Rapid phosphorylation of MAP-2-related cytoplasmic and nuclear Mr 300,000 protein by serine kinases after growth stimulation in quiescent cells☆

Antibody against brain microtubule-associated protein 2 (MAP-2) immunoprecipitated Mr 300,000 and 80,000 proteins of cultured fibroblasts and kidney cells. These proteins were not appreciably phosphorylated in quiescent cells, but were rapidly phosphorylated after growth stimulation by insulin, epidermal and fibroblast growth factors, transferrin, phorbol ester and diacylglycerol in the presence of Ca2+, in a manner similar to that of MAP-1-related Mr 350,000 protein (J. Cell Biol. 100, 748–753). A Ca2+ ionophore, which is known to make the quiescent cell competent but not to enter into the growth cycle, did not induce the phosphorylation. In a chase experiment, decay half lifes of labeled phosphoproteins were 5 h for Mr 350,000 and 300,000 proteins, and 1.5 h for Mr 80,000 protein. On subcellular fractionation, phosphorylated Mr 350,000 and 300,000 proteins were detected first mainly in the cytoplasm and then in the nucleus, while Mr 80,000 phosphoprotein was consistently detected in the cytoplasm. The phosphorylation of these proteins occurred on serine residues after stimulation with various factors. Thus, the phophorylation of cytoskeleton-associated Mr 350,000 and 300,000 proteins by serine kinases seems to be a common second process after growth stimulation and to link cytoplasmic and intranuclear events.