Subunit structure and amino acid composition of mouse submaxillary gland nerve growth factor

The subunit structure and amino acid composition of mouse submaxillary gland nerve growth factor (NGF) have been examined. Sedimentation equilibrium analyses of the protein in 0.1 N sodium acetate (pH 5.0) yielded a molecular weight of 29,000. In unbuffered 6 M guanidine hydrochloride (Gd-HCl), NGF undergoes apparent dissociation to half-molecules of mol wt 14,500. Reduction and alkylation of the disulfide bonds do not further reduce the molecular weight in Gd-HCl. Amino-terminal analyses by the cyanate method indicated that the native protein has two residues of serine as amino end groups. Fractionation of the soluble tryptic peptides of S-[¹⁴C]carboxymethyl NGF, by column chromatography and one-dimensional paper electrophoresis, resulted in the isolation of 15 peptides containing 5 residues of S-carboxymethylcysteine. Digestion and fractionation of the acid-insoluble portion of the tryptic digest with thermolysin, yielded one additional peptide containing a single residue of S-carboxymethylcysteine. On the basis of the lysine and arginine content of native NGF, 35 tryptic peptides, containing 12 residues of S-carboxymethylcysteine could be maximally expected. These results suggest that mouse NGF is composed of two very similar or identical subunits of 14,500 molecular weight that are associated in the native molecule by noncovalent forces.