The molecular basis of the heterogeneity of the gamma subunit of 7 S nerve growth factor.

The arginine esteropeptidase gamma subunit of the 7 S nerve growth factor complex contains six different molecular species which arise by post-translational modifications. An endopeptidase cleavage of the larger of the two peptide chains in two-chain gamma species produces the three-chain species and introduces a new COOH-terminal lysine residue. The charge heterogeneity of the various species arises from the removal, by a carboxypeptidase B-like enzyme in the submaxillary gland, of this lysine residue and/or the original COOH-terminal arginine residue of the shorter Mr = 10,000 chain. These latter interconversions of the gamma species have been reproduced in vitro using carboxypeptidase B. The significant decrease in the proportion of two-chain gamma species in saliva compared to submaxillary gamma subunit suggests that the endopeptidase cleavage can occur during secretion of saliva; however, the subcellular localization of the enzyme has not been ascertained.