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

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Abstract
The biosynthesis of .beta.-nerve growth factor (.beta.NGF) was studied in mouse submaxillary glands incubated with L-[35S]cystine. .beta.NGF was isolated from tissue extracts by the addition of antiserum against .beta.NGF 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 MW of 22,000 and a smaller peak comigrating with purified .beta.NGF chains (13,260). As time proceeds, the radioactivity in the 22,000 MW peak plateaus, while the label in .beta.NGF continues to increase, until by 4 h it greatly exceeds the radioactivity of the 22,000 MW species. When glands incubated for 10 min are transferred to medium containing a large excess of unlabeled L-cystine, the 22,000 MW peak gradually declines, and there is a corresponding increase in radioactivity at the .beta.NGF position. The 22,000 MW species isolated from sodium dodecyl sulfate gels possesses all the cystine-containing peptides of .beta.NGF, and possibly 2 additional ones. When immunoprecipitates from submaxillary glands labeled for 25 min are incubated with the .gamma. subunit (a specific arginyl-esteropeptidase associated with .beta.NGF in the 7S NGF complex), the radioactivity in the 22,000 MW species is converted to the .beta.NGF position. The results suggest that the 22,000 MW species is a biosynthetic precursor to .beta.NGF, and that the .gamma. subunit may function as a specific protease in the processing event.