Regulation of microtubule assembly by a Ca2+- and calmodulin-dependent protein kinase in the brain

Abstract

Microtubules play an important role in various cellular processes including mitosis, axonal flow and hormone secretion. Since removal of Ca2+ was necessary for in vitro microtubule assembly, it was postulated that the inhibitory effect of Ca2+ on microtubule assembly may mainly be mediated through calmodulin. In our previous work, we reported that a brain Ca2+-and calmodulin-dependent protein kinase (640K-enzyme) phosphor-ylated microtubule protein. To clarify the physiological significance of the 640K-enzyme, we examined the phosphorylation of microtubule associated proteins (MAPs) by the 640K-enzyme and its inhibitory effect on microtubule assembly. 1) The 640K-enzyme phosphorylated MAPs in a Ca2+- and calmodulin-dependent manner. 2) Among MAPs, MAP2 and tau factor were phosphorylated. 3) Ka value of the enzyme for calmodulin was 57.0 nM with MAPs as substrates. 4) The phosphorylation of MAPs led to an inhibition of microtubule assembly. 5) The critical tubulin concentration for microtubule assembly was unchanged by the MAPs phosphorylation. These results indicate that assembly and disassembly of brain microtubules are regulated by the Ca2+- and calmodulin-dependent protein kinase which requires only a nanomolar concentration of calmodulin for activation.