A role for class I PAKs in the regulation of the excitability of the actin cytoskeleton

P21 activated kinases (PAKs) are involved in a wide range of functions from the regulation of the cytoskeleton to the control of apoptosis and proliferation. Although many PAK substrates identified are implicated in the regulation of the actin cytoskeleton, a coherent picture of the total effect of PAK activation on the state of the actin cytoskeleton is unclear. We therefore set out to observe and quantify the effect of PAK inhibition on the actin cytoskeleton in greater detail. In Mouse Embryonic Fibroblasts, inhibition of PAK kinase activity, either by treatment with small molecule inhibitors or overexpression of mutant PAK constructs leads to the constitutive production of patches of the phosphoinositide PIP3 on the ventral surface of the cell. The formation of these patches remodels the actin cytoskeleton and polarises the cell. From the overexpression of truncated and mutant PAK constructs as well as an in vitro model of PAK recruitment to small GTPases we propose that this is due to a hyper recruitment of PAK and PAK binding partners in the absence of PAK kinase activity. This aberrant production of PIP3 suggests that, by limiting its own recruitment, the kinase activity of class I PAKs acts to downregulate PI3K activity, further highlighting class I PAKs as regulators of PI3K activity and therefore the excitability of the actin cytoskeleton.