lead to the relocation of proteins to other subcellular compartments. Ubiquitylation and sumoylation require the concerted functions of several proteins: an "activating enzyme" (E1), a conjugating enzyme (E2), and a ligase (E3), which performs the transfer of ubiquitin or SUMO to the substrate. To date, no proteins that catalyze the ligation of SUMO to proteins have been found. Now, Johnson and Gupta have characterized two proteins, Siz1 and Siz2, that bear sequence similarity to ubiquitin ligases. Siz1- and Siz2-doubly deficient yeast contained almost no sumoylated proteins, whereas Siz1-deficient yeast were unable to sumoylate septin proteins, whose function may be required during yeast budding and cytokinesis. Similarly, Siz1 was required for the sumoylation of septins in vitro, indicating that Siz1 is required for ligating SUMO to proteins in yeast. Siz1 could also catalyze the multimerization of SUMO proteins, leading to populations of proteins containing differing amounts of linked SUMO. Thus, Siz1 appears to function as an E3-like ligase in protein sumoylation.
E. S. Johnson A. A. Gupta, An E3-like factor that promotes SUMO conjugation to the yeast septins. Cell 106, 735-744 (2001). [Online Journal]