Ca2+- and calmodulin-stimulated endogenous phosphorylation of neurotubulin.

Ca2+ plays a major role in the functional use of tubulin in brain and other tissues. It activates an endogenous tubulin kinase system in brain cytosol, tubulin, and presynaptic nerve terminal fractions prepared from rat brain. Activation of the Ca2+ tubulin kinase system was modulated by the Ca2+ receptor protein calmodulin. The concentrations of Ca2+ and calmodulin required to produce a half-maximal stimulation of the tubulin kinase were 0.8 microM and 0.4 micrograms, respectively. Ca2+ -calmodulin tubulin kinase activity was very unstable after death, and procedures were developed to stabilize the activity of this enzyme system. Evidence is presented demonstrating that the Ca2+ -calmodulin tubulin kinase system is distinct from the previously described cyclic AMP-Mg2+ tubulin kinase. The results suggest that Ca2+- and calmodulin-stimulated phosphorylation of tubulin may be a major biochemical mechanism modulating some of calcium's effects on tubulin and may play a significant role in mediating some of calcium's actions on cell functions.