Dephosphorylation of Microtubule Proteins by Brain Protein Phosphatases 1 and 2A, and Its Effect on Microtubule Assembly

Publisher Website
Google Scholar
Abstract
Protein phosphatase C was purified 140-fold from bovine brain with 8% yield using histone H1 phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase (cyclic AMP-kinase). Brain protein phosphatase C was considerd to consist of 10 and 90%, respectively, of the catalytic subunits of protein phosphatases 1 and 2A on the basis of the effect of ATP and inhibitor-2. Protein phosphatase C dephosphorylated microtubule-association protein 2 (MAP2). .tau. factor, and tubulin phosphorylated by a multifunctional Ca2+/calmodulin-dependent protein kinase (calmodulin-kinase) and the catalytic subunit of cyclic AMP-kinase. The properties of dephosphorylation of MAP2, .tau. factor, and tubulin were compared. The Km values were in the ranges of 1.6-2.7 .mu.M for MAP2 and .tau. factor. The Km value for tubulin decreased from 25 to 10-12.5 .mu.M in the presence of 1.0 mM Mn2+. No difference in kinetic properties of dephosphorylation was observed between the substrates phosphorylated by the two kinases. Protein phosphatase C did not dephosphorylate the native tubulin, but universally dephosphorylated tubulin phosphorylated by the two kinases. The holoenzyme of protein phosphatase 2A from porcine brain could also dephosphorylate MAP2, .tau. factor, and tubulin phosphorylated by the two kinases. The phosphorylation of MAP2 and .tau. factor by calmodulin-kinase separately induced the inhibition of microtubule assembly, and the dephosphorylation by protein phosphatas. C removed its inhibitory effect. These data suggest that brain protein phosphatases 1 and 2A are involved in the switch-off mechanism of both Ca2+/calmodulin-dependent and cyclic AMP-dependet regulation of microtubule formation.

This publication has 55 references indexed in Scilit: