Ca²⁺-dependent phosphorylation by endogenous kinases of Mr 95 K and 50 K-55 K proteins in PC12 pheochromocytoma cells

Abstract

Endogenous protein phosphorylation of PC12 cells was investigated with the homogenate as well as intact cells. In the case of the homogenate, the major proteins that were phosphorylated in the presence of Ca²⁺ were found to be of Mr 95 K and Mr 50 K-55 K. Ca²⁺/calmodulin-dependent protein kinase appeared to be responsible for phosphorylation of Mr 50 K-55 K proteins and partly of Mr 95 K protein. The apparentK _m's for Ca²⁺ of Mr 95 K and 50 K-55 K protein phosphorylation were 2.2×10⁻⁷ M and around 1.5×10⁻⁶ M, respectively. Since several cell lines of neuroblastoma exhibited Mr 95 K protein phosphorylation of similar type, the protein phosphorylation may be a common process shared by neuronal cells. Depolarization of intact PC12 cells by high K⁺ concentrations induced Mr 95 K protein phosphorylation. The results suggest that a physiological increase by excitation in the intracellular Ca²⁺ concentration triggers phosphorylation of Mr 95 K protein in neuronal cells and this phosphorylation may play a role in the regulation of transmitter release.