The p21-activated kinases (PAKs) play an important role in diverse cellular processes, including cytoskeletal dynamics, growth/apoptotic signal transduction through mitogen-activated protein kinases, and regulation of transcription factors. PAKs are activated by the binding of active Cdc42 and Rac. Upon PAK activation, a conserved threonine residue (Thr423 in human PAK1) in the activation loop of kinase domain is autophosphorylated in an intermolecular manner, which has been shown to be essential for PAK activity. However, the molecular details of the PAK autoactivation process remain poorly understood. Professor Zhi-Xin Wang at the Institute of Biophysics, Chinese Academy of Sciences and his colleagues reported that the crystal structures of phosphorylated and unphosphorylated PAK1 kinase domain. The phosphorylated PAK1 kinase domain has the conformation typical of all active protein kinases. Interestingly, the structure of unphosphorylated PAK1 kinase domain reveals an unusual dimeric arrangement expected in an authentic enzyme-substrate complex (as shown in figure). The two PAK1 kinase domains formed an asymmetric face-to-face dimer in which one monomer (the putative ‘enzyme’) is bond to AMP-PNP and has an active conformation, whereas the other monomer (the putative ‘substrate’) is empty and adopts an inactive conformation. The activation loop of the ‘substrate’ kinase domain is projected into the active site of the ‘enzyme’. Thus, the structure of this asymmetric homodimer mimics a trans-autophosphorylation complex, which suggests that the unphosphorylated PAK1 could dynamically adopt both the active and inactive conformations in solution while phosphorylation shifts a pre-existing equilibrium towards the active state. These results shed new light on the trans-autophosphorylation mechanism of PAK1, and other kinases alike.

This work was a fruit of a scientific collaboration between Zhi-Xin Wang’s group at the Institute of Biophysics, Chinese Academy of Sciences and the Tsinghua University. It was published in the latest issue of Structure in an original paper entitled "Structural Insights into the Autoactivation Mechanism of p21-Activated Protein Kinase" and highlighted with a preview " Face-to-Face, Pak-to-Pak" (http://www.sciencedirect.com/science/article/pii/S0969212611004205).

 Figure: Overall structure of the unphosphorylated PAK1 kinase domain. There are two molecules within an asymmetric unit. Monomer A is shown in blue with activation segment in marine, and monomer B in yellow with activation segment in orange. ATP analogue MgAMP-PNP bound to monomer A and Thr423 of monomer B are highlighted in cyan. From Zhi-Xin Wang’s group