Serine͞threonine kinase Akt͞PKB is a downstream effector molecule of phosphoinositide 3-kinase and is thought to mediate many biological actions toward anti-apoptotic responses. We found that Akt formed a complex with a 90-kDa heat-shock protein (Hsp90) in vivo. By constructing deletion mutants, we identified that amino acid residues 229 -309 of Akt were involved in the binding to Hsp90 and amino acid residues 327-340 of Hsp90␤ were involved in the binding to Akt. Inhibition of Akt-Hsp90 binding led to the dephosphorylation and inactivation of Akt, which increased sensitivity of the cells to apoptosis-inducing stimulus. The dephosphorylation of Akt was caused by an increase in protein phosphatase 2A (PP2A)-mediated dephosphorylation and not by a decrease in 3-phosphoinositide-dependent protein kinase-1-mediated phosphorylation. These results indicate that Hsp90 plays an important role in maintaining Akt kinase activity by preventing PP2A-mediated dephosphorylation.

The colonization of land by plants was a key event in the evolution of life. Here we report the draft genome sequence of the filamentous terrestrial alga Klebsormidium flaccidum (Division Charophyta, Order Klebsormidiales) to elucidate the early transition step from aquatic algae to land plants. Comparison of the genome sequence with that of other algae and land plants demonstrate that K. flaccidum acquired many genes specific to land plants. We demonstrate that K. flaccidum indeed produces several plant hormones and homologues of some of the signalling intermediates required for hormone actions in higher plants. The K. flaccidum genome also encodes a primitive system to protect against the harmful effects of high-intensity light. The presence of these plant-related systems in K. flaccidum suggests that, during evolution, this alga acquired the fundamental machinery required for adaptation to terrestrial environments.

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Serine/threonine kinase Akt is thought to mediate many biological actions toward anti-apoptotic responses. Screening of drugs that could interfere with the Akt signaling pathway revealed that Hsp90 inhibitors (e.g. geldanamycin, radicicol, and its analogues) induced Akt dephosphorylation, which resulted in Akt inactivation and apoptosis of the cells. Hsp90 inhibitors did not directly affect Akt kinase activity in vitro. Thus, we examined the effects of Hsp90 inhibitors on upstream Akt kinases, phosphatidylinositide-3-OH kinase (PI3K) and 3-phosphoinositide-dependent protein kinase-1 (PDK1). Hsp90 inhibitors had no effect on PI3K protein expression. In contrast, treatment of the cells with Hsp90 inhibitors decreased the amount of PDK1 without directly inhibiting PDK1 kinase activity. We found that the kinase domain of PDK1 was essential for complex formation with Hsp90 and that Hsp90 inhibitors suppressed PDK1 binding to Hsp90. PDK1 degradation mechanisms revealed that inhibition of PDK1 binding to Hsp90 caused proteasome-dependent degradation of PDK1. Treatment of proteasome inhibitors increased the amount of detergent-insoluble PDK1 in Hsp90 inhibitor-treated cells. Therefore, the association of PDK1 with Hsp90 regulates its stability, solubility, and signaling. Because Akt binding to Hsp90 is also involved in the maintenance of Akt kinase activity, Hsp90 plays an important role in PDK1-Akt survival signaling pathway.The characterization of the survival signal transduction pathways stimulated by growth factors and cytokines has revealed that phosphatidylinositide-3-OH kinase (PI3K) 1 is involved in the pathway (1-3). PI3K is a heterodimeric lipid kinase, which consists of an 85-kDa regulatory subunit and a 110-kDa catalytic subunit. After stimulation with growth factors and cytokines, PI3K is activated by the interaction of the 85-kDa subunit with phosphotyrosines of activated intracellular domain of growth factor receptors or with the receptorassociated adapter proteins. Subsequently, PI3K associates with the plasma membrane where the 110-kDa catalytic subunit phosphorylates phosphoinositides. The generated phospholipid second messenger molecule, phosphatidylinositol 3,4,5-triphosphate (PtdIns(3,4,5)P 3 ), raises a diverse set of cellular responses (4 -6). The major targets of PtdIns(3,4,5)P 3 are pleckstrin homology (PH) domain-containing proteins, including serine/threonine kinase Akt (also known as protein kinase B (PKB) or RAC-PK) (7, 8).The interaction of PtdIns(3,4,5)P 3 with the PH domain of Akt recruits Akt to the plasma membrane, where it is phosphorylated at two key regulatory residue sites, Thr 308 and Ser 473 . Phosphorylation at both residues is necessary for full activation of Akt and the subsequent regulation of many PI3K-regulated biological responses, including glucose uptake, protein synthesis, and apoptosis inhibition (7,8). Akt phosphorylation at Thr 308 is catalyzed by the ubiquitously expressed 3-phosphoinositide-dependent protein kinase-1 (PDK1) (9 -11). The kinase responsible for phosph...