Heterotrimeric G Proteins

Preface Heterotrimeric (a/33,) G proteins function as cell surface signal transducers for a large number of hormones, neurotransmitters, and for autocrine and paracrine factors. It is now known that a hundred or so (not counting the olfactory) receptors are coupled to various effectors through G proteins. This large number of receptors couple to members of one of the four families of G proteins to transmit their signals. The specificity of the receptor-G protein interactions determines the transmission of the signal to different downstream pathways. As with all real life situations, the specificity of interactions between receptors and G proteins is varied and complex. Consequently, signals from a single receptor may be transmitted through several pathways simultaneously. Each of the four families of G proteins has many members. Cloning studies have indicated that there are twenty a, four/3, and six ~/subunits. It has been suggested that very large numbers of heterotrimeric G proteins with defined subunit compositions can be generated from these individual subunits. "Knockout" experiments indicate that G proteins of defined subunit composition communicate signals from different receptors. However, at this time there is not sufficient general information to indicate that the functional identity of a G protein is defined by the molecular identity of all of its subunits. Consequently, in spite of the molecular heterogeneity of the/3 and ~/subunits, the different G proteins are still classified by the identity of their