Inhibition of microtubule assembly by phosphorylation of microtubule-associated proteins

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Abstract
32P labeling of microtubular protein from pig brains by endogenous protein kinase activity results from a net increase in protein-bound phosphate and is not the result of a phosphate exchange reaction between ATP and phosphoprotein. Protein phosphorylation is maximal in the presence of 0.5 mM Mg2+ and 0.25 mM ATP, resulting in .apprx. 2.8 nmol of phosphate/mg of protein. Phosphorylation can be increased 2- to 3-fold by c[cyclic]AMP. The protein substrates for phosphorylation in either the absence or presence of cAMP are the microtubule-associated proteins [MAP] which copurify with tubulin and promote microtubule assembly. Phosphorylation of microtubule-associated proteins inhibits both the rate and extent of microtubule assembly when the protein is exposed to conditions which result in dissociation of rings. Phosphorylation apparently modifies MAP so that they have a reduced ability to form an assembly-competent complex with tubulin.