p21-activated Kinase (PAK1) Is Phosphorylated and Activated by 3-Phosphoinositide-dependent Kinase-1 (PDK1)

Departments of Immunology and Cell Biology, Scripps Research Institute, La Jolla, California 92037, USA.
Journal of Biological Chemistry (Impact Factor: 4.6). 12/2000; 275(52):41201-41209. DOI: 10.1074/jbc.M006553200

ABSTRACT In this study, we show that phosphorylated 3-phosphoinositide-dependent kinase 1 (PDK1) phosphorylates p21-activated kinase
1 (PAK1) in the presence of sphingosine. We identify threonine 423, a conserved threonine in the activation loop of kinase
subdomain VIII, as the PDK1 phosphorylation site on PAK1. Threonine 423 is a previously identified PAK1 autophosphorylation
site that lies within a PAK consensus phosphorylation sequence. After pretreatment with phosphatases, autophosphorylation
of PAK1 occurred at all major sites except threonine 423. A phosphothreonine 423-specific antibody detected phosphorylation
of recombinant, catalytically inactive PAK1 after incubation with wild-type PAK1, indicating phosphorylation of threonine
423 occurs by an intermolecular mechanism. The biological significance of PDK1 phosphorylation of PAK1 at threonine 423 in vitro is supported by the observation that these two proteins interact in vivo and that PDK1-phosphorylated PAK1 has an increased activity toward substrate. An increase of phosphorylation of catalytically
inactive PAK1 was observed in COS-7 cells expressing wild-type, but not catalytically inactive, PDK1 upon elevation of intracellular
sphingosine levels. PDK1 phosphorylation of PAK1 was not blocked by pretreatment with wortmannin or when PDK1 was mutated
to prevent phosphatidylinositol binding, indicating this process is independent of phosphatidylinositol 3-kinase activity.
The data presented here provide evidence for a novel mechanism for PAK1 regulation and activation.

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