Regulation of a Mitogen-Activated Protein Kinase Kinase Kinase, MLTK by PKN

PKNα is a fatty acid- and Rho-activated serine/threonine protein kinase having a catalytic domain homologous to members of the protein kinase C family. Recently it was reported that PKNα is involved in the p38 mitogen–activated protein kinase (MAPK) signaling pathway. To date, however, how PKNα regulates the p38γ MAPK signaling pathway is unclear. Here we demonstrate that PKNα efficiently phosphorylates MLTKα (MLK-like mitogen-activated protein triple kinase), which was recently identified as a MAPK kinase kinase (MAPKKK) for the p38 MAPK cascade. Phosphorylation of MLTKα by PKNα enhances its kinase activity in vitro. Expression of the kinase-negative mutant of PKNα inhibited the mobility shift of MLTKα caused by osmotic shock in SDS-PAGE. Furthermore, PKNα associates with each member of the p38γ MAPK signaling pathway (p38γ, MKK6, and MLTKα). These results suggest that PKNα functions as not only an upstream activator of MLTKα but also a putative scaffold protein for the p38γ MAPK signaling pathway.