The disk membrane as model for an excitable membrane

Excitable membranes appear to be biochemically characterized by the presence of a highly specific receptor protein, which undergoes conformational changes upon stimulation and returns to its original conformation for renewed excitability. Our present knowledge of the disk membrane and its receptor protein rhodopsin is reviewed in the light of this model. The influence of the membrane environment on rhodopsin has been determined in bovine rod outer segment membranes, in rhodopsin preparations which are depleted of the membrane phospholipids, and in reconstituted phospholipid-rhodopsin vesicles. Except for the absorbance spectrum in the visual region, all properties studied change upon lipid depletion. This process starts when 30% of the phospholipids have been removed. Upon reincorporation of lipid-free rhodopsin into lipid bilayer membranes, all properties are restored to the values observed in the native membranes. Hence, removal of the membrane causes deficiencies in the rhodopsin molecule, which are reversible under appropriate conditions. With respect to the function of rhodopsin in visual excitation, the photolytic sequence seems to be the most relevant parameter presently available. Experimental evidence indicates a partial unfolding of the protein upon illumination, culminating at the metarhodopsin I to II transition, followed by a refolding, which is largely completed in the free opsin stage. While the unfolding phase of the photolytic process apparently is independent of the microenvironment of the rhodopsin molecule, the refolding phase seems to require embedding in a partially hydrophobic environment which allows sufficient thermal motion, e.g. a semifluid biomembrane.