Emerging from the PAK: The p21-activated protein kinase family

The p21-activated protein kinases (PAKs) are members of a growing family of regulatory enzymes that may play roles in diverse phenomena such as cellular morphogenesis, the stress response and the pathogenesis of AIDS. PAKs were initially discovered as binding partners for small (21 kDa) GTPases that regulate actin polymerization, and recent evidence has shown that some members of the PAK family may be effectors for related GTPases that are involved in intracellular vesicle trafficking. Because the downstream signalling pathways for all such GTPases are poorly understood, intense studies are under way to discern the role of PAK and its cousins. In this review, the authors highlight some of the established properties of the extended PAK family and discuss current controversies regarding their possible roles as GTPase effectors.