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 responsib le for the transforming potential of this virus, Middle-T has been sho wn to interact with a variety of cellular proteins known to mediate mi togenic signaling, like phosphatase-2A, Src family kinases, phosphatid ylinositol 3-kinase (PI3-kinase), the adapter protein SHC, phospholipa se C gamma-1 and 14-3-3 family proteins, Association with SHC and PI 3 -kinase, respectively, stimulates two independent signaling pathways t hat are indispensible for viral oncogenicity. SHC activates the Ras/MA PK 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 re gulating early gene expression, Recently, the protein kinase B/Akt (PK B/Akt) has been identified as a target of PI 3-kinase in receptor tyro sine kinase signaling, Here we show that PKB/Akt is a target of wild t ype 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 domina nt-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 activate d by middle-T, Taken together these data suggest that middle-T activat es PKB/Akt in a PI 3-kinase-dependent manner, Furthermore, direct asso ciation with D3 phosphatidylinositides seems to be essential for activ ation of PKB/Akt.