Phosphorylation of tubulin and microtubule-associated proteins by the purified insulin receptor kinase.

The purified insulin receptor kinase catalyzed the phosphorylation of native tubulin and microtubule-associated proteins (MAPs; MAP2, tau) on tyrosine residues. Insulin (10(-7) M) stimulated the reaction by 4-10-fold by increasing Vmax with little change in Km. alpha-Tubulin was preferred as a substrate for the kinase compared to beta-tubulin. MAP2 was found to be the best substrate among the cytoskeletal proteins tested; in the presence of insulin, the Vmax for MAP2 was 6.3 nmol/min/mg, its Km was 5.1 microM, and 1.7 mol of phosphate were incorporated per mol of MAP2. Under the same conditions used for this phosphorylation of tubulin and MAPs, actin and tropomyosin were very poorly phosphorylated. These data, coupled with previous evidence for potential functional relationships between insulin action and microtubules, raise the possibility that microtubule proteins may be cellular targets for the insulin receptor kinase.