A role for class I p21-activated kinases in the regulation of the excitability of the actin cytoskeleton.

The p21-activated kinases (PAKs) are involved in a range of functions, including the regulation of the actin cytoskeleton. However, although many PAK substrates identified have been 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. Here, we show that, in mouse embryonic fibroblasts, inhibition of class I PAK kinase activity by small-molecule inhibitors leads to the constitutive production of the phosphoinositide phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P3] on the ventral surface of the cell. The formation of patches of PI(3,4,5)P3 remodels the actin cytoskeleton and polarises the cell. From the overexpression of truncated and mutated PAK1 and PAK2 constructs, as well as an in vitro model of PAK activation, we propose that this is driven by a hyper recruitment of class I PAK and PAK-binding partners. This aberrant production of PI(3,4,5)P3 suggests that, by limiting its own recruitment, the kinase activity of class I PAKs acts to downregulate phosphoinositide 3-kinase (PI3K) activity, further highlighting class I PAKs as regulators of PI3K activity and therefore the excitability of the actin cytoskeleton.