Physiological changes in bilayer thickness induced by cholesterol control GPCR rhodopsin function

We monitored the effect on function of the G-protein-coupled receptor (GPCR) rhodopsin from small, stepwise changes in bilayer thickness induced by cholesterol. Over a range of phosphatidylcholine bilayers with hydrophobic thickness from z21 A to 38 A, the metarhodopsin-I (MI)/metarhodopsin-II (MII) equilibrium was monitored with UV-visible spectroscopy while ordering of hydrocarbon chains was probed by 2H-NMR. Addition of cholesterol shifted equilibrium toward MII for bilayers thinner than the average length of hydrophobic transmembrane helices (27 A) and to MI for thicker bilayers, while small bilayer thickness changes within the range of the protein hydrophobic thickness drastically up-or downregulated MII formation. The cholesterol-induced shifts toward MII for thinner membranes correlated with the cholesterol-induced increase of bilayer hydro-phobic thickness measured by NMR, consistent with continuum elastic modeling. The energetic penalty of adding cholesterol to thick bilayers caused rhodopsin oligomerization and a shift toward MI. In membranes of physiological thickness, changes in bilayer mechanical properties induced by cholesterol potentiated the interplay between bilayer and protein thickness resulting in large swings of the MI-MII equilibrium. In membrane containing cholesterol, elastic deformations near the protein are a domi-nant energetic contribution to the functional equilibrium of the model GPCR rhodopsin.

Automated summary

We investigated the influence of cholesterol-induced changes in bilayer thickness on the function of G-protein-coupled receptor (GPCR) rhodopsin. The equilibrium between metarhodopsin-I (MI) and metarhodopsin-II (MII) was monitored using UV-visible spectroscopy, and the ordering of hydrocarbon chains was probed using 2H-NMR. Addition of cholesterol shifted the equilibrium towards MII for thinner bilayers and towards MI for thicker bilayers, and small changes in bilayer thickness within the protein hydrophobic thickness range had drastic effects on MII formation. The interplay between bilayer and protein thickness, induced by cholesterol, resulted in significant swings of the MI-MII equilibrium in membranes of physiological thickness.