Lateral diffusion of visual pigment in photorecptor disk membranes

Cone lamellae and red and green rod outer segment disks contain a large intrinsic membrane protein on their margins: an ultrastructural immunocytochemical study of frog retinas

In addition to rhodopsin, the disk membranes of rod outer segments (ROS) contain a large integral membrane protein (mol. wt 290,000). This protein was previously localized by immunocytochemistry to the margins and incisures of disks in frog red ROS by specific antibody applied to thin sections of bovine serum albumin embedded retinas (Papermaster et al., 1978b, J. Cell Biol. 78, 415-425). Upon further study of the reactions of this antibody with outer segments of other photoreceptor classes in frog retina, labeling of the short incisures and margins of green ROS and margins of cone outer segment lamellae is also observed. Thus the large protein participates in the structure of the edges of disks and lamellae of all photoreceptors in the frog. In addition, labeling of the inter-incisure surface of all photoreceptor classes was observed at high antibody concentration. In order to interpret this labeling, the effect of dilution on labeling density was determined and double reciprocal plots (Markham and Benton, 1931, J. Am. Chem. Soc. 53, 497) were employed to evaluate the relative affinity and heterogeneity. There was considerable deviation from linearity in the plots of labeling disk interiors compared to the relatively linear plots of disk incisure labeling which suggests that the interior sites contain a weakly cross-reacting antigen or that the serum contains a lower concentration of antibody weakly reactive with another antigen.

The lack of relationship between fluorescence polarization and lateral diffusion in biological membranes

An investigation has been carried out of the relationship between changes in the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and concomitant changes in the lateral diffusion of proteins and lipid probes in membranes. Plasma membranes from lymphocytes and a CH1 mouse lymphoma line were treated with up to 70 mol% (relative to the total membrane phospholipid) of oleic or linoleic fatty acids. Under these conditions the fluorescence polarization of DPH decreased by between 8 and 15% which, in the framework of the microviscosity approach, suggests a membrane fluidity change of between 20 and 50%. The lateral diffusion coefficients of surface immunoglobin and the lipid probes 3,3'-dioctadecylindocarbocyanine and pyrene were also measured in these membranes using the fluorescence photobleaching recovery technique and the rate of pyrene excimer formation. The diffusion rates were found to be unaffected by the presence of free fatty acids. Hence despite large 'microviscosity' changes as reported by depolarization of DPH fluorescence, lateral diffusion coefficients are essentially unchanged. This finding is consistent with the idea that perturbing agents such as free fatty acids do not cause a general fluidization of the membrane but act locally to alter, for example, protein function. It is also consistent with the suggestion that lateral mobility of membrane proteins is not modulated by the lipid viscosity.

Electric and optical anisotropy and their osmotically induced changes of photoreceptor disk membrane vesicles

Electro-optical characterization of the photoreceptor disk membrane vesicle is performed by examining the electric field and concentration dependence of the study-state birefringence of aqueous suspensions of the vesicles. The electric polarizability anisotropy is found to be negative and of large magnitude: alpha 1 - alpha 2 = -(1-3) X 10 cm3. The optical anisotropy is determined to be also negative but of small magnitude: g 1 - g 2 = -1 X 10(-7). The specific Kerr constant deduced from the concentration dependence of the Kerr constant is found to be very large: Ksp = 7 X 10(-4) e.s.u. Upon deforming the vesicles osmotically from the spherical shell to the disk structure, the steady-state birefringence increases by an order of magnitude which is attributed solely to the increase in optical anisotropy attending the corresponding change in the geometric eccentricity of the vesicle. A plausible birefringence mechanism based on the known structural features of the vesicles is proposed, which would account for these findings.

Rapid calcium release and proton uptake at the disk membrane of isolated cattle rod outer segments. 1. Stoichiometry of light-stimulated calcium release and proton uptake

We reported a rapid, light-stimulated release of calcium from isolated rod outer segments that is apparent only when both the disk membrane and the plasma membrane are made permeable to calcium by adding the ionophore A23187 [Kaupp, U. B., Schnetkamp, P. P. M., & Junge, W. (1979) Biochim. Biophys. Acta 552, 390-403]. In this paper, we have investigated the light-sensitive diskal binding sites and the calcium release mechanism in their dependence on the pH and the presence of mono- and divalent cations, including calcium itself. We have observed now that several different rod outer segment preparations (i.e., rod outer segments with an intact plasma membrane, broken cells, and sonicated material) possess a similar dependence of their calcium release on the ionic conditions, however, only if manipulated in a way that gives access to the outer conditions of sites within disks (namely, ionophore added in the case of intact rod outer segments). Monovalent cations, at concentrations between 20 and 40 mM, suppress light-induced calcium release. Divalent and trivalent cations are more efficient inhibitors by 1-2 and 2-3 orders of magnitude, respectively. These results suggest that calcium release is controlled by an interfacial potential. The optimum pH for calcium release is pH 6.3, and virtually no release occurs beyond pH 4.5 and 9. The drop for acidic pH is attributed to the pH dependence of calcium binding to disk membranes, and the drop for alkaline pH is attributed to the pH dependence of the metarhodopsin I/metarhodopsin II transition and the light-stimulated proton uptake. In general, calcium release parallels calcium binding as a function of pH and calcium concentrations, although the release saturates at lower calcium concentrations ((KDapp = 5 microM) than would be expected from the amount of calcium bound (KD = 30-60 microM). The maximum stoichiometry is approximately 1 mol of calcium release per mol of rhodopsin bleached. Concomitant measurements of the light-stimulated uptake of protons by the disk membrane revealed a maximal stoichiometry of 2.8 mol of protons taken up per mol of rhodopsin bleached. We present an integrated description of light-stimulated calcium release, proton uptake, and changes of the interfacial potential at the disk membrane.

Matrix control of protein diffusion in biological membranes

Lateral diffusion coefficients of fluorescently labeled lipids and integral membrane proteins were determined in the membranes of normal and spectrin-deficient spherocytic mouse erythrocytes by the technique of fluorescence redistribution after photobleaching. The results were used to generate a mathematical description of a matrix-control model of membrane protein diffusion. In the spherocytic cells, which lack the principal components of the cytoskeletal matrix of normal cells, the diffusion coefficients of lipid (1.5 +/- 0.5 X 10(-8) cm2/s) and protein (2.5 +/- 0.6 X 10(-9) cm2/s) differ only by a factor of 6, close to the difference predicted on the basis of size by the two-dimensional bilayer continuum model of Saffman and Delbrück [Saffman, P. G. l& Delbrück, M. (1975) Proc. Natl. Acad. Sci. USA 72, 3111-3113]. In contrast, the membranes of normal cells show a lipid diffusion coefficient (1.4 +/- 0.5 X 10(-8) cm2/s) that is some 300-fold greater than that of the membrane proteins (4.5 +/- 0.8 X 10(-11) cm2/s). Analysis of these results, based on the hypothesis that protein diffusion in normal membranes is sterically hindered by a labile matrix, yields an effective matrix surface viscosity consistent with the viscoelastic mechanical properties of the membranes. Thus, a relationship is established between the deformation characteristics of the membrane and the lateral mobility of proteins suspended in the membrane.

Transbilayer distribution of phospholipid fatty acyl chains in photoreceptor membrane

The transverse distribution of the fatty acyl chains of the major phospholipids over the two faces of the photoreceptor membranes has been determined in bovine rod outer segment (stacked disk) preparations. For this purpose, the fatty acid composition of the phospholipids has been analyzed before and after treatment with trinitrobenzenesulfonic acid and phospholipase D. The latter agents are used under conditions in which they have been demonstrated to attack only the outer (cytoplasmic) face of the membrane. After treatment with trinitrobenzenesulfonic acid or phospholipase D, the fatty acid composition of the unreacted phospholipids is the same as that before treatment, regardless of the extent of modification or hydrolysis attained. The fatty acid composition of phosphatidic acid, resulting from phospholipase D action, also remains unchanged during progressive hydrolysis. These results indicate that the fatty acyl chains of the major phospholipids have the same composition on either side of the disk membrane. Together with our previously published evidence for the distribution of the major phospholipids in rod outer segment disk membranes, this means that both the phospholipids and their fatty acyl chains have a remarkably symmetrical distribution over the two membrane faces. On the basis of literature data it is concluded that this approximate symmetry reflects the high mobility of the entire phospholipid pool of disk membranes, thus including appreciable transbilayer movements of the phospholipids.

Dynamic kerr effect measurements on photoreceptor disk membrane vesicles

Dynamic Kerr effect measurements were performed with dilute aqueous suspensions of monodisperse spherical vesicles (approximately 1 micron diameter), isolated from the rod outer segment of bovine retina. A large birefringence, amounting to the specific Kerr constant of 10(-3) esu, can be observed. When a sufficiently long duration pulse (1 s) is applied, the decay of birefringence can be represented by a single exponential profile, yielding a relaxation time of 100 +/- 20 ms in 1 mM imidazole buffer. This is consistent with the rotatory relaxation time of these spherical membrane vesicles. When a short duration is applied, the birefringence increases more steeply and the decay profile contains several components. The slowest (terminal) relaxation time is 86 +/- 15 ms is due to the same process as the one observed in the slow pulse case.

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.

[3H]ouabain autoradiography of frog retina

The kinetics and distribution of ouabain binding in retinas of Rana pipiens were examined quantitatively by scintillation counting and freeze-dry autoradiography. The time-course of binding at several concentrations was consistent with a bimolecular reaction. Estimated equilibrium binding levels gave a Michaelis-Menton relationship with a Km = 8.3 x 10(-8) M and a maximum binding level (Bmax) = 4.4 x 10(-8) mol/g protein. The distribution of binding sites measured autoradiographically varied considerably between layers. The photoreceptor, inner plexiform, and optic nerve fiber layers exhibited the heaviest binding. Within the photoreceptor layer, binding was nonuniform. Binding in the outer segment decreased distally, averaging approximately 4% of that in the proximal receptor layers (Bmax = 4.6 x 10(-6) M). The origin of the outer segment activity is uncertain at light microscope resolution, as it may be a result of inner segment calyceal processes. Binding within the proximal receptor layers was also nonuniform. Several peaks were observed, with those at the inner segment and synaptic layers being especially noticeable. Assuming an absence of glial cell binding in the proximal receptor layers, we calculated there to be 13 x 10(6) ouabain or Na+,K+ pump sites per rod receptor. Limited measurements suggest a Bmax of approximately 8 x 10(-6) M for the inner plexiform layer.

Effect of bleaching light on measurements of lateral diffusion in cell membranes by the fluorescence photobleaching recovery method

The Fc receptors for IgE on rat basophil leukemia 2H3 cells were labeled with either rhodamine- or fluorescein-conjugated IgE and then with Fab fragments of anti-IgE conjugated with the other dye. Fluorescence photobleaching recovery measurements of lateral diffusion were performed with one of the dyes before and after extensive bleaching of the other dye over an entire cell. Bleaching one dye did not affect subsequent measurements made with the other dye. We thus detect no evidence for photoinduced artifacts in fluorescence photobleaching recovery experiments.

Lateral diffusion in biological membranes. A normal-mode analysis of diffusion on a spherical surface

A new approach is described for the analysis of lateral diffusion in biological membranes. It is shown that a suitably defined first moment of the concentration distribution on a spherical surface decays as a single exponential with a relaxation rate proportional to the diffusion coefficient and inversely proportional to the square of the radius of the sphere. The approach is illustrated with an example of fluorescence redistribution after photobleaching of membrane proteins in a spectrin-deficient spherocytic mouse erythrocyte membrane.

Absorption of light in photoreceptors: effect of waveguiding property

The effect of waveguiding property (i.e., the intensity distribution) of the photoreceptor on the number of photons absorbed in a photoreceptor has been studied. It has been found that the effect is significant only for large values of the exposure and the maximum effect is less than 11% in the case of human rod photoreceptor. In the analysis, the funnelling effect, which follows from the coupling between the interior and exterior fields, has not been considered.

The carbohydrates in frog retinal rod outer segments

Frog retinal rod outer segments appear to contain uncharacterized chemical components whose mass is roughly equivalent to 12--51% of the rhodopsin mass. Available data suggest that such components include soluble proteins and complex polysaccharides, and that hyaluronic acid accounts for a substantial fraction of this mass. Electron microscopic histochemical staining studies suggest that these polysaccharide components are located within the ROS disks. The oligosaccharide moieties of rhodopsin also appear localized within the disks. The interdisk cytoplasm may contain carbohydrates, but their quantity and identity are uncertain. Rhodopsin oligosaccharides as well as some fraction of the intradisk polysaccharide appear to have extended saccharide chains preferentially oriented perpendicular to the surface of the disk membrane. Possible roles for these polysaccharides in disk development and photoexcitation are discussed. The immediate need for complete rod outer segment chemical composition data is emphasized.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.

Linear dichroism of rhodopsin in air-water interface films

Air-water interface films of purified cattle rhodopsin and defined phospholipids are formed by the osmotic lysis of reconstituted membrane vesicles. The interface films thus formed consist of a phospholipid monolayer containing vesicle membrane fragments. Rhodopsin molecules at the interface are restricted within the membrane fragments where they are spectrophotometrically intact and capable of undergoing photoregeneration and chemical regeneration. Multilayers of up to 8 layers can be built from these interface films. The visible absorption band of rhodopsin in these multilayers is linearly dichroic. Quantitative analysis of the linear dichroism reveals that the dipole moment of transition of the retinal chromophore in rhodopsin forms an angle of 15 degrees +/- 4 degrees with the plane of the membrane fragments in the interface film. This orientation of the chromophore relative to the plane of the membrane is essentially the same as that observed in the intact retina. Thus, the orientation of rhodopsin in the interface films is similar to that in the intact disc membranes.

On two-dimensional passive random walk in lipid bilayers and fluid pathways in biomembranes

The lateral mobility of pyrene, pyrene decanoic acid, and 1-palmitoyl-2-pyrene decanoyl-phosphatidyl choline (pyrene lecithin) in lipid bilayers is determined by the excimer formation technique. This method is applied to vesicles of lecithins differing in chain length and in the degree of saturation of the hydrocarbon chains. These values are compared with results in cephalins of different chain length and in dipalmitoyl phosphatidic acid at variable pH. The influence of cholesterol is investigated. The results are analyzed in terms of the Montroll model of two-dimensional random walk. The jump frequency of the probe molecule within the lipid lattice is obtained. The advantage of this measure of transport in lipid layers is that it does not involve lipid lattice parameters. The main results of the present work are: (i) The lateral mobility of a given solute molecule in lamellae of saturated lecithins is independent of hydrocarbon chain length and rather a universal function of temperature. (ii) In unsaturated dioleyl lecithin the amphiphatic molecules have lateral mobilities of the same size as in saturated lipids. The jump frequency of pyrene, however, is by a factor of two larger in the unsaturated lecithin. (iii) The jump frequencies in phosphatidyl ethanolamines are about equal to those in lecithins. (iv) In phosphatidic acid layers the hopping frequencies depend on the charges of the head groups of both the lipids and the probes. (v) Cholesterol strongly reduces the jump frequency in fluid layers. (vi) The lateral mobility in biological membranes is comparable to that in artificial lipid bilayers. The experimental results are discussed in terms of the free volume model of diffusion in fluids. Good agreement with the predictions made from this model is found. A striking result is the observation of a tilt in dioleyl-lecithin bilayer membranes from the hopping frequencies of pyrene and pyrene lecithin. A tilt angle of phi = 17 degrees is estimated.

Upper limit on translational diffusion of visual pigment in intact unfixed barnacle photoreceptors

Translational diffusion of pigment molecules in the disc membranes of amphibian rod outer segments is in the range of 10 microm/10 s. Recently, Goldsmith and Wehner set an upper limit of 10 microm/20 min to the diffusion in isolated formaldehyde-fixed rhabdoms of crayfish. We have now used the early receptor potential (ERP) to study the diffusion in intact, unfixed barnacle photoreceptors. The ERP from a cell fully adapted to blue light (most of the pigment in the rhodopsin state) was changed by 8-22% of its maximum change when the pigment in a 30 microm spot was (almost) completely shifted to the metarhodopsin state by red laser adaptation. Further red illumination of the same spot 30 min later produced only a limited further change in the ERP (attributable to light scatter), showing that R had not migrated into the spot. It is concluded that the visual pigment diffuses by less than 30 microm/30 min.

Immunocytochemical localization of a large intrinsic membrane protein to the incisures and margins of frog rod outer segment disks

Immunocytochemical techniques have localized a large protein which is an intrinsic membrane component of isolated frog rod outer segments (ROS). This large protein whose apparent mol wt is 290,000 daltons comprises about 1--3% of the ROS membrane mass. Its molar ratio to opsin is between 1:300 and 1:900. Adequate immune responses were obtained with less than 30 microgram (100 pmol) of antigen per rabbit. Antibodies to the large protein were used for its localization on thin sections of frog retina embedded in glutaraldehyde cross-linked bovine serum albumin (BSA). Specifically bound antibodies were detected by an indirect sequence with ferritin-conjugated antibodies. This technique detected the protein which is represented by 1,000--3,000 molecules per disk. This indicates that the procedure is sufficiently sensitive for analysis of membrane components in low molar proportions. The large protein was specifically localized to the incisures of ROS disks which divide the disks into lobes and to the disk margin. Thus, opsin is mobile within the membrane of the disk while the large protein is apparently constrained to the disk edges. This finding raises the possibility that special functions are also localized ot his unusual region of high curvature, and that collisions of bleached opsin with these edges are physiologically important in couter segment function.

What does Halobacterium tell us about photoreception?

A photosensory mechanism is proposed for Halobacterium halobium based on the observation of light-induced motor responses. Possible mechanisms of signal transduction in Halobacterium are discussed. Bacteriorhodopsin and the visual pigment rhodopsin are compared with respect to their structural and functional properties. The conclusion is drawn that Halobacterium may help to understand primary photochemical events of rhodopsin rather than the transduction mechanism of visual photoreceptors.

Calorimetric and freeze fracture analysis of lipid phase transitions and lateral translational motion of intramembrane particles in mitochondrial membranes

Differential scanning calorimetry combined with freeze fracture electron microscopy reveals that thermotropic lipid phase transitions and lateral translational motion of intramembrane particles occur in both membranes of whole, intact rat liver mitochondria and in isolated inner and outer membranes. The onset temperature of the liquid crystalline to gel state lipid phase transition in whole mitochondria and in the isolated outer membrane fraction is biphasic with an initial transition exotherm occurring at 9 degrees C. The onset temperature of the transition exotherm of the isolated inner membrane occurs at -4 degrees C. The onset temperature of the lipid transition endotherm is -15 degrees C for whole mitochondria, the inner membrane, ane the outer membrane fractions. These calorimetric analyses reveal that the bilayer lipid in the inner, energy transducing membrane is more fluid than in the outer membrane. Mitochondrial membranes cooled to temperatures in the region of their transition exotherms and then frozen reveal striking lateral separations between smooth, intramembrane particle-free regions (rich in gel state lipid) and particle-dense regions (rich in integral proteins) in their hydrophobic fracture faces. Such thermotropic lipid-protein lateral separations are completely reversible. These freeze fracture observations suggest that both mitochondrial membranes are naturally fluid to the extent that the integrat membrane proteins can diffuse laterally in the bilayer lipid.