Coding sequence of the precursor of the beta subunit of rat propionyl-CoA carboxylase

A cDNA encoding the cytoplasmic precursor of the beta subunit of the mitochondrial enzyme propionyl-CoA carboxylase (EC 6.4.1.3) was cloned and sequenced. The DNA sequence of 2070 nucleotides is almost identical in size to the major hybridizing mRNA from rat liver (2000 +/- 50 nucleotides), suggesting that the cloned DNA represents nearly all of the mRNA sequence. A polypeptide expressed in vitro from an mRNA transcript of this cDNA is indistinguishable in size from the beta subunit precursor (58,500 Da). An open reading frame of 1623 nucleotides, flanked by stop codons, encodes a polypeptide of 541 amino acids; the predicted amino acid sequence was confirmed as that of the beta subunit of propionyl-CoA carboxylase by matching it to the amino acid sequences of five peptides derived from pure mature rat enzyme. Although the exact length of the cleavable, NH2-terminal leader peptide has not been determined because the NH2-terminal residue of the mature subunit is blocked, the leader is most likely 40-42 amino acids in length and is highly positively charged. Computer-aided analysis of secondary structure suggests that the leader peptide consists of two alpha-helical segments, with the two most NH2-terminal arginine residues occupying opposite sites of the first helix; this helix has no apparent hydrophobic moment.