Cyclic AMP dependent endogenous phosphorylation of a microtubule associated protein

Abstract

Microtubules prepared from chick brain homogenates by successive cycles of assembly disassembly were found to contain 2 high mol wt proteins, designated microtubule associated protein1 and microtubule associated protein2. Microtubule associated protein2 (apparent mol wt 300,000 by sodium dodecyl sulfate polyacrylamide gel electrophoresis) was the preferred substrate for an endogenous cyclic AMP dependent protein kinase which appeared to be an integral component of the microtubules. The initial rate of phosphorylation of microtubule associated protein2 was enhanced 4- to 6 fold by cyclic AMP, with half maximal stimulation occurring at 2 X 10-7 M cyclic AMP. Under optimal conditions, a total of 1.0 and 1.9 mol of phosphate was incorporated per mole of microtubule associated protein2 in the absence and presence of cyclic AMP, respectively. Cyclic AMP also stimulated the phosphorylation of tubulin, but the rate of phosphate incorporation per mol of tubulin was only 0.15% that of microtubule associated protein2. The data raise the possibility that the cyclic AMP dependent phosphorylation of microtubule associated protein2 may play a role in microtubule assembly or function.