Farnesylation of Ras is important for the interaction with phosphoinositide 3-kinase gamma

The correct functioning of Ras proteins requires post-translational modific ation of the CTP hydrolases (GTPases). These modifications provide hydropho bic moieties that lead to the attachment of Ras to the inner side of the pl asma membrane. In this study we investigated the role of Ras processing in the interaction with various putative Ras-effector proteins. We describe a specific, GTP-independent interaction between post-translationally modified Ha- and Ki-Ras4B and the G-protein responsive phosphoinositide 3-kinase p1 10 gamma. Our data demonstrate that post-translational processing increases markedly the binding of Ras to p110 gamma in vitro and in Sf9 cells, where as the interaction with p110 alpha is unaffected under the same conditions. Using in vitro farnesylated Ras, we show that farnesylation of Ras is suff icient to produce this effect. The complex of p110 gamma and farnesylated R asGTP exhibits a reduced dissociation rate leading to the efficient shieldi ng of the GTPase from GTPase activating protein (GAP) action. Moreover, Ras processing affects the dissociation rate of the RasGTP complex with the Ra s binding domain (RBD) of Raf-1, indicating that processing induces alterat ions in the conformation of RasGTP. The results suggest a direct interactio n between a moiety present only on fully processed or farnesylated Ras and the putative target protein p110 gamma.