The presence of two major protein components in the bovine photoreceptor disc membrane

Lipid fluidity of the outer segment membranes from cephalopod retina

Membranes from the retina outer segments of squid (Loligo sp.) were isolated by sucrose continuous gradient. In these membranes, phosphatidylcholine and phosphatidylethanolamine were shown to be the most abundant lipids, while rhodopsin was the predominant protein. Steady-state fluorescence depolarization of the probe 1,6 diphenyl-1,3,5-hexatriene was employed to characterize the microviscosity of the photosensitive membranes and of the vesicles prepared from their lipid extract. The lipid fluidity parameter (r0/r-1)-1 in these membranes ranged between approximately 3.8 and approximately 0.95 (0-38 degrees C), while the value of the lipid extract vesicles ranged between approximately 3 and approximately 0.6. No phase transition was observed in the membrane and vesicle preparation in the range of temperatures under study [0-38 degrees C). Photosensitive membranes from cuttlefish (sepia officinalis) gave similar results. The (r0/r-1)-1 value in this species varied between approximately 4 and approximately 1.2 (0-38 degrees C) with a linear dependence on temperature. In these outer segments the bleaching of rhodopsin did not cause any change of the membrane lipid fluidity value. The present results are compared with those obtained in bovine rod outer segments.

Light-enhanced cross-linking of rhodopsin in rod outer segment membranes as detected by chemical probes

Bovine rod outer segment membranes were treated with cross-linking reagents before and after light exposure. Bleached membranes showed enhanced cross-linking with difluorodinitrobenzene or methyl acetimidate compared to dark-adapted membranes. The light-induced enhancement of cross-linking may be due to increased association of rhodopsin monomers in the light and/or due to increased reactivity of amino and sulfhydryl groups of bleached rhodopsin. In some instances, the band ascribed to the rhodopsin monomer in gel electrophoresis appears as a partially resolved doublet. Treatment of bleached rod outer segment membranes with methyl acetimidate improved the resolution of the doublet into two closely migrating bands.

A light scattering study on the ion permeabilities of dark-adapted bovine rod outer segment disk membranes

The ion permeability properties of dark adapted bovine rod outer segment disk membranes were studied using light scattering to monitor osmotic responses of disks to various salts and ionophores. A preparation procedure is presented which provides very fresh rod outer segment material with mostly intact stacked disks, but with perforated plasma membrane. It is shown that in this preparation the disks (or rod sacs) are the only osmotically responding compartments and that these responses can be readily monitored by means of light-scattering techniques. The disk membrane is found under the conditions tested, to possess no measurable permeability to cations Na+, Ca2+, Mg2+ nor the the anions Cl-, Br-, NO3-, SO4(2-), H2PO4- and HPO4(2-). There is a considerable K+ permeability, which can be completely abolished by millimolar amounts of divalent cations. The proton permeability of the disk membrane is found to depend dramatically upon the preparation procedure and duration. The fresher the material used the lower is the proton permeability measured. In our freshest preparations, even after freeze-thawing in liquid nitrogen, the disks exhibit an H+ permeability which is so low that it cannot be measured with the techniques used in this study. Even in mitochondrial or chloroplast membmranes, in which proton gradients and therefore a low proton conductance play an essential role, such low proton permeabilities have not been found. This would suggest that proton gradients across the disk membrane could play an important role in the physiological function of the photoreceptor cell. In summary it can be said that the disk membrane, apparently more than any other natural membrane system studied so far, is capable of retaining ion gradients for extended periods of time.