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.