MARK/Par-1 is a kinase involved in development of embryonic polarity. In neurons, MARK phosphorylates tau protein and causes its detachment from microtubules, the tracks of axonal transport. Because the target sites of MARK on tau occur at an early stage of Alzheimer neurodegeneration, we searched for interaction partners of MARK. Here we report that MARK2 is negatively regulated by PAK5, a neuronal member of the p21-activated kinase family. PAK5 suppresses the activity of MARK2 toward its target, tau protein. The inhibition requires the binding between the PAK5 and MARK2 catalytic domains, but does not require phosphorylation. In transfected Chinese hamster ovary (CHO) cells both kinases show a vesicular distribution with partial colocalization on endosomes containing AP-1/2. Although MARK2 transfected alone destabilizes microtubules and stabilizes actin stress fibers, PAK5 keeps microtubules stable through the downregulation of MARK2 but destabilizes the F-actin network so that stress fibers and focal adhesions disappear and cells develop filopodia. The results point to an inverse relationship between actin-and microtubule-related signaling by the PAK5 and MARK2 pathways that affect both cytoskeletal networks.
INTRODUCTIONThe observations reported here originated from a study of the neuronal microtubule-associated protein tau and its abnormal changes in Alzheimer's disease. The function of tau in healthy neurons is to stabilize microtubules and to ensure axonal transport along microtubules. In degenerating neurons, tau is hyperphosphorylated, detaches from microtubules, and aggregates into pathological "paired helical filaments." The detachment from microtubules is achieved most efficiently by phosphorylating the KXGS-motifs in the microtubule-binding domain of tau, and elevated phosphorylation at these sites occurs early in Alzheimer's disease (Augustinack et al., 2002). A search for the responsible kinase lead to the identification of the MARK family of protein kinases (Drewes et al., 1997). They are related to the Par-1 kinases in Caenorhabditis elegans and Drosophila melanogaster, which are involved in the determination of embryonic polarity (reviews, Pellettieri and Seydoux, 2002;Fortini, 2004), and indeed the activity of MARK is important for neuronal polarity as well (Biernat et al., 2002). MARK kinases consist of an N-terminal catalytic domain, followed by UBA, spacer, and tail domains. One requirement for activity is the phosphorylation of a threonine in the activation loop (T208 in MARK2), which keeps the active site accessible to the substrate. In the case of MARKs from mammalian brain this is achieved by the kinase MARKK (Timm et al., 2003). Its activation, like that of MARK, leads to detachment of tau and neuronal degeneration, and similar principles appear to hold for other organisms (Nishimura et al., 2004). On the other hand, as with other multidomain kinases, regulation is likely to take place on several levels (Huse and Kuriyan, 2002). Examples are the binding of a pseudosubstrate peptide int...
