Group II activators of G-protein signalling and proteins containing a G-protein regulatory motif.

Beyond the core triad of receptor, Gαßγ and effector, there are multiple accessory proteins that provide alternative modes of signal input and regulatory adaptability to G-protein signalling systems. Such accessory proteins may segregate a signalling complex to microdomains of the cell, regulate the basal activity, efficiency and specificity of signal propagation and/or serve as alternative binding partners for Gα or Gßγ independent of the classical heterotrimeric Gαßγ complex. The latter concept led to the postulate that Gα and Gßγ regulate intracellular events distinct from their role as transducers for cell surface seven-transmembrane span receptors. One general class of such accessory proteins is defined by AGS proteins or activators of G-protein signalling that refer to mammalian cDNAs identified in a specific yeast-based functional screen. The discovery of AGS proteins and related entities revealed a number of unexpected mechanisms for regulation of G-protein signalling systems and expanded functional roles for this important signalling system.

Coupling of beta2-adrenoceptors to XLalphas and Galphas: a new insight into ligand-induced G protein activation.

Galpha(s) and extra-large Galpha(s) (XLalpha(s)) can both transduce receptor activation into intracellular cAMP generation. It is unknown, however, whether these two GNAS-locus products display distinct properties with respect to receptor coupling. Here, we show that XLalpha(s) couples to the beta2-adrenoceptor more efficiently than Galpha(s). In transfected human embryonic kidney 293 cells and mouse embryonic fibroblasts null for both Galpha(s) and XLalpha(s) (2B2 cells), basal cAMP accumulation mediated by XLalpha(s) was higher than that mediated by Galpha(s). Inverse agonist treatment reduced Galpha(s)-mediated basal activity, whereas its effect was markedly blunted on XLalpha(s)-mediated basal activity. Rank order of ligand efficacies regarding cAMP accumulation was the same when the receptor was coupled to XLalpha(s) or Galpha(s). However, ligand-induced and XLalpha(s)-mediated cAMP generation was higher than that mediated by Galpha(s). The relatively high efficiency of XLalpha(s)-mediated cAMP generation was conditional, disappearing with increased level of receptor expression or increased efficacy of ligand. Full-agonist responses in XLalpha(s)- and Galpha(s)-expressing cells were comparable even at low receptor levels, whereas partial agonist responses became comparable only when the receptor expression was increased (>3 pmol/mg). Radioligand binding studies showed that the high-affinity component in agonist binding to beta2-adrenoceptor was more pronounced in cells expressing XLalpha(s) than those expressing Galpha(s). We discuss these findings in the framework of current receptor-G protein activation models and offer an extended ternary complex model that can fully explain our observations.