Formation and decay of the arrestin-rhodopsin complex in native disc membranes [Molecular Biophysics]

April 6th, 2015 by Beyriere, F., Sommer, M. E., Szczepek, M., Bartl, F. J., Hofmann, K. P., Heck, M., Ritter, E.

In the G protein-coupled receptor rhodopsin, light-induced cis/trans isomerization of the retinal ligand triggers a series of distinct receptor states culminating in the active Metarhodopsin II (Meta II) state, which binds and activates the G protein transducin (Gt). Long before Meta II decays into the aporeceptor opsin and free all-trans-retinal, its signaling is quenched by receptor phosphorylation and binding of the protein arrestin-1, which blocks further access of Gt to Meta II. Although recent crystal structures of arrestin indicate how it might look in a pre-complex with the phosphorylated receptor, the transition into the high-affinity complex is not understood. Here we have applied Fourier-Transform-Infrared (FTIR) Spectroscopy to monitor the interaction of arrestin 1 and phosphorylated rhodopsin in the native membrane environment. By isolating the unique infrared signature of arrestin binding, we directly observed the structural alterations in both reaction partners. In the high-affinity complex, rhodopsin adopts a structure similar to Gt-bound Meta II. In arrestin, a modest loss of β-sheet structure indicates an increase in flexibility but is inconsistent with a large-scale structural change. During Meta II decay, the arrestin-rhodopsin stoichiometry shifts from one-to-one to one-to-two. Arrestin stabilizes half the receptor population in a specific Meta II protein conformation while the other half decays to inactive opsin. Altogether these results illustrate the distinct binding modes used by arrestin to interact with different functional forms of the receptor.