The Yeast ULP2 (SMT4) Gene Encodes a Novel Protease Specific for the Ubiquitin-Like Smt3 Protein

Yeast Smt3 and its vertebrate homolog SUMO-1 are ubiquitin-like proteins (Ubls) that are reversibly ligated to other proteins. Like SMT3, SMT4 was first isolated as a high-copy-number suppressor of a defective centromere-binding protein. We show here that SMT4 encodes an Smt3-deconjugating enzyme, Ulp2. In cells lacking Ulp2, specific Smt3-protein conjugates accumulate, and the conjugate pattern is distinct from that observed in a ulp1ts strain, which is defective for a distantly related Smt3-specific protease, Ulp1. The ulp2Δ mutant exhibits a pleiotropic phenotype that includes temperature-sensitive growth, abnormal cell morphology, decreased plasmid and chromosome stability, and a severe sporulation defect. The mutant is also hypersensitive to DNA-damaging agents, hydroxyurea, and benomyl. Although cell cycle checkpoint arrest in response to DNA damage, replication inhibition, or spindle defects occurs with normal kinetics, recovery from arrest is impaired. Surprisingly, either introduction of a ulp1ts mutation or overproduction of catalytically inactive Ulp1 can substantially overcome the ulp2Δ defects. Inactivation of Ulp2 also suppresses several ulp1ts defects, and the double mutant accumulates far fewer Smt3-protein conjugates than either single mutant. Our data suggest the existence of a feedback mechanism that limits Smt3-protein ligation when Smt3 deconjugation by both Ulp1 and Ulp2 is compromised, allowing a partial recovery of cell function.