Ulp1 association with nuclear pore complexes is required for the maintenance of global SUMOylation

The SUMOylation state of a protein depends upon antagonistic SUMO conjugation and deconjugation activities that, in Saccharomyces cerevisiae, are primarily driven by the PIAS SUMO E3 ligases Siz1 and Siz2 and the SUMO isopeptidase Ulp1. Subcellular localization of these regulators determines where and when protein SUMOylation occurs. Ulp1 localizes at the nuclear basket of nuclear pore complexes (NPC), placing it at the interface of the nuclear periphery and the nucleoplasm; locales where most cellular SUMO-dependent processes occur. In cells lacking the nuclear basket component Nup60, we find that Ulp1/NPC association becomes temperature-dependent, and Ulp1/NPC dissociation at higher temperatures leads to Ulp1 proteasomal degradation. Ulp1 dissociation from NPC nuclear baskets also induces a signal that triggers Siz1 and Siz2 proteasomal degradation in a manner-dependent upon SUMOylation, SUMO chain assembly, and Ubc4-dependent ubiquitination. Ultimately, Siz protein degradation decreases cellular SUMOylation, and this reduction appears to promote cell viability under conditions where the association of Ulp1 with NPC nuclear baskets is perturbed. These observations suggest that SUMO homeostasis functions to regulate SUMO conjugate levels in direct proportion to Ulp1 levels at NPC nuclear baskets by modulating stability of the PIAS SUMO E3 ligases.