A possible role of rhodopsin in maintaining bilayer structure in the photoreceptor membrane

Abstract

Summary31p-NMR measurements demonstrate that at 37 ° C, independent of the photolytic state of the photopigment rhodopsin, the lipids in the photoreceptormembrane are almost exclusively organised in a bilayer. In strong contrast, the 31P-NMR spectra of the extracted lipids are characteristic for the hexagonal HII phase and an isotropic phase. The isotropic phase is characterised by freeze-fracture electron microscopy as particles and pits on smooth surfaces, possibly indicating inverted micelles. These results suggest a structural role for rhodopsin in maintaining the photoreceptor membrane lipids in a bilayer configuration.It has recently been demonstrated that the lipid bilayer, while still the basic feature of biological membranes, is not the only conformational state in which the lipids in membranes can occur. 31P-NMR studies have presented strong evidence that in the endoplasmic reticulum of rat, bovine and rabbit liver the phospholipids can undergo isotropic motion, which suggests that inverted micellar structures may be present in conjunction with the bilayer [ 1 ]. Model studies of water-phospholipid mixtures show that the preferred phase depends on the type of lipid [ 2--6 ]. Under most conditions the thermodynamically most stable phase of phosphatidylcholine is the bilayer. Unsaturated phosphatidylethanolamine also prefers the bilayer conformation at lower temperatures, but at increasing temperature it undergoes a phase Abbreviation: Mops, 3-(N-morpholino)propanesulfonic acid.

Barrier characteristics of membrane model systems containing unsaturated phosphatidylethanolamines

“…branes as comparable 3~P-NMR spectra and freeze-fracture morphology were observed in the total lipid extracts of rod outer segment [9] and Escherichia coli membranes [10]. Furthermore 3~p-NMR spectra of microsomes at 37"C indicate isotropic motion of part of the membrane phospholipids [11,19].…”

Structural properties of phospholipids in the rat liver inner mitochondrial membrane. A 31P-NMR study

“…It is therefore certainly possible that some proteins play structural bilayer~tabflizing roles, as has been suggested for rod outer segment membranes [26]. This possibility is reinforced by recent studies [27] indicating an asymmetric distribution of phospholipids across the inner mitochondrial membrane, with an enrichment of phosphatidylethanolamine and cardiolipin in the inner monolayer.…”

Differential scanning calorimetry of bovine rhodopsin in rod‐outer‐segment disk membranes

“…It is also possible that this is due to an indirect effect of the presence of rhodopsin or opsin on the lipid membrane matrix. It has previously been shown that rhodopsin and opsin stabilize the lamellar structure of the rod outer segment disk membrane (de Grip et al, 1979;Albert et al, 1984) and this could affect the denaturation of neighboring opsins.…”

“…The thermal stabilities of membrane-bound and detergentsolubilized rhodopsin and opsin have been investigated previously (Hubbard, 1958;de Grip, 1982;Fong et al, 1982). Hubbard (1958) showed membrane-bound rhodopsin to be considerably more stable than opsin by determining the amount of rhodopsin (or opsin) that survived heating for 10 min at various fixed temperatures.…”