Covalent Modification of G-Proteins by Affinity Labeling

The activation of heterotrimeric G-proteins is tightly regulated by the exchange of GTP for GDP in the α-subunit; mostly—but not exclusively—seven-transmembrane receptors function as the guanine nucleotide exchange factors (GEFs). A research goal may be to determine which G-protein α-subunit is activated by the receptor under investigation. In a membrane preparation obtained from cells or tissues this can be achieved in a seemingly straightforward manner by determining if the receptor increases the covalent incorporation of GTP analogs into G-protein α-subunits. Because the GTP analogs may be labeled to high specific radioactivity the α-subunit can then be identified with the use of specific antibodies (see Chapter 13 ). One of the compounds we present here (2′,3′-dialdehyde-GTP) can also be employed to block receptor-mediated G-protein activation and to disrupt the cognate signaling pathway.