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The heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) are generally localized to the inner surface of the plasma membrane where they transduce signals from transmembrane receptors to intracellular effectors. In the inactive state, the G protein exists as a trimer containing alpha, beta and gamma subunits. Interaction of the G protein with agonist-bound receptor stimulates the exchange of GTP for GDP on the alpha subunit whereupon the beta and the gamma subunits dissociate from the activated alpha as a tight complex. The GTP-bound alpha subunit and the free beta-gamma complex can act independently on downstream effectors to modulate second messenger concentrations. The hydrolysis of GTP to GDP deactivates alpha, allowing the reassociation of alpha and beta-gamma and the recoupling of the heterotrimer to the receptor. The three types of lipid modifications found on G proteins are depicted here on the heterotrimer associated with the plasma membrane. The alpha subunit contains both a palmitoyl group linked through a thioester and a myristoyl group attached via an amide bond to the amino terminus. The gamma subunit contains a geranylgeranyl isoprenoid linked through a thioether bond, as well as a methylester at the carboxyl terminus. All the lipids are illustrated inserted into the membrane bilayer, however additional membrane proteins may also be involved in the membrane association. Upper portion of graphic appeared in Current
Opinion in Cell Biology. 6:219-225, 1994. |
