Introduction
The Ras switch operates by alternating between an active, GTP-bound state and an inactive, GDP-bound state.  Understanding the mechanisms that regulate activation / inactivation of Ras-like GTPases is of obvious biological significance and is a subject of intense investigation.  The fact that Ras family effector proteins will specifically recognize the GTP bound form of the protein has been exploited experimentally to develop a powerful affinity purification assay that monitors Ras protein activation.  The assay uses the Ras-binding domain (RBD) of the Ras effector kinase Raf1.  The Raf-RBD domain has been shown to bind specifically to the GTP-bound form of Ras proteins.  The fact that the Raf-RBD has a high affinity for GTP-Ras and that its binding results in a significantly reduced intrinsic and catalytic rate of hydrolysis of Ras make it an ideal tool for affinity purification of GTP-Ras from cell lysates.  The Raf-RBD is in the form of a GST fusion protein, which allows one to "pull-down" the Raf-RBD/GTP-Ras complex with glutathione affinity beads.  The assay therefore provides a simple means of quantitating Ras activation in cells.  The amount of activated Ras is determined by a western blot using a Ras specific antibody.