Immunological characterization of guanine nucleotide-binding proteins: effects of a monoclonal antibody against the gamma subunit of transducin on guanine nucleotide-binding protein-receptor interactions
Halpern, J.L.; Moss, J.
Molecular Pharmacology 37(6): 797-800
ISSN/ISBN: 0026-895X PMID: 1694260 Accession: 007430570
Guanine nucleotide-binding proteins (G proteins) transduce signals from agonist- and light-sensitive receptors. In the visual excitation system, the photon receptor rhodopsin is coupled to the G protein Gt (transducin). Gt is composed of .alpha., .beta., and .gamma. subunits; the .alpha. subunit binds guanine nucleotide, whereas the .beta. and .gamma. subunits, which are tightly associated, appear to facilitate interaction of .alpha. with receptor and pertussis toxin-catalyzed ADP-ribosylation of .alpha. To study the function of transducin, monoclonal antibodies were developed against the purified protein. Monoclonal antibody 2H3 reacted with Gt.gamma. but not G.gamma. from bovine brain or rabbit liver. In the absence of photolyzed rhodopsin, both intact 2H3 and Fab fragments of 2H3 were able to inhibit completely, in a concentration-dependent manner, ADP-ribosylation of transducin by pertussis toxin. 2H3 had no effect on ADP-ribosylation in the presence of photolyzed rhodopsin. The GTPase activity of transducin, which is dependent on rhodopsin, was inhibited only 50% by 2H3. These data are consistent with the hypothesis that an epitope recognized by 2H3 may be important in the formation of the .alpha.beta.gamma. complex or that interaction of 2H3 with .gamma. may alter conformation of the latter and, thereby, inhibit complex formation. Further, reactions of .gamma. with 2H3 appear to be prevented by interaction with rhodopsin, suggesting that its interaction either shields or alters the epitope recognized by 2H3.