GPCRs

Probing the molecular mechanisms of GPCR functional selectivity in live cells

A fundamental unanswered question in GPCR signaling is the role of structural conformations of the GPCR in G protein selection. An emerging view from several studies is that GPCRs are not simple ‘on-off’ switches, but adopt multiple conformations in a ligand-dependent manner. In turn, these ligands elicit diverse functional responses through the activation of distinct G protein heterotrimers or G protein independent effectors such as arrestins. The conformational plasticity of GPCRs addresses the phenomenon of functional selectivity, wherein the same GPCR can elicit diverse ligand-dependent responses. Currently there is no direct method to link ligand-specific GPCR conformations observed in structural studies to differential downstream responses. The laboratory has used protein engineering to develop a GPCR conformational biosensor (Malik et al., JBC 2013) that can detect G protein-specific conformations.  We have used this technology to demonstrate a Gi-specific conformation in the β2-adrenergic receptor induced by the β-blocker metoprolol (Fig. 1). Currently several laboratories are using our sensors to profile the functional selectivity of a range of GPCRs.

We have also used protein engineering to map the rules of the GPCR-G protein interaction. This study has yielded surprising insights into the affinities of GPCRs for G proteins, and the functional consequences of these interaction profiles in downstream responses. Our findings provide a strong foundation for GPCR-G protein pairing, and in due process have yielded a simple yet powerful genetically encoded technique to ‘tune’ G protein selection in live cells. Current efforts are focused on using this technique to (a) link G protein selection to physiological responses in cells and model organisms; and (b) define a quantitative ‘bias factor’ for ligands that is independent of the cell type being examined.