Comparison of 7S nerve growth factor and nerve growth factor I from mouse submandibular glands

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Abstract
7S nerve growth factor (7S NGF) and nerve growth factor I (NGFI) are NGF-containing protein complexes isolated from mouse submandibular glands by different protocols, and reports suggest that the molecules differ chemically. In this study, we compared the molecular properties and subunit compositions of the two proteins. Purified 7S NGF and NGF1 electrophoresed to identical positions on polyacrylamide gels in nondissociating buffers, with electrophoretic mobilities indistinguishable from that of unpurified NGF in salivary gland extracts. Ultraviolet absorption curves were identical, and sedimentation coefficients were similar (7.3 .+-. 0.25 S for 7S NGF; 7.2 .+-. 0.2 S for NGFI) as determined by sedimentation velocity analysis. By sedimentation equilibrium analysis, molecular weights of 135,000-140,000 were obtained for both complexes at protein concentrations in the centrifuge cell > 85 .mu.g/mL; when protein concentrations within the centrifuge cell ranged from .apprx. 30 to 100 .mu.g/mL at equilibrium, both complexes dissociated. Molecular weight values determined by gel filtration on Bio-Gel P300 and Sephadex G200 resins were similar for both proteins, and the values determined on Sephadex agreed with those obtained by ultracentrifugation. The subunit compositions of the complexes were also similar as determined by nonequilibrium isoelectric focusing, NGF1 being composed of proteins that migrated to positions identical with those of the .alpha., .beta., and .gamma. subunits of 7S NGF. Furthermore, the stoichiometry of the subunits was similar in the two complexes as determined by radioimmunoassay to each of the subunits and by densitometric analysis of electrophoresis gels. Both methods showed that the complexes contain approximately 2 mol of the .alpha. and .gamma. subunits per mole of .beta.-NGF. By all the criteria we tested, 7S NGF and NGF1 were indistinguishable proteins.