Structural Basis of Arrestin Selectivity for Active Phosphorylated G Protein-Coupled Receptors

Arrestins are a small family of proteins that bind G protein-coupled receptors (GPCRs). Arrestin binds to active phosphorylated GPCRs with higher affinity than to all other functional forms of the receptor, including inactive phosphorylated and active unphosphorylated. The selectivity of arrestins suggests that they must have two sensors, which detect receptor-attached phosphates and the active receptor conformation independently. Simultaneous engagement of both sensors enables arrestin transition into a high-affinity receptor-binding state. This transition involves a global conformational rearrangement that brings additional elements of the arrestin molecule, including the middle loop, in contact with a GPCR, thereby stabilizing the complex. Here, we review structural and mutagenesis data that identify these two sensors and additional receptor-binding elements within the arrestin molecule. While most data were obtained with the arrestin-1-rhodopsin pair, the evidence suggests that all arrestins use similar mechanisms to achieve preferential binding to active phosphorylated GPCRs.

Concentration-dependent tetramerization of bovine visual arrestin

The oligomeric states of bovine visual arrestin in solution were studied by small-angle x-ray scattering. The Guinier plot of arrestin at the concentration ranging from 0.4 mg/ml to 11.1 mg/ml was approximated with a straight line, and the apparent molecular weight was evaluated by the concentration-normalized intensity at zero angle (I(0)/conc). Using ovalbumin as a molecular weight standard, it was found that arrestin varied from monomer to tetramer depending on the concentration. The I(0)/conc decreased at high-salt concentration, but was independent of temperature. The simulation analysis of the concentration-dependent increase of I(0)/conc demonstrated that the tetramerization is highly cooperative, and arrestin at the physiological concentration is virtually in the equilibrium between monomer and tetramer. The concentration of arrestin monomer, which is considered to be an active form, remains at an almost constant level even if the total concentration of arrestin fluctuates within the physiological range. The scattering profile of arrestin tetramer in solution was in good agreement with that in the crystal, indicating that the quaternary structure in solution is essentially identical to that in crystal. Small-angle x-ray scattering was applied to a binding assay of phosphorylated rhodopsin and arrestin in the detergent system, and we directly observed their association as the increase of I(0)/conc.