THE HUMAN UNP LOCUS AT 3P21.31 ENCODES 2 TISSUE-SELECTIVE, CYTOPLASMIC ISOFORMS WITH DEUBIQUITINATING ACTIVITY THAT HAVE REDUCED EXPRESSIONIN SMALL-CELL LUNG-CARCINOMA CELL-LINES

The human Unp gene at 3p21.3 has sequence similarity to ubiquitin prot eases and has been suggested to play a role in carcinogenesis of the l ung (Gray et al., 1995). To investigate this possibility, we isolated cDNAs from several human tissue libraries and found evidence for two m ajor isoforms, encoding proteins predicted to either contain an intern al 47 amino acid segment or not. Both are functional in deubiquitinati on assays, and mutation of a critical conserved cysteine residue to al anine abolished activity. Unp specifies two closely-migrating transcri pts whose relative abundance varies among human adult tissues. Antibod ies specific to UNP confirm the presence of at least two endogenous pr otein isoforms of approximately 105-110 kDa in cell lysates, as predic ted from the cDNA sequences. Cellular fractionation and immunocytochem istry revealed UNP expression localized primarily in the cytoplasm. Wh en we examined a panel of lung-derived cell lines for both UNP mRNA an d protein expression, we found reduced levels of UNP protein in all fo ur small cell lung carcinoma cell lines tested. These findings directl y contradict and offer alternative interpretations to a number of prev iously published observations on Unp.