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J. Biol. Chem., Vol. 281, Issue 33, 23978-23989, August 18, 2006

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Role of the Phox Homology Domain and Phosphorylation in Activation of Serum and Glucocorticoid-regulated Kinase-3^*

From the Samuel Lunenfeld Research Institute and Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 1X5, Canada

Serum and glucocorticoid-regulated kinases (SGKs) form a family of serine/threonine protein kinases that exhibit structural and sequence similarity to the protein kinase B (PKB)/Akt family. The major difference between these two families is the absence of a lipid-binding, pleckstrin homology domain in the SGKs. Despite the absence of the pleckstrin homology domain, activation of the three human isoforms is, like PKB, dependent upon the phosphatidylinositol 3'-kinase (PI3K) pathway that is induced by growth factors and mitogens. Full-length SGK3 contains a complete Phox homology (PX) domain that targets the protein to endosomes. Both a functional PX domain and PI3K activation are necessary for phosphorylation of SGK3 at two regulatory sites (Thr-320 and Ser-486) and subsequent induction of kinase activity. PDK1 phosphorylates endosome-associated SGK3 at Thr-320, whereas diversion of SGK3 to the plasma membrane, where PDK1 normally activates PKB, interferes with PDK1 phosphorylation of SGK3. A chimeric protein in which the carboxyl-terminal hydrophobic motif (HM) of SGK3 has been exchanged for the HM of PRK2 is constitutively active. Finally, we demonstrate that SGK3 activation becomes PX domain-independent once the HM is phosphorylated. Taken together, these data indicate that the targeting of SGK3 to endosomes, mediated by its PX domain, is essential for proper SGK3 activation, likely due to co-localization of SGK3 with an endosomal, PI3K-dependent and staurosporine-sensitive HM kinase.

Received for publication, May 5, 2006 , and in revised form, June 19, 2006.

^* This work was supported by a Canadian Institutes of Health Research studentship (to M. T.) and by Canadian Institutes of Health Research and Terry Fox Foundation grants (to J. R. W.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1.

¹ To whom correspondence should be addressed: Samuel Lunenfeld Research Institute, 600 University Ave., Toronto, Ontario M5G 1X5, Canada. Tel.: 416-946-2962; Fax: 416-946-2984; E-mail: woodgett{at}mshri.on.ca.

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