Regulation of a Third Conserved Phosphorylation Site in SGK1*

SGK1 (serum- and glucocorticoid-induced kinase 1) is a member of the AGC branch of the protein kinase family. Among well described functions of SGK1 is the regulation of epithelial transport through phosphorylation of the ubiquitin protein ligase Nedd4-2 (neuronal precursor cell expressed developmentally down-regulated 4-2). The activation of SGK1 has been widely accepted to be dependent on the phosphorylation of Thr²⁵⁶ in the activation loop and Ser⁴²² in the hydrophobic motif near the C terminus. Here, we report the identification of two additional phosphorylation sites, Ser³⁹⁷ and Ser⁴⁰¹. Both are required for maximum SGK1 activity induced by extracellular agents or by coexpression with other protein kinases, with the largest loss of activity from mutation of Ser³⁹⁷. Coexpression with active Akt1 increased the phosphorylation of Ser³⁹⁷ and thereby SGK1 kinase activity. SGK1 activation was further augmented by coexpression with the protein kinase WNK1 (with no lysine kinase 1). These findings reveal further complexity underlying the regulation of SGK1 activity.