Differential sensitivity of phosphatidylinositol 3-kinase p110γ to isoforms of G protein βγ dimers

The ability of G protein alpha and betagamma subunits to activate the p110gamma isoform of phosphatidylinositol 3-kinase ( PtdIns 3-kinase) was examined using pure, recombinant G proteins and the p101/p110gamma form of PtdIns 3-kinase reconstituted into synthetic lipid vesicles. GTP-activated G(s), G(i), G(q), or G(o) alpha subunits were unable to activate PtdIns 3-kinase. Dimers containing Gbeta(1-4) complexed with gamma(2)-stimulated PtdIns 3-kinase activity about 26-fold with EC50 values ranging from 4 to 7 nM. Gbeta(5)gamma(2) was not able to stimulate PtdIns 3-kinase despite producing a 10-fold activation of avian phospholipase Cbeta. A series of dimers with beta subunits containing point mutations in the amino acids that undergo a conformational change upon interaction of betagamma with phosducin (beta1H311Agamma2, beta1R314Agamma2, and beta1W332Agamma2) was tested, and only beta1W332Agamma2 inhibited the ability of the dimer to stimulate PtdIns 3-kinase. Dimers containing the beta(1) subunit complexed with a panel of different Ggamma subunits displayed variation in their ability to stimulate PtdIns 3-kinase. The beta(1)gamma(2), beta(1)gamma(10), beta(1)gamma(12), and beta(1)gamma(13) dimers all activated PtdIns 3-kinase about 26-fold with 4 - 25 nM EC50 values. The beta(1)gamma(11) dimer, which contains the farnesyl isoprenoid group and is highly expressed in tissues containing the p101/p110gamma form of PtdIns 3-kinase, was ineffective. The role of the prenyl group on the gamma subunit in determining the activation of PtdIns 3-kinase was examined using gamma subunits with altered CAAX boxes directing the addition of farnesyl to the gamma(2) subunit and geranylgeranyl to the gamma(1) and gamma(11) subunits. Replacement of the geranylgeranyl group of the gamma(2) subunit with farnesyl inhibited the activity of beta(1)gamma(2) on PtdIns 3-kinase. Conversely, replacement of the farnesyl group on the gamma(1) and gamma(11) subunit with geranylgeranyl restored almost full activity. These findings suggest that all beta subunits, with the exception of beta(5), interact equally well with PtdIns 3-kinase. In contrast, the composition of the gamma subunit and its prenyl group markedly affects the ability of the betagamma dimer to stimulate PtdIns 3-kinase.