ABSTRACT
The ralA and ralB genes belong to the ras superfamily. Like G proteins, the proteins encoded by members of this superfamily bind GTP and GDP with high affinity and cycle between the active GTP-bound and inactive GDP-bound states (for review see Reference 1). The former is promoted by the replacement of GDP with GTP, the latter by hydrolysis of GTP to GDP. The Ras proteins can be distinguished from the G protein family by their smaller size (20 to 30 kDa compared to 40 to 50 kDa). They also appear to exist in cells as monomers, in contrast to the heterotrimeric G proteins that contain β- and γ-subunits. Finally, the intrinsic GTPase activity of the Ras superfamily members can be stimulated by specific GTPase-activating proteins (GAPs).
