Evidence for pro-beta-nerve growth factor, a biosynthetic precursor to beta-nerve growth factor

The biosynthesis of beta-nerve growth factor (betaNGF) was studied in mouse submaxillary glands incubated with L-[(35)S]cystine. betaNGF was isolated from tissue extracts by the addition of antiserum against betaNGF and the washed immunoprecipitates were analyzed by sodium dodecyl sulfate gel electrophoresis. With short labeling periods (10 and 25 min) there is a major labeled species with an apparent molecular weight of 22,000 and a smaller peak comigrating with purified betaNGF chains (13,260). As time proceeds, the radioactivity in the 22,000 molecular weight peak plateaus, while the label in betaNGF continues to increase, until by 4 hr it greatly exceeds the radioactivity of the 22,000 molecular weight species. When glands incubated for 10 min are transferred to medium containing a large excess of unlabeled L-cystine, the 22,000 molecular weight peak gradually declines, and there is a corresponding increase in radioactivity at the betaNGF position. The 22,000 molecular weight species isolated from sodium dodecyl sulfate gels possesses all the cystine-containing peptides of betaNGF, and possibly two additional ones. When immunoprecipitates from submaxillary glands labeled for 25 min are incubated with the gamma subunit (a specific arginyl-esteropeptidase associated with betaNGF in the 7S NGF complex), the radioactivity in the 22,000 molecular weight species is converted to the betaNGF position. The results suggest that the 22,000 molecular weight species is a biosynthetic precursor to betaNGF, and that the gamma subunit may function as a specific protease in the processing event.