Differential phosphorylation of microtubule proteins by ATP and GTP

Purified brain microtubule protein is phosphorylated by endogenous protein kinase activities in the presence of [-³²P] ATP or [-³²P] GTP. Here we show that certain microtubule-associated proteins are phosphorylated differently by GTP or ATP as direct phosphoryl donors, suggesting the presence of distinct kinase activities, with different specificities, associated with microtubule protein.