THE CARBOXYL EXTENSION OF A UBIQUITIN-LIKE PROTEIN IS RAT RIBOSOMAL-PROTEIN S30

The amino acid sequence of the rat 40 S ribosomal subunit protein S30 was deduced from the sequence of nucleotides in a recombinant cDNA and confirmed by the determination of the 18 residues at the NH2 terminus of the protein. Unlike the majority of ribosomal proteins, which are unprocessed primary products of the translation of their mRNAs, S30 is formed by cleavage from a larger hybrid protein. The NH2-terminal pol ypeptide has 38% identity with ubiquitin and contains the characterist ic carboxyl-terminal Gly-Gly dipeptide of this family of proteins. S30 has 59 amino acids and the molecular weight is 6,643; the ubiquitin-l ike sequence has 74 residues and the molecular weight is 7,634. The hy brid protein is encoded in each of the 8-10 members of the family of r at S30 genes; there is, however, only a single species of mRNA which c ontains the sequences for both proteins. The coding sequence of the hy brid protein occurs in the reverse polarity in the genome of the Finke l-Biskis-Reilly murine sarcoma virus.