A novel mammalian Smt3-specific isopeptidase 1 (SMT3IP1) localized in the nucleolus at interphase

A novel Smt3-specific isopeptidase, SMT3IP1, was cloned using a yeast two-h ybrid screen with Smt3b as bait. The clone, named SMT3IP1 (Smt3-specific is opeptidase 1), which bound to Smt3b but not SUMO-1 in the two-hybrid system , was distantly related to budding yeast Saccharomyces cerevisiae Ulp1, hum an SENP1 or human SUSP1. The catalytic domains in the C-terminal region wer e very similar, but the N-terminal region was quite different to other enzy mes. The cysteine, histidine and asparatic acid residues in the catalytic d omains were conserved. SMT3IP1 expressed by the baculovirus-expression syst em had the ability to cleave SUMO-1 or Smt3b from SUMO-1/RanGAP1 or Smt3b/R anGAP1 conjugates, respectively, and the activity was a little stronger tow ards the Smt3b conjugate than towards the SUMO-1 conjugate. Furthermore, th e enzyme bound more strongly to Smt3a and Smt3b than to SUMO-1 in vitro. Th e enzyme did not cleave Nedd8 from Nedd8/cullin-1. Nor did it cleave ubiqui tin from ubiquitinated p53. SMT3IP1 was localized almost exclusively at the nucleolus during interphase. The N-terminal sequence was responsible for t he nucleolar localization of this enzyme. Whether SMT3IP1 functions in the nucleolus or just stays there before it functions in the nucleus, as shown in the case of CDC14 phosphatase, remains to be elucidated.