Isolation of cDNA clone encoding rat senescence marker protein-30 (SMP30) and its tissue distribution

We have isolated and characterized two cDNA clones encoding senescence marker protein-30 (SMP30), the amounts of which are known to decrease androgen-independently with aging in the livers of rats. Of these cDNA clones, one consisted of 1588 bp nucleotides and the other of 1195 bp nucleotides generated by alternative polyadenylation. These two cDNA clones shared the same open reading frame, but the larger species had 393 bp nucleotides of 3' untranslated region in addition to the first polyadenylation site of smaller species. Northern hybridization analysis showed that two species of mRNA (1.7 kb and 1.4 kb) located in the liver and kidney were consistent with these short and long forms of cDNA. The open reading frame, 897 bp could encode 299 amino acids. The estimated molecular weight and pI of the deduced polypeptide were 33,387 and 5.1, respectively. Furthermore, immunohistochemical analysis confirmed that SMP30 was preferentially localized in the hepatocytes and renal proximal tubular epithelium. Genomic Southern hybridization analysis demonstrated that SMP30 was widely conserved among higher animals. A computer-assisted homology analysis of nucleic acid and protein databases revealed no remarkable homology with other known proteins. Therefore, SMP30 seems to be a novel protein. In addition, the existence of putative A-U rich mRNA degradation signals and protein degradation signals (PEST sequence) in the structure of SMP30 may suggest important regulatory function of this unique protein manifested by changes in its concentrations.