Seeking Ligand Bias: Assessing GPCR Coupling to Beta-Arrestins for Drug Discovery.

G protein-coupled receptors (GPCR) are the major site of action for endogenous hormones and neurotransmitters. Early drug discovery efforts focused on determining whether ligands could engage G protein coupling and subsequently activate or inhibit cognate "second messengers." Gone are those simple days as we now realize that receptors can also couple βarrestins. As we delve into the complexity of ligand-directed signaling and receptosome scaffolds, we are faced with what may seem like endless possibilities triggered by receptor-ligand mediated events.