A Mr 70,000 phosphoprotein of sympathetic neurons regulated by nerve growth factor and by depolarization

The effect of nerve growth factor (NGF) on protein phosphorylation was investigated in cultures of dissociated, purified chick sympathetic neurons labeled with inorganic [32P]phosphate or [35S]methionine. For at least 90 min after dissociation and purification of the neurons, overall protein phosphorylation was similar in the absence and presence of added NGF, indicating that the neurons were not unspecifically affected by this period of NGF deprivation. Addition of NGF resulted in a marked decrease in the phosphorylation of a 70,000-dalton protein, designated p70 . p70 existed in five isoelectric variants, referred to as p70 /1-5. p70 /1 was unphosphorylated and was the least acidic variant. p70 /2-5 contained progressively more phosphate and they were increasingly acidic. NGF, via dephosphorylation (or via a highly specific and very limited proteolysis), induced the conversion of p70 /5 and p70 /4 to p70 /2. The effect of NGF on p70 involved phosphothreonine residues to a greater extent than phosphoserine residues and occurred rapidly, being detectable after 5 min and complete after 15 min. 8-Br-cAMP did not mimic the effect of NGF on p70 . Depolarization of the neurons with high K+ and addition of the calcium ionophore A23187 produced effects on the phosphorylated p70 variants similar to those induced by NGF. The response of p70 to two distinct "survival factors," NGF and depolarization, may suggest a role for this phosphoprotein in the survival of sympathetic neurons.