Preparation of osmotically intact rod outer segment disks by Ficoll flotation

Maximizing the Rod Outer Segment Yield in Retinas Extracted from Cattle Eyes

The retina is a thin neuronal multilayer responsible for the detection of visual information. The first step in visual transduction occurs in the photoreceptor outer segment. The studies on photoreception and visual biochemistry have often utilized rod outer segments (OS) or OS disks purified from mammalian eyes. Literature reports several OS and disk purification procedures that rarely specify the procedure utilized to collect the retina from the eye. Some reports suggest the use of scissors, while others do not mention the issue as they declare to utilize frozen retinas. Because the OS are deeply embedded in the retinal pigmented epithelium (RPE), the detachment of the retina by a harsh pull-out can cause the fracture of the photoreceptor cilium. Here, we present a protocol maximizing OS yield. Eye semi-cups, obtained by hemisecting the eyeball and discarding the anterior chamber structures and the vitreous, are filled with Mammalian Ringer. After 10-15 min of incubation, the retinas spontaneously detach with their wealth of OS almost intact. The impressive ability of the present protocol to minimize the number of OS stuck inside the RPE, and therefore lost, compared with the classic procedure, is shown by confocal laser scanning microscopy analysis of samples stained ex vivo with a dye (MitoTracker deep red) that stains both retinal mitochondria and OS. Total protein assay of OS disks purified by either procedure also shows a 300% total protein yield improvement. The advantage of the protocol presented is its higher yield of photoreceptor OS for subsequent purification procedures, while maintaining the physiological features of the retina.

Differential expression of the five redox complexes in the retinal mitochondria or rod outer segment disks is consistent with their different functionality

PURPOSE

The retinal rod outer segment (OS) disk membranes, devoid of mitochondria, conducts oxidative phosphorylation (OxPhos). This study aimed at identifying which proteins expressed in the retinal rod OS disks determined the considerable adenosine-5'-triphosphate production and oxygen consumption observed in comparison with retinal mitochondria.

PROCEDURES

Characterization was conducted by immunogold transmission electron microscopy on retinal sections. OxPhos was studied by oximetry and luminometry. The proteomes of OS disks and mitochondria purified from bovine retinas were studied by mass spectrometry. Statistical and bioinformatic analyses were conducted by univariate, multivariate, and machine learning methods.

RESULTS

Weighted gene coexpression network analysis identified two protein expression profile modules functionally associated with either retinal mitochondria or disk samples, in function of a strikingly different ability of each sample to utilized diverse substrate for F1Fo-ATP synthase. The OS disk proteins correlated better than mitochondria with the tricarboxylic acids cycle and OxPhos proteins.

CONCLUSIONS

The differential enrichment of the expression profile of the OxPhos proteins in the disks versus mitochondria suggests that these proteins may represent a true proteome component of the former, with different functionality. These findings may shed new light on the pathogenesis of rod-driven retinal degenerative diseases.

Proteome of Bovine Mitochondria and Rod Outer Segment Disks: Commonalities and Differences

The retinal rod outer segment (OS) is a stack of disks surrounded by the plasma membrane, housing proteins related to phototransduction, as well as mitochondrial proteins involved in oxidative phosphorylation (OxPhos). This prompted us to compare the proteome of bovine OS disks and mitochondria to assess the significant top gene signatures of each sample. The two proteomes, obtained by LTQ-Orbitrap Velos mass spectrometry, were compared by statistical analyses. In total, 4139 proteins were identified, 2045 of which overlapping in the two sets. Nonhierarchical Spearman's correlogram revealed that the groups were clearly discriminated. Partial least square discriminant plus support vector machine analysis identified the major discriminative proteins, implied in phototransduction and lipid metabolism, respectively. Gene Ontology analysis identified top gene signatures of the disk proteome, enriched in vesiculation, glycolysis, and OxPhos proteins. The tricarboxylic acid cycle and the electron transport proteins were similarly enriched in the two samples, but the latter was up regulated in disks. Data suggest that the mitochondrial OxPhos proteins may represent a true OS proteome component, outside the mitochondrion. This knowledge may help the scientific community in the further studies of retinal physiology and pathology.

Functional expression of oxidative phosphorylation proteins in the rod outer segment disc

The rod Outer Segment (OS) disc, an organelle devoid of mitochondria, is specialized in phototransduction, a process requiring a continual chemical energy supply. We have shown that OS discs express functional mitochondrial electron transport chains, Fo F1 -ATP synthase and the tricarboxylic acid cycle enzymes, all mitochondrial features. Here, we focus on oxygen consumption and adenosine triphosphate (ATP) synthesis by OS discs analysing electron transport chain I-III-IV and II-II-IV pathways, supported by reduced nicotinamide adenine dinucleotide and succinate, respectively. Interestingly, respiratory capacity of discs was measurable also in the presence of 3-hydroxy-butyrrate, a typical metabolic substrate for the brain. Data were supported by a two-dimensional electrophoresis analyses conducted as our previous one, but focused to those mitochondrial proteins that are involved in oxidative phosphorylation. Carbonic anhydrase was also found active in OS discs. Moreover, colocalization of Rhodopsin with respiratory complex I and ATP synthase seems a further step in the characterization of some proteins typical of the mitochondrial inner membranes that are expressed in the rod discs. The existence of oxygen utilization in the outer retina, likely supplying ATP for phototransduction, may shed light on some retinal pathologies related to oxidative stress of the outer retina.

Quantifying the differential effects of DHA and DPA on the early events in visual signal transduction

A range of evidence from animal, clinical and epidemiological studies indicates that highly polyunsaturated acyl chains play important roles in development, cognition, vision and other aspects of neurological function. In a number of these studies n3 polyunsaturated fatty acids (PUFAs) appear to be more efficacious than n6 PUFAs. In a previous study of retinal rod outer segments obtained from rats raised on either an n3 adequate or deficient diet, we demonstrated that the replacement of 22:6n3 by 22:5n6 in the n3 deficient rats led to functional deficits in each step in the visual signaling process (Niu et al., 2004). In this study, we examined rhodopsin and phosphodiesterase function and acyl chain packing properties in membranes consisting of phosphatidylcholines with sn-1=18:0, and sn-2=22:6n3, 22:5n6, or 22:5n3 in order to determine if differences in function are due to the loss of one double bond or due to differences in double bond location. At 37 °C the n6 lipid shifted the equilibrium between the active metarhodopsin II (MII) state and inactive metarhodopsin I (MI) state towards MI. In addition, 22:5n6 reduced the rates of MII formation and MII-transducin complex formation by 2- and 6-fold, respectively. At a physiologically relevant level of rhodopsin light stimulation, the activity of phosphodiesterase was reduced by 50% in the 22:5n6 membrane, relative to either of the n3 membranes. Activity levels in the two n3 membranes were essentially identical. Ensemble acyl chain order was assessed with time-resolved fluorescence measurements of the membrane probe diphenylhexatriene (DPH). Analysis in terms of the orientational distribution of DPH showed that acyl chain packing in the two n3 membranes is quite similar, while in the 22:5n6 membrane there was considerably less packing disorder in the bilayer midplane. These results demonstrate that the n3 bond configuration uniquely optimizes the early steps in signaling via a mechanism which may involve acyl chain packing deep in the bilayer.

Conservation and divergence of Grb7 family of Ras-binding domains

Ras proteins are signal-transducing GTPases that cycle between inactive GDP-bound and active GTP-bound forms. Ras is a prolific signaling molecule interacting with a spectrum of effector molecules and acting through more than one signaling pathway. The Ras-effector proteins contain a Ras-associating (RA) domain through which these associate with Ras in a GTP-dependent manner. The RA domain is highly conserved among the members of the growth factor receptor-bound (Grb) 7 family of proteins which includes Grb7, Grb10 and Grb14. Our laboratory has reported an unusual observation that RA domain of Grb14 binds to the C-terminal nucleotide binding site of cyclic nucleotide gated channel (CTRCNGA1) and inhibits the channel activity. Molecular modeling of the CTR-CNGA1 displays 50%-70% tertiary structural similarity towards Ras proteins. We named this region as Ras-like domain (RLD). The interaction between RA-Grb14 and RLD-CNGA1 is mediated through a simple protein-protein interaction temporally and spatially regulated by light and cGMP. It is interesting to note that Grb14 binds to GTPase-mutant Rab5, a Ras-related small GTPase whereas Grb10 binds only to GTP-bound form of active Rab5 but not to GTPase-defective mutant Rab5. These results suggest that Grb14 might have been evolved later in the evolution that binds to both Ras and nucleotide binding proteins such as CNGA1. Our studies also suggest that eukaryotic CNG channels could be evolved through a gene fusion between prokaryotic ion channels and cyclic nucleotide binding proteins, both of which might have undergone several sequence variations for functional adaptation during evolution.

Extramitochondrial tricarboxylic acid cycle in retinal rod outer segments

Vertebrate retinal rod Outer Segments (OS) are the site of visual transduction, an energy demanding process for which mechanisms of ATP supply are still poorly known. Glycolysis or diffusion of either ATP or phosphocreatine from the Inner Segment (IS) does not seem to display adequate timing to supply ATP for phototransduction. We have previously reported data suggesting an aerobic metabolism in OS, which would largely account for the light-stimulated ATP need of the photoreceptor. Here, by oxymetry and biochemical analyses we show that: (i) disks isolated by Ficoll flotation consume O(2) in the presence of physiological respiring substrates either in coupled or uncoupled conditions; (ii) OS homogenates contain the whole biochemical machinery for the degradation of glucose, i.e. glycolysis and the tricarboxylic acid cycle (TCA cycle), consistently with the results of our previous proteomic study. Activities of the 8 TCA cycle enzymes in OS were comparable to those in retinal mitochondria-enriched fractions. Disk and OS preparations were subjected to TEM analysis, and while they can be considered free of inner segment contaminants, immunogold with specific antibodies demonstrate the expression therein of both the visual pigment rhodopsin and F(o)F(1)-ATP synthase. Finally, double immunofluorescence on mouse retina sections demonstrated a colocalization of some respiratory complex mitochondrial proteins with rhodopsin in rod OS. Data, suggestive of the exportability of the mitochondrial machinery for aerobic metabolism, may shed light on those retinal pathologies related to energy supply impairment in OS and to mutations in TCA enzymes.

Catalytic properties of the retinal rod outer segment disk ADP-ribosyl cyclase

Cyclic ADP-ribose (cADPR) is a second messenger modulating intracellular calcium levels. We have previously described a cADPR-dependent calcium signaling pathway in bovine rod outer segments (ROS), where calcium ions play a pivotal role. ROS ADP-ribosyl cyclase (ADPR-cyclase) was localized in the membrane fraction. In the present work, we examined the properties of the disk ADPR-cyclase through the production of cyclic GDP-ribose from the NAD(+) analogue NGD(+). The enzyme displayed an estimated K(m) for NGD(+) of 12.5 ± 0.3 μM, a V(max) of 26.50 ± 0.70 pmol cyclic GDP-ribose synthesized/min/mg, and optimal pH of 6.5. The effect of divalent cations (Zn(2+), Cu(2+), and Ca(2+)) was also tested. Micromolar Zn(2+) and Cu(2+) inhibited the disk ADPR-cyclase activity (half maximal inhibitory concentration, IC50=1.1 and 3.6 μM, respectively). By contrast, Ca(2+) ions had no effect. Interestingly, the properties of the intracellular membrane-associated ROS disk ADPR-cyclase are more similar to those of the ADPR-cyclase found in CD38-deficient mouse brain, than to those of CD38 or CD157. The novel intracellular mammalian ADPR-cyclase would elicit Ca(2+) release from the disks at various rates in response to change in free Ca(2+) concentrations, caused by light versus dark adaptation, in fact there was no difference in disk ADPR-cyclase activity in light or dark conditions. Data suggest that disk ADPR-cyclase may be a potential target of retinal toxicity of Zn(2+) and may shed light to the role of Cu(2+) and Zn(2+) deficiency in retina.

Rhodopsin-regulated insulin receptor signaling pathway in rod photoreceptor neurons

The retina is an integral part of the central nervous system and retinal cells are known to express insulin receptors (IR), although their function is not known. This article describes recent studies that link the photoactivation of rhodopsin to tyrosine phosphorylation of the IR and subsequent activation of phosphoinositide 3-kinase, a neuron survival factor. Our studies suggest that the physiological role of this process is to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis. We focus mainly on our recently identified regulation of the IR pathway through the G-protein-coupled receptor rhodopsin. Various mutant and knockout proteins of phototransduction cascade have been used to study the light-induced activation of the retinal IR. Our studies suggest that rhodopsin may have additional previously uncharacterized signaling functions in photoreceptors.

Helix formation in arrestin accompanies recognition of photoactivated rhodopsin

Binding of arrestin to photoactivated phosphorylated rhodopsin terminates the amplification of visual signals in photoreceptor cells. Currently, there is no crystal structure of a rhodopsin-arrestin complex available, although structures of unbound rhodopsin and arrestin have been determined. High-affinity receptor binding is dependent on distinct arrestin sites responsible for recognition of rhodopsin activation and phosphorylation. The loop connecting beta-strands V and VI in rod arrestin has been implicated in the recognition of active rhodopsin. We report the structure of receptor-bound arrestin peptide Arr(67-77) mimicking this loop based on solution NMR data. The peptide binds photoactivated rhodopsin in the unphosphorylated and phosphorylated form with similar affinities and stabilizes the metarhodopsin II photointermediate. A largely alpha-helical conformation of the receptor-bound peptide is observed.

Lipid second messengers and related enzymes in vertebrate rod outer segments

Rod outer segments (ROSs) are specialized light-sensitive organelles in vertebrate photoreceptor cells. Lipids in ROS are of considerable importance, not only in providing an adequate environment for efficient phototransduction, but also in originating the second messengers involved in signal transduction. ROSs have the ability to adapt the sensitivity and speed of their responses to ever-changing conditions of ambient illumination. A major contributor to this adaptation is the light-driven translocation of key signaling proteins into and out of ROS. The present review shows how generation of the second lipid messengers from phosphatidylcholine, phosphatidic acid, and diacylglycerol is modulated by the different illumination states in the vertebrate retina. Findings suggest that the light-induced translocation of phototransduction proteins influences the enzymatic activities of phospholipase D, lipid phosphate phosphatase, diacylglyceride lipase, and diacylglyceride kinase, all of which are responsible for the generation of the second messenger molecules.

Evidence for aerobic metabolism in retinal rod outer segment disks

The disks of the vertebrate retinal rod Outer Segment (OS), devoid of mitochondria, are the site of visual transduction, a very energy demanding process. In a previous proteomic study we reported the expression of the respiratory chain complexes I-IV and the oxidative phosphorylation Complex V (F(1)F(0)-ATP synthase) in disks. In the present study, the functional localization of these proteins in disks was investigated by biochemical analyses, oxymetry, membrane potential measurements, and confocal laser scanning microscopy. Disk preparations, isolated by Ficoll flotation, were characterized for purity. An oxygen consumption, stimulated by NADH and Succinate and reverted by rotenone, antimycin A and KCN was measured in disks, either in coupled or uncoupled conditions. Rhodamine-123 fluorescence quenching kinetics showed the existence of a proton potential difference across the disk membranes. Citrate synthase activity was assayed and found enriched in disks with respect to ROS. ATP synthesis by disks (0.7 micromol ATP/min/mg), sensitive to the common mitochondrial ATP synthase inhibitors, would largely account for the rod ATP need in the light. Overall, data indicate that an oxidative phosphorylation occurs in rod OS, which do not contain mitochondria, thank to the presence of ectopically located mitochondrial proteins. These findings may provide important new insight into energy production in outer segments via aerobic metabolism and additional information about protein components in OS disk membranes.

Insulin growth factor 1 receptor/PI3K/AKT survival pathway in outer segment membranes of rod photoreceptors

PURPOSE

The authors previously reported that physiological light induces the tyrosine phosphorylation of insulin receptors (IRs), which leads to the activation of the phosphoinositide 3-kinase (PI3K) and Akt (serine/threonine protein kinase B) survival pathway in rod photoreceptor cells. Tissue-specific deletion of IRs from photoreceptors resulted in stress-induced photoreceptor degeneration. Insulin growth factor 1 receptor (IGF-1R) is highly related in sequence and structure to the IR and shares 70% sequence identity overall and 84% identity within the tyrosine kinase domain. The role of IGF-1R in photoreceptor function is unknown. In this study the authors examined IGF-1R signaling in rod outer segment (ROS) membranes.

METHODS

IGF-1R localization was examined in the plasma and disc membranes of ROS. Activation of the IGF-1R/PI3K/Akt pathway was analyzed using specific antibodies against phospho-tyrosine, IGF-1R, and phospho-Akt. PI3K activity was determined in the anti-phospho-tyrosine and anti-IGF-1R immunoprecipitates. Glutathione-S-transferase fusion proteins containing two Src homology 2 (SH2) domains of the p85 subunit of PI3K and their mutants were used to study the molecular interaction between IGF-1R and p85. In vivo IGF-1R signaling was studied in rats exposed to physiological light or to constant light.

RESULTS

IGF-1R is predominately localized to plasma membranes of ROS. These studies indicate that light stress results in an increase in tyrosine phosphorylation of IGF-1R and an increase in PI3K enzyme activity in anti-phosphotyrosine and anti-IGF-1R immunoprecipitates of ROS and retinal homogenates. The authors observed that light stress induces tyrosine phosphorylation of IGF-1R in ROS membranes, which leads to the binding of p85 through N-SH2 and C-SH2 domains. Finally, the authors observed a significant activation of Akt in light-stressed retinas, indicating activation of the Akt survival pathway downstream of IGF-1R activation.

CONCLUSIONS

Light stress induced the activation of PI3K through activation and binding of IGF-1R, which leads to activation of the Akt survival pathway in photoreceptors.

Proteomic analysis of the retinal rod outer segment disks

The initial events of vision at low light take place in vertebrate retinal rods. The rod outer segment consists of a stack of flattened disks surrounded by the plasma membrane. A list of the proteins that reside in disks has not been achieved yet. We present the first comprehensive proteomic analysis of purified rod disks, obtained by combining the results of two-dimensional gel electrophoresis separation of disk proteins to MALDI-TOF or nLC-ESI-MS/MS mass spectrometry techniques. Intact disks were isolated from bovine retinal rod outer segments by a method that minimizes contamination from inner segment. Out of a total of 187 excised spots, 148 proteins were unambiguously identified. An additional set of 61 proteins (partially overlapping with the previous ones) was generated by one-dimensional (1D) gel nLC-ESI-MS/MS method. Proteins involved in vision as well as in aerobic metabolism were found, among which are the five complexes of oxidative phosphorylation. Results from biochemical, Western blot, and confocal laser scanning microscopy immunochemistry experiments suggest that F 1F o-ATP synthase is located and catalytically active in ROS disk membranes. This study represents a step toward a global physiological characterization of the disk proteome and provides information necessary for future studies on energy supply for phototransduction.

Differential distribution of proteins and lipids in detergent-resistant and detergent-soluble domains in rod outer segment plasma membranes and disks

Membrane heterogeneity plays a significant role in regulating signal transduction and other cellular activities. We examined the protein and lipid components associated with the detergent-resistant membrane (DRM) fractions from retinal rod outer segment (ROS) disk and plasma membrane-enriched preparations. Proteomics and correlative western blot analysis revealed the presence of alpha and beta subunits of the rod cGMP-gated ion channel and glucose transporter type 1, among other proteins. The glucose transporter was present exclusively in ROS plasma membrane (not disks) and was highly enriched in DRMs, as was the cGMP-gated channel beta-subunit. In contrast, the majority of rod opsin and ATP-binding cassette transporter A4 was localized to detergent-soluble domains in disks. As expected, the cholesterol : fatty acid mole ratio was higher in DRMs than in the corresponding parent membranes (disk and plasma membranes, respectively) and was also higher in disks compared to plasma membranes. Furthermore, the ratio of saturated : polyunsaturated fatty acids was also higher in DRMs compared to their respective parent membranes (disk and plasma membranes). These results confirm that DRMs prepared from both disks and plasma membranes are enriched in cholesterol and in saturated fatty acids compared to their parent membranes. The dominant fatty acids in DRMs were 16 : 0 and 18 : 0; 22 : 6n3 and 18 : 1 levels were threefold higher and twofold lower, respectively, in disk-derived DRMs compared to plasma membrane-derived DRMs. We estimate, based on fatty acid recovery that DRMs account for only approximately 8% of disks and approximately 12% of ROS plasma membrane.

Confocal laser scanning microscopy of retinal rod outer segment intact disks: new labeling technique

Vertebrate retinal rod outer segment disks house the proteins involved in the phototransduction cascade that converts light into neuronal signal. We develop a technique for the immunofluorescent labeling of osmotically intact isolated rod outer segment disks for confocal laser scanning microscopy imaging. Osmotically intact Ficoll-flotation isolated bovine disks are directly labeled with antibodies in solution. For the first time, osmotically intact single disks can be visualized. Thus, imaging of purified disks, based on advanced optical techniques, may serve as a powerful complement to other methods in studies on phototransduction. In fact, even though much is known about the rod outer segment photoresponse, some unanswered questions remain, particularly about ATP supply, light adaptation, and morphogenesis.

G-protein-coupled receptor rhodopsin regulates the phosphorylation of retinal insulin receptor

We have shown previously that phosphoinositide 3-kinase in the retina is activated in vivo through light-induced tyrosine phosphorylation of the insulin receptor (IR). The light effect is localized to photoreceptor neurons and is independent of insulin secretion (Rajala, R. V., McClellan, M. E., Ash, J. D., and Anderson, R. E. (2002) J. Biol. Chem. 277, 43319-43326). These results suggest that there exists a cross-talk between phototransduction and other signal transduction pathways. In this study, we examined the stage of phototransduction that is coupled to the activation of the IR. We studied IR phosphorylation in mice lacking the rod-specific alpha-subunit of transducin to determine if phototransduction events are required for IR activation. To confirm that light-induced tyrosine phosphorylation of the IR is signaled through bleachable rhodopsin, we examined IR activation in retinas from RPE65(-/-) mice that are deficient in opsin chromophore. We observed that IR phosphorylation requires the photobleaching of rhodopsin but not transducin signaling. To determine whether the light-dependent activation of IR is mediated through the rod or cone transduction pathway, we studied the IR activation in mice lacking opsin, a mouse model of pure cone function. No light-dependent activation of the IR was found in the retinas of these mice. We provide evidence for the existence of a light-mediated IR pathway in the retina that is different from the known insulin-mediated pathway in nonneuronal tissues. These results suggest that IR phosphorylation in rod photoreceptors is signaled through the G-protein-coupled receptor rhodopsin. This is the first study demonstrating that rhodopsin can initiate signaling pathway(s) in addition to its classical phototransduction.

Crystals of Native and Modified Bovine Rhodopsins and Their Heavy Atom Derivatives

Rhodopsin, the pigment protein responsible for dim-light vision, is a G protein-coupled receptor that converts light absorption into the activation of a G protein, transducin, to initiate the visual response. We have crystallised detergent-solubilised bovine rhodopsin in the native form and after chemical modifications as needles 10-40 microm in cross-section. The crystals belong to the trigonal space group P3(1), with two molecules of rhodopsin per asymmetric unit, related by a non-crystallographic 2-fold axis parallel with the crystallographic screw axis along c (needle axis). The unit cell dimensions are a=103.8 A, c=76.6 A for native rhodopsin, but vary over a wide range after heavy atom derivatisation, with a between 101.5 A and 113.9 A, and c between 76.6 A and 79.2 A. Rhodopsin molecules are packed with the bundle of transmembrane helices tilted from the c-axis by about 100 degrees . The two molecules in the asymmetric unit form contacts along the entire length of their transmembrane helices 5 in an antiparallel orientation, and they are stacked along the needle axis according to the 3-fold screw symmetry. Hence hydrophobic contacts are prominent at protein interfaces both along and normal to the needle axis. The best crystals of native rhodopsin in this crystal form diffracted X-rays from a microfocused synchrotron source to 2.55 A maximum resolution. We describe steps taken to extend the diffraction limit from about 10 A to 2.6 A.

DHA-rich phospholipids optimize G-Protein-coupled signaling

OBJECTIVE

To assess the effects of n-3 polyunsaturated phospholipid acyl chains on the initial steps in G-protein-coupled signaling.

STUDY DESIGN

Isolated components of the visual signal transduction system, rhodopsin, G protein (G(t)), and phosphodiesterase (PDE), were reconstituted in membranes containing various levels of n-3 polyunsaturated phospholipid acyl chains. In addition, rod outer segment disk membranes containing these components were purified from rats raised on n-3-deficient and n-3-adequate diets. The conformation change of rhodopsin, coupling of rhodopsin to G(t), and PDE activity were each measured separately.

RESULTS

The ability of rhodopsin to form the active metarhodopsin II conformation and bind G(t) were both compromised in membranes with reduced levels of n-3 polyunsaturated acyl chains. The activity of PDE, directly related to the integrated cellular response, was reduced in all membranes lacking or deficient in n-3 polyunsaturated acyl chains. PDE activity in membranes containing 22:5n-6 PC was 50% lower than in membranes containing either 22:6n-3 PC or 22:5n-3 PC.

CONCLUSIONS

The earliest events in G-protein-coupled signaling; receptor conformation change, receptor-G-protein binding, and PDE activity are reduced in membranes lacking n-3 polyunsaturated acyl chains. Efficient and rapid propagation of G-protein-coupled signaling requires polyunsaturated n-3 phospholipid acyl chains.

Cholesterol-dependent association of caveolin-1 with the transducin alpha subunit in bovine photoreceptor rod outer segments: disruption by cyclodextrin and guanosine 5'-O-(3-thiotriphosphate)

Evidence suggests that caveolins, 21-24 kDa cholesterol-binding proteins that generally reside in specialized detergent-resistant membrane microdomains, act as signaling scaffolds. Detergent-resistant membranes isolated from rod outer segments (ROS) have been previously shown to contain the photoreceptor G-protein, transducin. In this report we show, by subcellular fractionation, that caveolin-1 is an authentic component of purified ROS. We demonstrate that caveolin-1 in ROS almost exclusively resides in low-buoyant-density, cholesterol-rich, detergent-resistant membranes that can be disrupted by cholesterol depletion using methyl-beta-cyclodextrin (MCD). Cholesterol depletion was also observed to extract a pool of transducin alpha (Talpha) from ROS membranes. Immunoprecipitation with anti-caveolin-1 revealed the association of Talpha in the absence of Tbetagamma. Treatment of ROS with MCD resulted in a 2-fold decrease in recovery of Talpha in anti-caveolin-1 immunoprecipitates. This interaction was also completely disrupted when ROS were exposed to light in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), a nonhydrolyzable GTP analogue. In addition, caveolin-1/Talpha association in the immune complex was disrupted by a peptide based on the primary sequence of the caveolin-1 scaffolding domain. Finally, we confirm the colocalization of caveolin-1 and Talpha in photoreceptors by immunofluorescence microscopy. These results strongly suggest that the association between Talpha and caveolin-1 occurs in cholesterol-rich, detergent-resistant membranes and is likely to be dependent upon the activation state of Talpha.

Structure and orientation of a G protein fragment in the receptor bound state from residual dipolar couplings

Residual dipolar couplings for a ligand that is in fast exchange between a free state and a state where it is bound to a macroscopically ordered membrane protein carry precise information on the structure and orientation of the bound ligand. The couplings originate in the bound state but can be detected on the free ligand using standard high resolution NMR. This approach is used to study an analog of the C-terminal undecapeptide of the alpha-subunit of the heterotrimeric G protein transducin when bound to photo-activated rhodopsin. Rhodopsin is the major constituent of disk-shaped membrane vesicles from rod outer segments of bovine retinas, which align spontaneously in the NMR magnet. Photo-activation of rhodopsin triggers transient binding of the peptide, resulting in measurable dipolar contributions to 1J(NH) and 1J(CH) splittings. These dipolar couplings report on the time-averaged orientation of bond vectors in the bound peptide relative to the magnetic field, i.e. relative to the membrane normal. Approximate distance restraints of the bound conformation were derived from transferred NOEs, as measured from the difference of NOESY spectra recorded prior to and after photo-activation. The N-terminal eight residues of the bound undecapeptide adopt a near-ideal alpha-helical conformation. The helix is terminated by an alpha(L) type C-cap, with Gly9 at the C' position in the center of the reverse turn. The angle between the helix axis and the membrane normal is 40 degrees (+/-4) degrees. Peptide protons that make close contact with the receptor are identified by analysis of the NOESY cross-relaxation pattern and include the hydrophobic C terminus of the peptide.

Manipulation of cholesterol levels in rod disk membranes by methyl-beta-cyclodextrin: effects on receptor activation

The effect of cholesterol on rod outer segment disk membrane structure and rhodopsin activation was investigated. Disk membranes with varying cholesterol concentrations were prepared using methyl-beta-cyclodextrin as a cholesterol donor or acceptor. Cholesterol exchange followed a simple equilibrium partitioning model with a partition coefficient of 5.2 +/- 0.8 in favor of the disk membrane. Reduced cholesterol in disk membranes resulted in a higher proportion of photolyzed rhodopsin being converted to the G protein-activating metarhodopsin II (MII) conformation, whereas enrichment of cholesterol reduced the extent of MII formation. Time-resolved fluorescence anisotropy measurements using 1,6-diphenyl-1,3,5-hexatriene showed that increasing cholesterol reduced membrane acyl chain packing free volume as characterized by the parameter f(v). The level of MII formed showed a positive linear correlation with f(v) over the range of 4 to 38 mol % cholesterol. In addition, the thermal stability of rhodopsin increased with mol % of cholesterol in disk membranes. No evidence was observed for the direct interaction of cholesterol with rhodopsin in either its agonist- or antagonist-bound form. These results indicate that cholesterol mediates the function of the G protein-coupled receptor, rhodopsin, by influencing membrane lipid properties, i.e. reducing acyl chain packing free volume, rather than interacting specifically with rhodopsin.

Effect of ethanol and osmotic stress on receptor conformation. Reduced water activity amplifies the effect of ethanol on metarhodopsin II formation

The combined effects of ethanol and osmolytes on both the extent of formation of metarhodopsin II (MII), which binds and activates transducin, and on acyl chain packing were examined in rod outer segment disc membranes. The ethanol-induced increase in MII formation was amplified by the addition of neutral osmolytes. This enhancement was linear with osmolality. At 360 milliosmolal, the osmolality of human plasma, 50 mM ethanol was 2.7 times more potent than at 0 osmolality, demonstrating the importance of water activity in in vitro experiments dealing with ethanol potency. Ethanol disordered acyl chain packing, and increasing osmolality enhanced this acyl chain disordering. Prior osmotic stress data showed a release of 35 +/- 2 water molecules upon MII formation. Ethanol increases this number to 49 water molecules, suggesting that ethanol replaces 15 additional water molecules upon MII formation. Amplification of ethanol effects on MII formation and acyl chain packing by osmolytes suggests that ethanol increases the equilibrium concentration of MII both by disordering acyl chain packing and by disrupting rhodopsin-water hydrogen bonds, demonstrating a direct effect of ethanol on rhodopsin. At physiologically relevant levels of osmolality and ethanol, about 90% of ethanol's effect is due to disordered acyl chain packing.

Can phosphorylation and dephosphorylation of rod outer segment membranes affect phosphatidate phosphohydrolase and diacylglycerol lipase activities?

Phosphatidate phosphohydrolase (PAPase) and diacylglycerol lipase (DGL) enzymatic activities were found to be differently affected by preincubation of rod outer segments (ROS) under protein phosphorylation or dephosphorylation conditions in darkness or in light. Under protein kinase C (PKC) phosphorylation conditions, PAPase and DGL were inhibited in darkness and in light. The inhibitory effect on PAPase and DGL activities by PKC phosphorylation in the presence of light was more pronounced when the activities were compared with the activities in control membranes determined in the presence of EGTA. The addition of PKC activators such as phorbol-12,13-dibutyrate and dioctanoylglycerol (DOG) instead of DG produced the same pattern of changes in enzymatic activities. Pretreatment of ROS membranes with cAMP-dependent protein kinase (PKA) produced a significant increase in both enzymatic activities in the presence of light. No changes were observed when ROS proteins were phosphorylated by PKA in the dark. Dephosphorylation of ROS membranes with alkaline phosphatase resulted in a decrease in PAPase activity that was more marked under light than under dark conditions. DGL activity was not modified under dephosphorylation conditions. These findings suggest that the metabolization of phosphatidic acid in isolated ROS is differently affected by protein phosphorylation and dephosphorylation reactions.

Zinc causes an apparent increase in rhodopsin phosphorylation

PURPOSE

Rhodopsin is a zinc-binding protein. We investigated the effect of low concentrations of zinc on the initial phosphorylation of rhodopsin.

METHODS

Dark-adapted bovine rod outer segments (ROS) were incubated with (gamma 32P) ATP and 5 mM magnesium in the presence and absence of micromolar amounts of zinc. The ROS were exposed to light to initiate phosphorylation under conditions which allow only limited initial phosphorylation.

RESULTS

We found that zinc enhanced the rhodopsin phosphorylation apparent on autoradiographies by several fold. Phosphorylation reactions conducted in the presence of potent phospho-opsin phosphatase inhibitors show a comparable zinc-enhanced phosphorylation of rhodopsin. Under our reaction conditions, ROS membranes also appear more red upon initial exposure to light when zinc is present.

CONCLUSIONS

Zinc can increase initial rhodopsin phosphorylation, apparently acting at the substrate rhodopsin and not at relevant phosphatases or rhodopsin kinase. How zinc binding to rhodopsin might increase its ability to serve as a substrate for phosphorylation is under investigation.

Primary alcohols modulate the activation of the G protein-coupled receptor rhodopsin by a lipid-mediated mechanism

The visual pigment rhodopsin is a prototypical member of the G protein-coupled receptor superfamily. In this study, we have investigated the effect of a series of n-alcohols on the formation of metarhodopsin II (MII), the photoactivated conformation of rhodopsin, which binds and activates transducin. When rhodopsin was photolyzed in the presence of several n-alcohols, increased MII formation was observed in the order ethanol > butanol > hexanol, whereas longer chain n-alcohols inhibited MII formation with decanol > octanol. The magnitude of the stimulatory effects was greater in a more highly unsaturated phospholipid. Alcohols, which enhanced MII formation also increased phospholipid acyl chain packing free volume, while those that decreased this bilayer property inhibited MII formation. An apparent discontinuity in the effect of these alcohols results when their potency is calculated in terms of the total aqueous alcohol concentration. In sharp contrast, a continuous variation in their behavior is observed, when their potency is calculated in terms of the amount of alcohol partitioned in the membrane. Our findings strongly support a lipid-mediated mechanism of action for alcohols on rhodopsin and, by analogy, for other G protein-coupled receptors.

Inositol-1,4,5-trisphosphate receptors in the vertebrate retina

Evidence has shown an activation of phosphatidylinositol 4,5-bisphosphate (PIP2) specific phospholipase C (PtdIns-PLC) by light in the vertebrate retina and rod outer segments (ROS), suggesting important roles for its two metabolites, 1,2-diacylglycerol (DG) and inositol-1,4,5-trisphosphate [Ins(1,4,5)P3]. DG activates protein kinase C (PKC) and Ins(1,4,5)P3 releases bound intracellular calcium. Since Ca2+ plays an important role in light adaptation, the presence of Ins(1,4,5)P3 receptors in ROS may indicate a regulatory role of Ins(1,4,5)P3 to the free Ca2+ content. In the present study, we investigated the Ins(1,4,5)P3 receptors in whole retinal membranes and several subcellular fractions prepared from bovine retinas. Scatchard analyses of binding data for retinal membrane preparations showed a single, high-affinity binding site with equilibrium dissociation constant (Kd) of 24 +/- 2 nM and maximal binding capacity (Bmax) of 353 +/- 15 fmol/mg protein at pH 7.4. Specific binding was found in both small and large synaptosomal preparations representing inner and outer plexiform layers, respectively. A detectable, but low abundance of Ins(1,4,5)P3-specific binding in ROS was observed at both pH 7.4 and 8.3, but no specific binding of Ins(1,4,5)P3 was found in isolated outer segment discs. The binding of Ins(1,4,5)P3 in ROS was reduced by addition of ATP, suggesting a regulatory role for this nucleotide. Addition of calcium, sodium, and potassium ions also reduced specific binding of Ins(1,4,5)P3. Immunocytochemical studies indicate intense staining in the inner segment and extending to the ROS. Inner and outer plexiform layers were also stained. These findings show that the Ins(1,4,5)P3 receptor is present in photoreceptor cells and inner and outer plexiform layers in the vertebrate retina.

Rhodopsin mobility, structure, and lipid-protein interaction in squid photoreceptor membranes

Treatment of outer segment membranes from Loligo forbesi with endoprotease-V8 from Staphylococcus aureus results in cleavage of the C-terminal extension of the squid rhodopsin, with accompanying reduction of the apparent molecular weight from 47,000 to 36,000 on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Negative-stain electron microscopy of the intact membranes shows that small clusters of the rhodopsin C-termini form structures extending from the membrane surface and that these are absent after protease treatment. Fourier transform infrared spectra of the amide I band of the protein indicate that removal of the C-terminal extension increases the relative alpha-helical content of squid rhodopsin to a level comparable to that for bovine rhodopsin in disk membranes, and to an extent which suggests that the alpha-helical structure lies mainly in the M(r) 36,000 (transmembrane) section of the protein. Saturation-transfer electron spin resonance (ESR) spectroscopy of the spin-labeled protein reveals that the rotational diffusion of squid rhodopsin in outer segment membranes that have been extensively washed with urea to remove peripheral proteins is much slower than that of bovine rhodopsin in rod outer segment disk membranes. This reduction in rotational mobility is also found with purified squid rhodopsin reconstituted in egg phosphatidylcholine and in urea-washed outer segment membranes which have been treated with endoprotease-V8 to remove the C-terminal extension of squid rhodopsin. In the latter case, the saturation-transfer ESR spectra are virtually identical to those of the non-proteolyzed membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct zinc binding to purified rhodopsin and disc membranes

Using the radionuclide 65Zn, we have demonstrated the direct binding of zinc to purified rhodopsin. 65Zn is eluted with detergent-solubilized rhodopsin from concanavalin A columns and remains bound to the visual pigment through a subsequent gel-filtration step. Zinc binding to purified disc membranes is highly specific and, of the ions tested, copper is the best competitor. Equilibrium-dialysis experiments indicate that zinc binding to detergent-solubilized forms of rhodopsin may increase on bleaching the photopigment. These results may have important implications for studies that indicate that zinc plays a role in retinal degeneration and normal photoreceptor physiology.

Surface-bound biomembranes incorporating receptors: electrochemical and structural characterization

A generic method is described for forming surface-bound structures that incorporate protein receptors in a membrane-like environment. Silane reagents (octadecyltrichlorosilane and dimethyloctadecylchlorosilane) were used to produce primed substrates bearing full and partial monolayers, respectively. Biomembranes were formed by dialysis of detergent-solubilized membranes in the presence of two different alkylsilanized substrates: Si/SiO2 electrodes and glass microspheres. Electrochemical analysis of the capacitance was used to determine apparent thickness and degree of surface coverage at each stage in the deposition process. Elemental analysis on glass beads gave the hydrocarbon incorporation. Glass bead substrates were also examined by Fourier transform infrared spectroscopy to evaluate the alkylsilanized substrate before and after dialysis. Both vertebrate rhodopsin and the nicotinic acetylcholine receptor could be incorporated into structures with composition and dimensions similar to natural bilayer membranes. The techniques reported here are applicable for coupling membrane receptors to a variety of transducing substrates used in biosensors.

Light-induced proton permeability changes in retinal rod photoreceptor disk membranes

We have used the membrane-permeant charged fluorescent dye, 3,3'-dipropylthiadicarbocyanine iodide (diS-C3[5]), to monitor electrical potentials across the membranes of isolated bovine disks. Calibration curves obtained from experiments where a potential was created across the disk membrane by a potassium concentration gradient and valinomycin showed an approximately linear relation between dye fluorescence and calculated membrane potential from 0 to -120 mV. Light exposure in the presence of the permeant buffer, imidazole, caused a rapid decay of the membrane potential to a new stable level. Addition of CCCP, a proton ionophore, in the dark produced the same effect as illumination. When the permeant buffer, imidazole, was replaced by the impermeant buffer, Hepes, neither light nor CCCP discharged the gradient. We interpret the changes in membrane potential measured upon illumination to be the result of a light-induced increase in the permeability of the disk membrane to protons. A permeant buffer is required to prevent the build-up of a pH gradient which would inhibit the sustained proton flow needed for an observable change in membrane potential.

Asymmetric distribution of phospholipids in acetylcholine receptor-rich membranes from T. marmorata electric organ

1. The distribution of phospholipids between the two leaflets of the lipid bilayer in acetylcholine receptor (AChR)-rich membranes from T. marmorata has been examined with two complementary techniques: chemical derivatization with the membrane-impermeable reagent trinitrobenzenesulphonate (TNBS) and B.cereus phospholipase C hydrolysis. 2. AChR-membranes were reacted with TNBS at 0-4 and 37 degrees C and the accessibility of their aminophospholipids was compared to that of rod outer segment and erythrocyte membranes. The results indicate that more of the total ethanolamine glycerophospholipid (EGP) than of the total phosphatidylserine (PS) is located in the outer monolayer. 3. Nearly half the phospholipid content of AChR membranes is hydrolyzed by phospholipase C with a half-time of ca. 1.6 min at 25 degrees C. Consistent with the TNBS results, more of the total EGP than of the total PS is degraded. Beyond 3 min the reaction slows down, relatively smaller additional amounts of lipids are hydrolyzed, and all phospholipid classes are attacked to a similar extent, indicating that after half the lipid is removed all phospholipids become accessible to the enzyme. 4. The results indicate that the outer leaflet of the bilayer is richer in ethanolamine and choline glycerophospholipids, whereas phosphatidylinositol, most of the sphingomyelin, and ca 65% of the PS are located on the inner leaflet.

Nucleotide binding to the rod outer segment rim protein

The rod photoreceptor outer segment maintains a remarkable morphology. Two of the proteins which have been implicated in the maintenance of this structure are the 240 kDa spectrin-like protein, and the 220 kDa glycoprotein often referred to as the rim protein. We have probed rat rod outer segment proteins with light-activated (azido-labeled) radioactive nucleotides and found a nucleotide binding site(s) on the rim protein which has a preference for guanine nucleotides. Binding to this site is stimulated by the divalent cations zinc, manganese and magnesium, but not calcium. This site is under investigation and may play a role in stabilizing protein structure.

Synthesis of very long chain (up to 36 carbon) tetra, penta and hexaenoic fatty acids in retina

The synthesis of very long chain (C24 to C36) polyunsaturated (four, five and six double bonds) fatty acids (VLCPUFA) is investigated in bovine retina using [14C]acetate. Saturates on the one hand (mainly palmitate), and polyenes on the other (mainly VLCPUFA), incorporate most of the label found in lipids. Phosphatidylcholine (PC) is the most highly labelled lipid class, since both types of 14C-labelled fatty acids, but especially this novel series of VLCPUFA, are concentrated in this phospholipid. Radioactivity from [14C]acetate is found in very long chain tetra, penta and hexaenoic fatty acids of PC. The labelling of 20:4(n - 6), 20:5(n - 3), 22:5(n - 6) and 22:6(n - 3) is much lower than that of longer polyenes of each of these series, indicating that VLCPUFA are synthesized in situ by successive elongations of the above polyenes, pre-existing in retina lipids. In various subcellular fractions isolated from retinas after incubations with [14C]acetate (including cytosol, microsomes, mitochondria and photoreceptor membranes), the labelling of the VLCPUFA of PC is very high, even at relatively short intervals of incubation. The results suggest that not only the synthesis but also the intracellular traffic among membranes of VLCPUFA-containing species of PC are very active processes in the retina.

Labeling of lipids of retina subcellular fractions by [1-14C]eicosatetraenoate (20:4(n-6)) docosapentaenoate (22:5(n-3)) and docosahexaenoate (22:6(n-3))

(1-14C)-labeled (n-6) eicosatetraenoate, (n-3) docosapentaenoate and (n-3) docosahexaenoate (20:4, 22:5 and 22:6, respectively) are efficiently taken up and actively esterified into the lipids of bovine retina after 2 h incubation. Photoreceptor membranes, mitochondria, microsomes and postmicrosomal supernatants, which display significant differences in phospholipid and fatty acid compositions, are isolated after such incubations to study the labeling of lipids. The lipid classes preferentially labeled with the acids (1) largely differ among and within subcellular fractions, while (2) some common features in the treatment of the three polyenes are observed in each fraction. In all of them, the three acids are actively incorporated in phosphatidylcholine; ethanolamine glycerophospholipid, phosphatidylserine (PS) and phosphatidylinositol (PI) are highly labeled with 22:6, 22:5 and 20:4 respectively; within ethanolamine glycerophospholipid, the three label phosphatidylethanolamine in preference to plasmenylethanolamine. Most of the 14C esterified in mitochondria is in phospholipids. The endoplasmic reticulum produces in addition highly labeled triacylglycerols, also found in cytosol. High levels of 14C-labeled diacylglycerols are observed exclusively in photoreceptor membranes, where the specific radioactivity of PI is very high. The total amounts of 14C incorporated (1) are in general similar within a given fraction for the three polyenes, but (2) largely differ among fractions. The labeling of the highly unsaturated phospholipids of photoreceptor membranes is the lowest, while the postmicrosomal supernatant (whose lipids are relatively the poorest in polyenoic fatty acids) contains most of the labeled lipids isolated from retinas under these conditions. The results indicate that polyunsaturated species of retina phospholipids undergo an active synthesis and turnover, as well as an intense intracellular traffic among membranes.

Calmodulin and calmodulin binding proteins in amphibian rod outer segments

The calmodulin (CaM) content of fully intact frog rod outer segments (ROS) has been measured. The molar ratio between rhodopsin and total CaM in ROS is 800:1. This is in good agreement with the data reported for bovine ROS CaM [Kohnken, R. E., Chafouleas, J. G., Eadie, D. M., Means, A. R., & McConnell, D.G. (1981) J. Biol. Chem. 256, 12517-12522]. In the absence of Ca2+, the ROS membrane fraction contains only 4% of total ROS CaM. In contrast, in the presence of Ca2+, 15% of total ROS CaM is found in the membrane fraction. For half-maximal binding of CaM to CaM-depleted ROS membranes, 3 X 10(-7) M Ca2+ is required. This CaM binding is inhibited by trifluoperazine. CaM binding proteins in the ROS membrane fraction are identified by using two different methods: the overlay method and the use of 3,3'-dithiobis(sulfosuccinimidyl propionate) (DTSSP), a bifunctional cross-linking reagent. Ca2+-dependent CaM binding proteins with apparent molecular weights of 240,000, 140,000, 53,000, and 47,000 are detected in the ROS membrane fraction by the overlay method. Anomalous, Ca2+-independent CaM binding to rhodopsin is also detected with this method, and this CaM binding is inhibited by the presence of Ca2+. With the bifunctional cross-linking reagent, DTSSP, three discrete proteins with molecular weights of 240,000, 53,000, and 47,000 are detected in the native ROS membrane fraction. CaM binding to rhodopsin is not detected with this method. Moreover, while the Mr 140,000 band is not detected with DTSSP, a smeared band with a molecular weight between 78,000 and 93,000 is identified (with DTSSP) in the ROS membrane fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Active labeling of phosphatidylcholines by [1-14C]docosahexaenoate in isolated photoreceptor membranes

Isolated bovine rod outer segments and photoreceptor disks actively incorporated [1-14C]docosahexaenoate (22:6) into phospholipids when incubated in the presence of CoA, ATP, and Mg2+. About 80% of the esterified fatty acid was in phosphatidylcholine (PC). Microsomal and mitochondrial fractions incorporated as much 22:6 as rod outer segments, but it was distributed among various phospholipids and neutral glycerides. The isolated photoreceptor membrane thus contains an acyl-CoA synthetase which activates the fatty acid and a docosahexaenoyl-CoA-lysophosphatidylcholine acyltransferase activity. The specific radioactivity of PC was higher in rod outer segments than in the other subcellular fractions. About 2/3 of the label in photoreceptor membrane PC was in its dipolyunsaturated molecular species and 1/3 in hexaenes. Dipolyunsaturated PCs showed high turnover rates of 22:6 in all three subcellular membranes, especially in mitochondria. Retinal membranes in vitro seem to take up free [14C]22:6 from the medium by simple diffusion or partition into the membrane lipid. The ability of these membranes to activate and esterify [1-14C]22:6 indicates that docosahexaenoate-containing molecular species of retina lipids, including those of photoreceptor membranes, are subject to acylation-deacylation reactions in situ.

Permeability of rod outer segment disk membranes as probed by Ficoll density gradient centrifugation and by turbidimetry

Osmotic volume changes of isolated disks prepared from bovine rod outer segments were investigated utilizing the capacity of osmotically intact disks to float on 2.5% (w/v) Ficoll under hypotonic conditions. The amount of floating disks decreases steadily and approximately linearly when the osmolarity is increased stepwise with an osmotically active solute. The slope of the decrease becomes smaller for partially permeable substances and no decrease in observed for fully permeable solutes. All ions assayed by this method as well as sucrose were osmotically fully active to disk membranes. The buffer molecules Tris and imidazole partially penetrated the disk membranes at pH 7, whereas the zwitterionic buffer Hepes was impermeable. Ethanol showed absolutely no osmotic activity to disk membranes and, therefore, can easily penetrate these membranes. concurrent turbidimetric measurements and, therefore, can easily penetrate these membranes. Concurrent turbidimetric measurements confirmed these results. Upon prolonged exposure of disk membranes to 5% (w/v) Ficoll, a considerable modification of the osmotic properties of disk membranes is observed, indicating a significant interaction of 5% (w/v) Ficoll with disk membranes. No influence of 2.5% (w/v) Ficoll on the osmotic properties of disk membranes was detected. The restriction of the use of Ficoll for disk membrane preparative purposes will be discussed.