A New Look at Receptor-Mediated Activation of a G-protein

Abstract

The intimate kinetic processes of hormonal activation of a G-proteinmediated pathway are complex and involve the interplay of a receptor with the G-protein, the activation of the G-proteins by GTP, the G-protein subunit dissociation reaction, the turnoff of the activated α subunit by its endogenous GTPase, the reassociation of the G-protein subunits, and the reversible and presumably cyclical oscillation of the receptor between a minimum of two states — one with low and the other with high affinity for the hormone. Work by Lefkowitz’s group in the late 1970s provided solid evidence that for the β-adrenergic receptor (βAR), the activation-triggering state of the receptor is the state with high agonist affinity or R_H (DeLean et al. 1980; Kent et al. 1980). Work by Cassel and Selinger (1978) and later that by Michel and Lefkowitz (1982) showed that agonist stimulation accelerates an exchange reaction in which bound nucleotide, be it GDP- or GTP-like, is rapidly released from the system in an agonist-specific manner. Facilitation of guanine nucleotide binding by a receptor was confirmed with purified components in the mid 1980s (Cerione et al. 1985, 1986). This led to the now widely held concept that one role, and very likely the sole role, of the receptor is to promote nucleotide exchange at the GDP-GTP binding site, with the implicit assumption that the automatic fate of a G-protein with GTP bound to it is its activation (“self-activation”), and that consequently the receptor is not required for the activation reaction proper.