Protein kinase B/Akt is activated by polyomavirus middle-T antigen via a phosphatidylinositol 3-kinase-dependent mechanism

The middle tumor antigen (middle-T) of mouse polyomavirus is responsible for the transforming potential of this virus, Middle-T has been shown to interact with a variety of cellular proteins known to mediate mitogenic signaling, like phosphatase-2A, Src family kinases, phosphatidylinositol 3-kinase (PI3-kinase), the adapter protein SHC, phospholipase C gamma-1 and 14-3-3 family proteins, Association with SHC and PI 3-kinase, respectively, stimulates two independent signaling pathways that are indispensible for viral oncogenicity. SHC activates the Ras/MAPK pathway via Grb2/SOS resulting in changes in early gene expression, The downstream targets of PI 3-kinase are less well studied but seem to impinge on serum response factor (SRF) which is also involved in regulating early gene expression, Recently, the protein kinase B/Akt (PKB/Akt) has been identified as a target of PI 3-kinase in receptor tyrosine kinase signaling, Here we show that PKB/Akt is a target of wild type middle-T, but not of mutants unable to activate PI 3-kinase, These data were confirmed using inhibitors of PI 3-kinase as well as dominant-negative alleles of the catalytic subunit of this lipid kinase, In addition, mutants of PKB/Akt lacking a pleckstrin homology domain and therefore unable to bind to D3 phospatidylinositides were not activated by middle-T, Taken together these data suggest that middle-T activates PKB/Akt in a PI 3-kinase-dependent manner, Furthermore, direct association with D3 phosphatidylinositides seems to be essential for activation of PKB/Akt.