Importance of Hydrophobic Motif Interactions

INTRODUCTION The Serum and Glucocorticoid regulated kinases (SGK) are a family of serine/threonine protein kinases that show extensive sequence homology to the Protein Kinase B (PKB/Akt) enzymes. Three isoforms exist in humans: SGK1, SGK2 and SGK3. Like PKB, SGK1 acts downstream of phosphatidylinositol-3-kinase (PI3K) (1). PKB requires phosphorylation of its activation loop and of a hydrophobic motif (HM) near to it’s C-terminus to achieve full activation. Despite the similarity with PKB, the SGK3 mechanism of activation is unclear, partly because SGK3 is localized to endosomes while PKB is activated at the plasma membrane. Another AGC kinase, PKC related kinase 2 (PRK2), possesses an aspartic acid instead of a phosphorylatable residue in its HM, thereby mimicking a phosphorylated HM. A fusion of PKBβ and the HM of PRK2 was used in crystallographic studies to obtain an active conformation of activated PKB (2). Here, we generated a chimeric protein in which the HM of SGK3 is replaced by the HM of PRK2 in an attempt to to generate an activated allele of SGK3 for in vivo studies.