Nerve growth factor (NGF) and epidermal growth factor (EGF) produce stable alterations in PC12 cells that persist in the detergent-insoluble cytoskeleton, resulting in the phosphorylation of a 250,000-mol-wt cytoskeletally associated protein in situ. Treatment of PC12 cells with NGF or EGF, followed by detergent lysis of the cells and incubation of the resulting cytoskeletons with gamma-32P-ATP, permitted detection of hormonally stimulated, energy-dependent events, which result in the enhanced phosphorylation of a cytoskeletally associated protein as an immediate consequence of receptor occupancy. These events were elicited only upon treatment of intact cells at physiological temperatures. The NGF- and EGF-stimulated events occurred rapidly; however, they were a transient effect of hormone action. NGF and EGF were found to act through independent mechanisms to stimulate the in situ phosphorylation of the 250,000-mol-wt protein, as the effects of NGF, but not EGF, were blocked by methyltransferase inhibitors. The 250,000-mol-wt protein was phosphorylated on serine and threonine residues in response to both NGF and EGF although in somewhat different proportions. The data suggest that the hormone-stimulated labeling of the 250,000-mol-wt protein may be the result of either the direct activation of a protein kinase, the redistribution of the kinase relative to its substrates as a consequence of hormone action, or the coincident occurrence of these events.

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