Structure of a cell surface G protein–coupled receptor (green) bound to β-arrestin (purple). G protein–coupled receptors that are in the active state for a long period of time become phosphorylated, triggering binding of an arrestin and inhibition of further signaling by the receptor.

[Data from Y. Kang et al., 2015, Nature 523:561-567, PDB ID 4zwj, and custom PDB.]