PAKs (p21-activated kinases) are key regulators of actin dynamics, cell proliferation and cell survival. PAKs are Ser/Thr kinases that are classified into two groups on the basis of their structural and functional features: group I (PAK1–3) and group II (PAK4–6). Group I PAKs have an auto-inhibitory domain (also called an inhibitory switch domain) and a kinase domain (catalytic domain, CD) and are activated by the binding of the active (that is, GTP-bound) forms of Rho GTPases, such as Cdc42 and Rac1. Group II PAKs have no auto-inhibitory domains and are not activated by active Rho GTPases. Because the deregulation of PAKs is closely associated with various human diseases,small-molecule inhibitors of these kinases have great potential as therapeutic agents. In addition, these compounds can also be used as powerful tools in studies aimed at understanding the PAK signaling pathway. PAKs are considered prime regulators of the actin cytoskeleton and motility. Due to their central role in actin remodelling and their ability to activate Matrix metalloproteinases (MMPs), Rho GTPases play an important role in tumor cell invasion and metastasis. The current evidence suggests the involvement of PAKs in motility, cell survival, anchorage-independent growth, angiogenesis, invasion, migration and regulation of cell cycle and mitosis. Consequently, PAKs have also been implicated in a number of pathological conditions including cancer.
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