Molecular cloning and nucleotide sequence of cDNA for mRNA of mitochondrial cytochrome P450(SCC) of bovine adrenal cortex

We have isolated cDNA clones of the mRNA for cytochrome P-450(SCC), which catalyzes the side-chain cleavage reaction of cholesterol in bovine adrenal cortex mitochondria, by using oligonucleotides as probes. Sequence analysis of the cloned cDNAs enabled us to deduce the primary structure of the precursor form of P-450(SCC), which consisted of 520 amino acids and contained an extrapeptide of 39 amino acids at the NH₂ terminus. The amino acid sequence from the 40th to 55th amino acid residue in the predicted structure completely coincided with the sequence of the NH₂-terminal portion of purified P-450(SCC). The amino acid composition calculated from the predicted structure showed an excellent agreement with that determined with the purified protein. The extrapeptide of the precursor molecule resembles those of a few nuclear-encoded yeast mitochondrial proteins reported so far. Although P-450(SCC) is a component of mitochondria, comparison of its primary structure with those of other forms of cytochrome P-450 shows that P-450(SCC) is structurally more related to microsomal cytochrome P-450s than to a bacterial cytochrome P-450, P-450(cam). A homologous sequence observed with various forms of cytochrome P-450 is also highly conserved in the P-450(SCC) molecule. Only two cysteinyl residues are present in the mature form of P-450(SCC), one of which is located in the middle of the conserved sequence, confirming the function of this cysteinyl residue as the fifth ligand of the heme.