Electrophoretic heterogeneity and polypeptide chain structure of the gamma-subunit of mouse submaxillary 7 S nerve growth factor.

The gamma-subunit of the high molecular weight form of mouse submaxillary gland nerve growth factor is known to be electrophoretically heterogeneous. The protein is a mixture of molecules containing either two or three chains, attached covalently by disulfide bonds, that arise from one or two endoproteolytic cleavages. Analysis of the amino acid composition and partial sequence of the resultant fragments has defined the location of the two cleavage sites. Further heterogeneity is generated by partial removal of the carboxyl-terminal basic residues located at the internal cleavage sites, presumably by an endogenous exopeptidase(s) with carboxypeptidase B specificity. Permutations of these endo- and exoproteolytic cleavages generate six possible microheterogeneous forms, which can account for the five major bands observed on electrophoresis at pH 7.05 and the three principal forms seen on isoelectric focusing.