THE PHOSPHORYLATION OF STATHMIN BY MAP KINASE

Stathmin, a ubiquitous cytosolic phosphoprotein which may play a role in integrating the effects of diverse signals regulating proliferation , differentiation and other cell functions, was found to be phosphoryl ated rapidly and stoichiometrically by mitogen-activated protein (MAP) kinase in vitro. Ser-25 was identified as the major site and Ser-38 a s a minor site of phosphorylation, while the p42 and p44 isoforms of M AP kinase were the only significant stathmin kinases detected in PC12 cells after stimulation by nerve growth factor (NGF). The results sugg est that MAP kinases are the enzymes responsible for increasing the le vel of phosphorylation of Ser-25, which has been observed previously i n PC12 cells following stimulation by NGF.