Multi-subunit collaboration enables Smc5/6 to function as a composite SUMO E3 complex.

The SUMO E3 enzymes control the efficiency and specificity of protein SUMOylation, providing regulatory means for many cellular processes. While most SUMO E3s fulfill their roles as single proteins, the conserved Nse2 E3 is an obligatory subunit of the genome-protecting complex Smc5/6. How the Smc5/6 complex functions in SUMOylation and the roles of its non-SUMO E3 subunits in this process remain to be elucidated. Here we examine the budding yeast Smc5/6 in SUMOylation reactions and in cellular SUMOylation assays. Biochemical data show that DNA stimulates Smc5/6's E3 activity by fostering enzyme and substrate proximity. Mutational analyses reveal that four non-SUMO E3 subunits utilize their DNA-binding abilities to support this stimulation. Moreover, ATP binding by SMC subunits favors SUMOylation by enhancing Smc5/6 association with DNA and chromatin and by enabling conformational changes. Our findings thus provide evidence for a specialized DNA- and ATP-stimulated composite SUMO E3 complex that uses inter-subunit collaboration to achieve efficient SUMOylation in genome regulation.