CDNA CLONING OF A NEW PUTATIVE ATPASE SUBUNIT P45 OF THE HUMAN 26S PROTEASOME, A HOMOLOG OF YEAST TRANSCRIPTIONAL FACTOR SUG1P

The nucleotide sequence of a cDNA that encodes a new regulatory subuni t, named p45, of the 26S proteasome of human hepatoblastoma HepG2 cell s has been determined. The polypeptide predicted from the open reading frame consists of 406 amino acid residues with a calculated molecular weight of 45770 and isoelectric point of 8.35. The sequences of sever al fragments of bovine p45, determined by protein chemical analyses, s panning 27% of the complete structure, were found to be in excellent a ccord with those deduced from the human cDNA sequence. Computer analys is showed that p45 belongs to a family of putative ATPases which inclu des regulatory components of 26S proteasomes. The overall structure of p45 was found to be homologous to that of yeast Sug1p, which has been identified as a transcripional factor. It is closely similar, but not identical to the sequence reported for Trip1, a functional homolog of Sug1p in human tissues. These results are consistent with the possibi lity that Sug1-like proteins with distinct sequence function in transc ription and protein degradation in human cells. However, the alternati ve hypothesis, that the same gene locus encodes both p45 and Trip1, ca nnot be excluded on the basis of such closely similar sequences. In ei ther case, both proteins are likely to function equivalently well in e ither transcription or protein degradation.