The p35/Cdk5 kinase is a neuron-specific Rac effector that inhibits Pak1 activity

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Abstract
Cyclin-dependent kinase 5 (Cdk5) and its neuron-specific regulator p35 (refs 1–4) are essential for neuronal migration and for the laminar configuration of the cerebral cortex5,6,7. In addition, p35/Cdk5 kinase concentrates at the leading edges of axonal growth cones and regulates neurite outgrowth in cortical neurons in culture8. The Rho family of small GTPases is implicated in a range of cellular functions, including cell migration and neurite outgrowth9,10,11,12,13. Here we show that the p35/Cdk5 kinase co-localizes with Rac in neuronal growth cones. Furthermore, p35 associates directly with Rac in a GTP-dependent manner. Another Rac effector, Pak1 kinase14,15, is also present in the Rac–p35/Cdk5 complexes and co-localizes with p35/Cdk5 and Rac at neuronal peripheries. The active p35/Cdk5 kinase causes Pak1 hyperphosphorylation in a Rac-dependent manner, which results in downregulation of Pak1 kinase activity. Because the Rho family of GTPases and the Pak kinases are implicated in actin polymerization16,17,18, the modification of Pak1, imposed by the p35/Cdk5 kinase, is likely to have an impact on the dynamics of the reorganization of the actin cytoskeleton in neurons, thus promoting neuronal migration and neurite outgrowth.