Crystal structure and functional analysis of Ras binding to its effector phosphoinositide 3-kinase γ

Ras activation of phosphoinositide 3-kinase (PI3K) is important for survival of transformed cells. We find that PI3K gamma is strongly and directly activated by H-Ras G12V in vivo or by GTP gammaS-loaded H-Ras in vitro. We have determined a crystal structure of a PI3Ky/Ras.GMPPNP complex. A critical loop in the Ras binding domain positions Ras so that it uses its switch I and switch II regions to bind PI3K gamma. Mutagenesis shows that interactions with both regions are essential for binding PI3K gamma. Ras also forms a direct contact with the PI3K gamma catalytic domain. These unique Ras/PI3K gamma interactions are likely to be shared by PI3K alpha. The complex with Ras shows a change in the PI3K conformation that may represent an allosteric component of Ras activation.