Characterization of the structure and regulation of two novel isoforms of serum- and glucocorticoid-induced protein kinase

The catalytic domain of serum- and glucocorticoid-induced protein kinase (S GK) is 54% identical with protein kinase B (PKB) and, like PKB, is activate d in vitro by 3-phosphoinositide-dependent protein kinase-1 (PDK1) and in v ivo in response to signals that activate phosphatidylinositol (PI) 3-kinase . Here we identify two novel isoforms of SGK, termed SGK2 and SGK3, whose c atalytic domains share 80% amino acid sequence identity with each other and with SGK (renamed SGK1). Like SGK1, the mRNA encoding SGK3 is expressed in all tissues examined, but SGK2 mRNA is only present at significant levels in liver, kidney and pancreas and, at lower levels, in the brain. The level s of SGK2 mRNA in H4IIE cells and SGK3 mRNA in Rat2 fibroblasts are not inc reased by stimulation with serum or dexamethasone, whereas the level of SGK 1 mRNA is increased greatly. SGK2 and SGK3 are activated in vitro by PDK1, albeit more slowly than SGK1, and their activation is accompanied by the ph osphorylation of Thr(193) and Thr(253) respectively, the residues equivalen t to the Thr in the 'activation loop' of PKB that is targeted by PDK1. The PDK1-catalysed phosphorylation and activation of SGK2 and SGK3, like SCK1, is greatly potentiated by mutating Ser(356) and Ser(419) respectively to As p, these residues being equivalent to the C-terminal phosphorylation site o f PKB. Like SGK1, SGK2 and SGK3 are activated 5-fold via a phosphorylation mechanism when cells are exposed to H2O2 but, in contrast with SGK1, activa tion is only suppressed partially by inhibitors of PI 3-kinase. SGK2 and SG K3 are activated to a smaller extent by insulin-like growth factor-1 (2-fol d) than SGK1 (5-fold). Like PKB and SGK1, SGK2 and SGK3 preferentially phos phorylate Ser and Thr residues that lie in Arg-Xaa-Arg-Xaa-Xaa-Ser/Thr moti fs.