CDNA CLONING OF RAT PROTEASOME SUBUNIT RC7-I, A HOMOLOG OF YEAST PRE1ESSENTIAL FOR CHYMOTRYPSIN-LIKE ACTIVITY

The nucleotide sequence of a cDNA that encodes a new subunit, named RC 7-I, of the 20 S proteasome of rat hepatoma cells has been determined. The polypeptide predicted from the open reading frame consists of 201 amino acid residues with a calculated molecular weight of 22,912 and isoelectric point of 7.25. Approximately 80% of the partial amino acid sequences of several fragments of RC7-I, determined by protein chemic al analyses, were found to be in excellent accordance with those deduc ed from the cDNA sequence. Computer analysis showed that RC7-I belongs to the beta-type subgroup of proteasomes with similarity to the beta- subunit of the archaebacterial proteasome, differing clearly from alph a-type subunits of the proteasome gene family. The overall structure o f RC7-I was found to be homologous to that of yeast PRE1, which is nec essary for chymotryptic activity.