Functional Consequences of Lipid-Mediated Interaction Between Rhodopsin Molecules

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In the retina, rhodopsin is densely packed in a rod outer segment disk membrane rich in phospholipids with PC and PE headgroups. Increasing rhodopsin packing density in model PC membranes has been shown to alter metarhodopsin-II (MII) formation. This observation is deemed to be the consequence of rhodopsin association promoted by non-specific properties of the membrane. Here, we studied the effect of rhodopsin packing density on MII formation in membranes characterized by different intrinsic curvature and different interfacial hydrogen bonding propensity. Rhodopsin was reconstituted into a series of POPC bilayers doped with DOPC, di-and mono-methylated DOPE or DOPE at rhodopsin/lipid ratios ranging from 1:250 to 1:70. The level of rhodopsin activation and rate of MII formation were determined by steady-state and time-resolved UV/vis spectroscopy. In PC membranes, lower rhodopsin concentrations shift the MI/MII equilibrium towards MII and result in a faster rate of MII formation, in agreement with previous findings. On the contrary, in membranes rich in lipids with PE headgroups, the MII concentration is independent of rhodopsin packing density and rates of MII formation, while reduced, show only a slight dependence on rhodopsin crowding. In addition, at low or high protein density, the amount of MII formed depends to a larger extent on the ability of the annular PE headgroups to establish hydrogen bonds with the MII state than on changes in membrane curvature elasticity. These results show clearly that MII formation and interaction between rhodopsin molecules depend strongly on interactions between annular lipids and rhodopsin, highlighting the fundamental role of the first layer of lipids surrounding the protein.