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

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Abstract
Ca2+ played a major role in the functional use of tubulin in brain and other tissues. It activated 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 .mu.M and 0.4 .mu.g, respectively. Ca2+-calmodulin tubulin kinase activity was very unstable after death and procedures were developed to stabilize the activity of this enzyme system. Ca2+-calmodulin tubulin kinase system was distinct from the cAMP-Mg2+ tubulin kinase. Ca2+- and calmodulin-stimulated phosphorylation of tubulin may be a major biochemical mechanism modulating some of Ca effects on tubulin and may play a significant role in mediating some of Ca actions on cell functions.