This laboratory has cloned and characterized a retroviral oncogene v-akt and its cellular homolog c-akt. Sequence analysis of c-akt and biochemical characterization of its product revealed that it encodes a protein kinase C (PKC)related serine threonine kinase, expressed in most tissues with highest levels in thymus and testes. The c-akt protein contains three characteristic regions: The N-terminal region which exhibits distant similarity to the src homology 2 (SH2) domain of cytoplasmic tyrosine kinases, a region rich in glutamic acid residues, which is predicted to form an amphipathic alpha-helix, and the kinase region with a 69 amino acid C-terminal tail. v-akt arose by recombination between c-akt and Gag and encodes a tripartite protein composed of Gag, X and c-akt components, where X is derived in part from the 5' untranslated region of the gene. The v-akt protein is myristylated at its aminoterminus and it is associated with the plasma membranes and the nucleus. Its subcellular distribution, therefore, is clearly different from that of c-akt which is localized primarily in the cytosol. The differences in posttranslational modification and subcellular distribution between v-akt and c-akt translate into dramatic differences in oncogenic potential between the two proteins. The c-akt (and v-akt) kinase is activated rapidly, following exposure of NIH3T3 cells to mitogenic concentrations of the platelet derived growth factor (PDGF). The PDGF induced activation of Akt follows the activation of Ras and is mediated by a combination of Ras-dependent and Ras-independent signals. Although Akt function maps downstream of Ras, however, Akt does not activate the MAP kinase cascade. Recent studies revealed that the SH2-like domain, we originally described in Akt, now referred to as the AH/PH (Akt homology/Pleckstrin homology) domain, is conserved during evolution, and is present in a large number of signalling proteins in addition to Akt. Moreover, they revealed that the AH/PH domain interacts with the kinase domain of Akt and contributes to the regulation of its catalytic activity. In view of these findings, we propose to use molecular genetics and tissue culture technologies to define the role of Akt in signal transduction and oncogenesis. In these studies we will emphasize the functional significance of the AH/VPH domain, a novel domain of protein-protein interaction, which together with the previously described SH2 and SH3 domains, may regulate kinase signalling.
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