[Structures and functions of small GTPase and heterotrimeric G proteins]

Small GTPase ras and heterotrimeric G proteins composed of alpha, beta and gamma subunits are members of a superfamily of regulatory GTP hydrolases. They function as molecular switches which cycle between an inactive GDP-bound state and an active GTP-bound state, and are involved in regulatory biological processes from the outside of the cell to its interior. Binding of GTP triggers conformational changes in switch regions, which enable alpha subunit and ras to interact with effector molecules. Beta gamma dimers dissociated from alpha subunit are signaling molecules in their own rights. These G proteins activate various signal transduction pathways including activation of MAP kinases, phosphoinositide 3-kinases and small GTPases.