Protease that catalyzes two essential functions in the SUMO pathway: processing of full-length SUMOs to their mature forms and deconjugation of SUMO from targeted proteins. Cleaves precursors of SUM1 and SUM2, but not of SUM3 or SUM5. Able to release SUM1 and SUM2 from conjugates, but unable to cleave SUM3. Protease activity mainly directed at deconjugating SUM1 and SUM2 from their target proteins. Regulates salt stress responses and flowering time. Redundant with ULP1D. The N-terminal regulatory domain is not required for peptidase activity in vitro. No visible phenotype in terms of overall growth, salt sensitivity and flowering time. Early flowering time and salt sensitivity in ulp1d/ots1 and ulp1c/ots2 double mutants. Belongs to the peptidase C48 family.