N-terminal interaction domain implicates PAK4 in translational regulation and reveals novel cellular localization signals.

The serine/threonine kinase PAK4 is a Rho GTPases effector protein implicated in many critical biological processes, including regulation of cell morphology and motility, embryonic development, cell survival, response to infection, and oncogenic transformation. Consistently with its pro-oncogenic features, PAK4 was found to be overexpressed in many cancer cell lines and tissues, and to be necessary to promote activation of survival pathways. PAK4, like other Paks, is now considered a promising target for specific therapy. Little is known on its modes of regulation, molecular partners, and substrates. Because the N-terminal regulatory moiety plays important roles in PAK4 activity and functions, even independently of GTPase interactions, in this study we employed an affinity chromatography approach to identify N-terminal domain binding partners. Within this protein region we identified a novel interaction domain involved in association with ribonucleoprotein (RNP) complexes, suggesting PAK4 implications in translational regulation. Indeed, we found that active PAK4 can affect (cap-independent) translation from specific IRES sequences in vivo, and that the N-terminal domain is critical for this regulation. Further, we could establish that within the RNP interacting sequence PAK4 regulatory domain contains targeting elements that drive cytoplasmic localization and act as nuclear export signal. Functional implication of endogenous PAK4 protein, which was found in both cytoplasmic and nuclear fractions, in IRES-mediated translation further underlines the significance of the reported findings. Our data reveal novel means for PAK4 regulation of gene expression, and provide new elements to understand the molecular mechanisms that determine PAK4 cellular localization and functions.