| Modification Notes |
Modification of cellular proteins by the ubiquitin-like protein SUMO provides an essential function for nuclear processes and cell cycle progression in yeast (1-3). In Saccharomyces cerevisiae, both sumoylating and desumoylating activities are essential for viability (2). Yeast contains two SUMO-1-deconjugating enzymes, Ulp1 and Ulp2, that localize to nuclear pores and in the nucleoplasm, respectively (4). The yeast Ubl-specific protease, Ulp1, catalyzes two essential functions in the SUMO pathway: processing of full-length SUMO to its mature form and deconjugation of SUMO from targeted proteins (1). Ulp1 cleaves proteins from Smt3 and SUMO-1, but not from ubiquitin (5). Proteins related to Ulp1 are present in many organisms, including several human pathogens (5). The catalytic C-domain of Ulp1 must be excluded from the nucleoplasm for cell viability (4). This is achieved by the noncatalytic N-domain, which tethers Ulp1 to the nuclear pores (4). This location could allow Ulp1 to remove SUMO-1 from sumoylated cargo proteins during their passage through the nuclear pore channel (4). However, the bulk of cellular Ulp1 does not associate with nucleoporins, but instead associates with three karyopherins Pse1, Kap95, and Kap60 (4). |
