Many biological signals are processed by the binding of chemicals to cell surface receptors. Signals are switched to intracellular language via guanine nucleotide binding regulatory proteins (G-proteins) which are present in all eukaryotic cells. Thus, G-proteins serve as interfaces between receptor-response coupling. Two forms of G-proteins have been reported: conventional G-proteins which are heterotrimeric and consist of alpha, beta, and gamma subunits, and monomeric small molecular weight G-proteins which are generally found as single polypeptides. Recently, high molecular weight G-proteins have also been described. The family of G-proteins contains multiple genes that encode the alpha, beta, or gamma subunits. G-proteins play a pivotal role in excitation-contraction coupling in smooth muscle function and control metabolic and secretory processes. In this review article, we have given a brief overview on the characteristics and methodology for the identification of G-proteins. The heterotrimeric G-proteins are generally identified by Western blotting and ADP-ribosylation with bacterial toxins. The monomeric and high molecular weight G-proteins have been identified by S-35 GTPsigmaS overlay technique and photoaffinity labeling, respectively. Recently, the use of molecular genetic probes has made it possible to investigate the expression of the message for various G-proteins.
