Guanine-nucleotide binding regulatory proteins as targets for novel drugs

Guanine-nucleotide regulatory binding proteins (G-proteins) serve to transduce information from agonist-bound receptor complexes to either effector enzymes or ion-channels. Drugs that perturb the function of G-proteins may do so by one of four mechanisms. (i) They may exert negative intrinsic activity toward the G-protein. For instance, we have shown that incubation of isolated plasma-membranes with the beta-adrenoceptor blocking drug sotalol blocked both GTP-stimulated and isoprenaline-stimulated adenylyl cyclase. This suggests that the empty beta-adrenoceptor is capable of tonically stimulating G(s-alpha) and therefore adenylyl cyclase; that is, empty beta-adrenoceptors promote GDP-GTP exchange. (ii) They may perturb the GDP-GTP exchange reaction. For instance, certain PDE inhibitors, including SKF 94836 and rolipram, stimulate a marked increase in the pertussis toxin-catalysed NAD+-dependent ADP-ribosylation of G(i-alpha). This effect is similar to that of GDP, which promotes stabilisation of the alpha-beta-gamma holomer of G(i). The effect of these PDE inhibitors is completely reversed by GppNHp, which triggers alpha-beta-gamma dissociation by binding to the guanine-nucleotide binding domain of the G-protein. PDE inhibitors may serve as a class of drugs which perturb GDP-GTP exchange. (iii) They may trigger uncoupling of receptor-G-protein complexes. For instance, the polycationic drug mastoparan binds to the C-terminal end of the G-protein and mimics the effect of receptor activation by promoting GTP-gamma-S binding, a reduction in pertussis toxin-catalysed ADP-ribosylation, and inhibition of adenylyl cyclase activity. Other agents, such as polyanionic drugs, bind to the receptor to promote uncoupling of receptor-mediated activation of certain G-proteins. (iv) They may alter the cross-talk mechanisms that operate between different receptor signalling systems. For instance, protein kinase C promotes the phosphorylation and inactivation of G(i). This leads to an unopposed stimulation of adenylyl cyclase via G(S) and, therefore, enhanced sensitivity to agents such as glucagon. Protein kinase C inhibitors may be usefully exploited to modulate these processes which appear to be abberant in certain disease-states.