Heptahelical terpsichory. Who calls the tune?

The discovery that arrestins can function as ligand-regulated signaling scaffolds has revealed a previously unappreciated level of complexity in G protein-coupled receptor (GPCR) signal transduction. Because arrestin-bound GPCRs are uncoupled from G proteins, arrestin binding can be viewed as switching receptors between two temporally and spatially distinct signaling modes. Recent work has established two factors that underscore this duality of GPCR signaling and suggest it may ultimately have therapeutic significance. The first is that signaling by receptor-arrestin "signalsomes" does not require heterotrimeric G protein activation. The second is that arrestin-dependent signals can be initiated by pathway-specific "biased agonists," creating the potential for drugs that selectively modulate different aspects of GPCR function. Currently, however, little is known about the physiological relevance of G protein-independent signals at the cellular or whole animal levels, and additional work is needed to determine whether arrestin pathway-selective drugs will find clinical application.