Interaction of PKN with α-Actinin

PKN is a fatty acid- and Rho-activated serine/threonine protein kinase, having a catalytic domain homologous to protein kinase C family. To identify components of the PKN-signaling pathway such as substrates and regulatory proteins of PKN, the yeast two-hybrid strategy was employed. Using the N-terminal region of PKN as a bait, cDNAs encoding actin cross-linking protein α-actinin, which lacked the N-terminal actin-binding domain, were isolated from human brain cDNA library. The responsible region for interaction between PKN and α-actinin was determined by in vitro binding analysis using the various truncated mutants of these proteins. The N-terminal region of PKN outside the RhoA-binding domain was sufficiently shown to associate with α-actinin. PKN bound to the third spectrin-like repeats of both skeletal and non-skeletal muscle type α-actinin. PKN also bound to the region containing EF-hand-like motifs of non-skeletal muscle type α-actinin in a Ca2+-sensitive manner and bound to that of skeletal muscle type α-actinin in a Ca2+-insensitive manner. α-Actinin was co-immunoprecipitated with PKN from the lysate of COS7 cells transfected with both expression constructs for PKN and α-actinin lacking the actin-binding domain. In vitro translated full-length α-actinin containing the actin-binding site hardly bound to PKN, but the addition of phosphatidylinositol 4,5-bisphosphate, which is implicated in actin reorganization, stimulated the binding activity of the full-length α-actinin with PKN. We therefore propose that PKN is linked to the cytoskeletal network via a direct association between PKN and α-actinin.