Regulation of the kinase MARK from Rattus norvegicus (Berkenhout, 1769): activation by MARKK and inhibition by PAK5

Abstract

MARK/Par1, a kinase involved in establishing cell polarity, phosphorylates microtubule-associated protein (tau/MAP2/MAP4) at KXGS motifs, and causes their detachment from microtubules. This leads to the destabilization of the microtubules, the tracks of axonal transport. The phosphorylation of the target sites on tau by MARK occurs at an early stage of Alzheimer neurodegeneration. This study addresses the regulation of MARK. It is activated by phosphorylation of an upstream kinase MARKK. It is inhibited by binding of PAK5, a kinase which normally regulates the actin cytoskeleton.

The upstream kinase, MARKK, is a member of the Ste20 kinase family. It phosphorylates MARK within the activation loop (T208 in MARK2). In cells the activity of MARKK enhances the dynamics of the microtubule network through the activation of MARK and leads to the phosphorylation and detachment of tau or equivalent MAPs from microtubules. Overexpression of MARKK eventually leads to microtubule breakdown and cell death, but in neuronal cells the primary effect of endogenous MARKK is to allow the development of neurites during differentiation.

A brain specific kinase PAK5, a member of the p21-activated kinase family, is found to inhibit MARK activity by direct binding, not by phosphorylation. The binding occurs through the catalytic domains of the two kinases. In cells this leads to the protection of the microtubule network. However, PAK5 itself destabilizes the F-actin network so that stress fibers and focal adhesions disappear and cells develop filopodia. In transfected Chinese hamster ovary cells both PAK5 and MARK show a vesicular distribution. Vesicles of inactive PAK5 are also transported to the microtubule organizing center.

Yeast two hybrid data also reveal the interaction between PAK5 and MARKK. This interaction occurs through the binding of the non-catalytic domains of the two kinases. In CHO cells overexpression of PAK5 inhibits the destructive effect of MARKK on the microtubule network.