G protein-coupled receptors (GPCRs) are versatile signaling proteins that mediate complex cellular responses to hormones and neurotransmitters. Recent advances in GPCR crystallography have provided inactive and active state structures for rhodopsin and the β2 adrenergic receptor (β2AR). Although these structures suggest a two-state 'on-off' mechanism of receptor activation, other biophysical studies and observed signaling versatility suggest that GPCRs are highly dynamic and exist in a multitude of functionally distinct conformations. To fully understand how GPCRs work, we must characterize these conformations and determine how ligands affect their energetics and rates of interconversion. This brief review will compare and contrast the dynamic properties of rhodopsin and β2AR that shed light on the role of structural dynamics in their distinct signaling behaviors.
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