Amino Acid Sequences of Mouse 2.5S Nerve Growth Factor. II. Isolation and Characterization of the Thermolytic and Peptic Peptides and the Complete Covalent Structure

The isolation and characterization of the thermolytic peptides of S-carboxymethyl nerve growth factor and the peptic peptides of native nerve growth factor are described. The peptides derived from the latter digestion were also used for the determination of the pairing of the half-cystinyl residues. In addition, the two fragments produced by cyanogen bromide cleavage were isolated and used, together with the analyses of the protein Sequencer, to establish the structure of the amino-terminal portion of the protein. It was determined that some preparations of nerve growth factor contained polypeptide chains that were eight residues shorter than the primary subunit. These chains have been designated A (long) and B (short) and apparently arise from limited proteolysis. Carboxypeptidase B digestion of S-carboxymethyl nerve growth factor defined the carboxyl-terminal portion of the molecule. These data are combined with those derived from the tryptic and chymotryptic peptides described in the preceding communication to construct an internally consistent primary and secondary structure of nerve growth factor. In addition, each of the side chains of Glx and Asx residues was identified as to its acidic or neutral character. Thus, mouse nerve growth factor consists, in the unaltered state, of a dimer of two polypeptide chains containing 118 amino acids, each with a molecular weight of 13, 259. The disulfide pairs are formed by Cys-15 and Cys-80, Cys-58 and Cys-108, and Cys-68 and Cys-110.