3-PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE-1 (PDK1) - STRUCTURAL ANDFUNCTIONAL HOMOLOGY WITH THE DROSOPHILA DSTPK61 KINASE

Background: The activation of protein kinase B (PKB, also known as c-A kt) is stimulated by insulin or growth factors and results from its ph osphorylation at Thr308 and Ser473. We recently identified a protein k inase, termed PDK1, that phosphorylates PKB at Thr308 only in the pres ence of lipid vesicles containing phosphatidylinositol 3,4,5-trisphosp hate (PtdIns(3,4,5)P-3) or phosphatidylinositol 3,4-bisphosphate (PtdI ns(3,4)P-2). Results: We have cloned and sequenced human PDK1. The 556 -residue monomeric enzyme comprises a catalytic domain that is most si milar to the PKA, PKB and PKC subfamily of protein kinases and a carbo xy-terminal pleckstrin homology (PH) domain. The PDK1 gene is located on human chromosome 16p13.3 and is expressed ubiquitously in human tis sues. Human PDK1 is homologous to the Drosophila protein kinase DSTPK6 1, which has been implicated in the regulation of sex differentiation, oogenesis and spermatogenesis. Expressed PDK1 and DSTPK61 phosphoryla ted Thr308 of PKB alpha only in the presence of PtdIns(3,4,5)P-3 or Pt dIns(3,4)P-2. Overexpression of PDK1 in 293 cells activated PKB alpha and potentiated the IGF1-induced phosphorylation of PKB alpha at Thr30 8. Experiments in which the PH domains of either PDK1 or PKB alpha wer e deleted indicated that the binding of PtdIns(3,4,5)P-3 or PtdIns(3,4 )P-2 to PKB alpha is required for phosphorylation and activation by PD K1. IGF1 stimulation of 293 cells did not affect the activity or phosp horylation of PDK1. Conclusions: PDK1 is likely to mediate the activat ion of PKB by insulin or growth factors. DSTPK61 is a Drosophila homol ogue of PDK1. The effect of PtdIns(3,4,5)P-3/PtdIns(3,4)P-2 in the act ivation of PKB alpha is at least partly substrate directed. (C) Curren t Biology Ltd ISSN 0960-9822.