Retinal Rod Outer Segment Disk Research Articles

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

PurposeThe 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.ProceduresCharacterization 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.ResultsWeighted 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.ConclusionsThe 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.

Modeling of phototransduction processes in the photoreceptor disk membranes by the Monte Carlo method

The Monte Carlo method was used to model the diffusion behaviors of functionally important proteins of the phototransduction system in retinal rod outer segment disk membranes. The results expand our knowledge of the mechanisms of inactivation of the main phototransduction heterotrimeric GTP-binding protein transducin.

Immunochemical or fluorescent labeling of vesicular subcellular fractions for microscopy imaging

We describe a procedure for the labeling of membranous vesicular purified subcellular fractions, to image them, typically by confocal laser scanning microscopy. Being intracellular organelles, these fractions, once purified cannot be attached to glass slides as for cells. Fractions are labeled "in batch" without prior embedding or freezing. Each labeling step performed by passages of resuspension/centrifugation is followed by washings. Then samples are dispersed on the glass slides. Mammalian retinal rod outer segment disks, intact brain stem myelin vesicles, and brain synaptosomes were chosen, as these subcellular fractions can be purified by well established procedures. These fractions were immunolabeled with specific antibodies. Moreover, by the earlier procedure, we show that the mitochondrial vital membrane potential probe MitoTracker Deep Red 633 stains myelin vesicles and rod disks before fixation, consistently with our previous reports of a respiring capacity of these membranes. Therefore, the technique seems adequate to become an instrument to study the structure and the function of these and other subcellular fractions.

Cytochrome C oxidase expression in retinal rod outer segment disks

Abstract Purpose The rod outer segment (ROS) plays a pivotal role in the phototransduction cascade, a process which requires a high level of energy consumption. In rods ATP production is confined to the mitochondria in the inner part, and the distances between sites of energy production and expenditure do not seem adequate to supply phototransduction. The goal of this study was to test the hypothesis that Cytochrome c oxydase (Cox) is located and catalytically active in ROS disk membranes. Methods Osmotically intact disks were isolated from 20 bovine retinal ROS obtained by sucrose gradient centrifugation in the presence of protease inhibitor cocktail and Ampicillin under dim red light at 4°C. Cox expression was studied by confocal laser scanning microscopy and semiquantitative Western‐immunoblotting, with a specific antibody. Cox activity in disks was investigated spectrophotometrically. Results Results from immunohystochemical confocal laser scanning microscopy, Western blot, and biochemical experiments suggest that Cox is uniformly distributed onto disks and therein catalytically active to an extent comparable to that of isolated mitochondria. Conclusion The expression and activity of Cox in bovine ROS disks is suggestive of an aerobic metabolism in ROS. This study opens new scenarios on the energy production in rods and on the possible role of its deficiency in some important retinal diseases which have not been clarified yet.

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 1F 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 μmol 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.

Rhodopsin Activity Varies in Proteoliposomes Prepared by Different Techniques

A variety of techniques are currently in use for preparing protein-containing lipid vesicles known as proteoliposomes. However, the functionality of membrane protein in proteoliposomes prepared by various techniques has rarely been evaluated directly. We prepared rhodopsin-containing proteoliposomes consisting of asolectin or native retinal rod outer segment disk lipids using n-octyl beta-d-glucopyranoside and the detergent dialysis (DD) and rapid dilution (RD) techniques and measured the activity of rhodopsin using equilibrium UV/vis and flash photolysis spectroscopy. A significant difference in rhodopsin activity was observed in proteoliposomes prepared by these techniques. The equilibrium constant of metarhodopsin I-metarhodopsin II is 30-45% higher, and the apparent rate constant of MII formation is up to 3-fold faster in proteoliposomes prepared by RD vs DD. The DD technique produced larger yet more heterogeneous vesicles, while the RD technique yielded smaller and more homogeneous vesicles, as determined by electron microscopy and isopicnic centrifugation. Both proteoliposomes and empty lipid vesicles lacking rhodopsin were formed in the DD preparation, while only proteoliposomes were formed in the RD preparation. Under identical conditions, proteoliposomes prepared by RD have a higher L/P ratio, which is consistent with the higher level of rhodopsin activity in RD proteoliposomes. Overall, the results presented here suggest that the RD technique has an advantage over the DD technique in terms of preserving optimal rhodopsin activity and controlling the lipid to protein ratio in the final proteoliposomes.

Calorimetric Studies of Bovine Rod Outer Segment Disk Membranes Support a Monomeric Unit for Both Rhodopsin and Opsin

The photoreceptor rhodopsin is a G-protein coupled receptor that has recently been proposed to exist as a dimer or higher order oligomer, in contrast to the previously described monomer, in retinal rod outer segment disk membranes. Rhodopsin exhibits considerably greater thermal stability than opsin (the bleached form of the receptor), which is reflected in an ∼15°C difference in the thermal denaturation temperatures (Tm) of rhodopsin and opsin as measured by differential scanning calorimetry. Here we use differential scanning calorimetry to investigate the effect of partial bleaching of disk membranes on the Tm of rhodopsin and of opsin in native disk membranes, as well as in cross-linked disk membranes in which rhodopsin dimers are known to be present. The Tms of rhodopsin and opsin are expected to be perturbed if mixed oligomers are present. The Tm remained constant for rhodopsin and opsin in native disks regardless of the level of bleaching. In contrast, the Tm of cross-linked rhodopsin in disk membranes was dependent on the extent of bleaching. The energy of activation for denaturation of rhodopsin and cross-linked rhodopsin was calculated. Cross-linking rhodopsin significantly decreased the energy of activation. We conclude that in native disk membranes, rhodopsin behaves predominantly as a monomer.

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.

Peripherin-2: An Intracellular Analogy to Viral Fusion Proteins

The C-terminus of the intracellular retinal rod outer segment disk protein peripherin-2 binds to membranes, adopts a helical conformation, and promotes membrane fusion, which suggests an analogy to the structure and function of viral envelope fusion proteins. Nuclear magnetic resonance (NMR) data and fluorescence data show that a 63-residue polypeptide comprising the C-terminus of bovine peripherin-2 (R284-G346) binds to the membrane mimetic, dodecylphosphocholine micelles. High-resolution NMR studies reveal that although this C-terminal fragment is unstructured in solution, the same fragment adopts helical structure when bound to the micelles. The C-terminus may be a member of the class of intrinsically unstructured protein domains. Using methods developed for the G-protein coupled receptor rhodopsin, a model for the structure of the transmembrane domain of peripherin-2 was constructed. Previously published data showed that both peripherin-2 and viral fusion proteins are transmembrane proteins that promote membrane fusion and have a fusion peptide sequence within the protein that independently promotes membrane fusion. Furthermore, the fusion-active sequence of peripherin-2 exhibits a sequence motif that matches the viral fusion peptide of influenza hemagglutinin (HA). These observations collectively suggest that the mechanism of intracellular membrane fusion induced by peripherin-2 and the mechanism of enveloped viral fusion may have features in common.

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.

High reolution structural studies on peripherin, an intergal membrane protein

Peripherin is a tetraspannin transmembrane protein localized in retinal rod outer segment disk membranes whose complete 3-dimensional structure is the subject of this nuclear magnetic resonance (NMR) analysis. Peripherin promotes membrane fusion and may be involved in fusion events with rod cell membranes during the rod outer segment disk renewal processes. The 63-residue C-terminal extramembraneous region of peripherin (PERCTER) is crucial to peripherin-mediated fusion and contains both an incisure affinity region and an OS targeting signal. Several mutations in this C-terminal region have been linked to various forms of retinal degenerative disease. The first target of our structural analysis has been the PERCTER, expressed and labeled with 15N, and subjected to structural determination by solution NMR. For this analysis, 3-dimensional HSQC-NOESY and HSQC-TOCSY experiments were utilized, along with 2-dimensional 1H NOESY and TOCSY experiments at various mixing times. This analysis is followed by a similar study of the PERCTER in the presence of a membrane-mimetic, DPC micelles. The next stage of our examination is to obtain NMR data from the isotope-labeled full length peripherin. Detailed understanding of the peripherin structure provides greater insight into retinal degenerative diseases and the mechanisms of membrane fusion. NIH EY10420-14, E. Matilda Ziegler Vision Award.

1H and 13C NMR of Multilamellar Dispersions of Polyunsaturated (22:6) Phospholipids

The polyunsaturated fatty acid docosahexaenoic acid (DHA) makes up ∼50% of the lipid chains in the retinal rod outer segment disk membranes and a large fraction of the lipid chains in the membranes of neuronal tissues. There is an extensive literature concerned with the dietary requirements for essential fatty acids and the importance of DHA to human health, but relatively little research has been done on the physical properties of this important molecule. Using 1H and 13C MAS NMR measurements of dispersions of 1-palmitoyl-2-docosahexaenoyl-phosphatidylcholine in excess phosphate buffer, we have unambiguously assigned most of the resonances in both the 1H and 13C NMR spectra. We were able to use cross-polarization spectroscopy to follow the transfer of polarization from specific 1H nuclei not only to their directly bonded 13C but also to those 13C that are in close proximity, even though they are not directly bonded. Cross-peaks in two-dimensional cross-polarization spectra revealed a close association between the choline headgroup and at least part of the DHA chain but not with the palmitate chain. Finally, we examined the dynamics of the different parts of this lipid molecule, using rotating frame spin-lattice relaxation measurements, and found that methylene groups of both chains experience important motions with correlation times in the 10- μs range, with those for the palmitate chain being ∼50% longer than those of the DHA chain. The choline headgroup and the chain terminal groups have significantly shorter correlation times, and that part of the dipolar interaction that is fluctuating at these correlation times is significantly smaller for these groups than it is for the palmitate and DHA chain methylenes.

Rhodopsin-cholesterol interactions in bovine rod outer segment disk membranes

Cholesterol modulates the function of rhodopsin in the retinal rod outer segment (ROS) disk membranes. One mechanism for such modulation is cholesterol modulation of the properties of the membrane bilayer. This has been explored previously. Another possible mechanism is an interaction between the sterol and the protein, which has not been previously explored. In this study, the fluorescent sterol, cholestatrienol, was used to probe interactions between cholesterol and rhodopsin in bovine ROS disk membranes. Cholestatrienol was incorporated into the disk membranes by exchange from donor phospholipid vesicles. Fluorescence energy transfer from protein tryptophans to cholestatrienol was observed indicating close approach of this fluorescent sterol to the tryptophan. The effectiveness of the energy transfer was measured by the quenching of tryptophan fluorescence by cholestatrienol. The quenching of tryptophan fluorescence was directly related to the cholestatrienol content of the membranes. Cholesterol was incorporated into the disk membranes by exchange from donor phospholipid vesicles. The effect of increasing membrane cholesterol on the ability of cholestatrienol to quench rhodopsin tryptophan fluorescence was determined. This quenching was inversely proportional to the membrane cholesterol content. Furthermore the observed quenching was greater than could be explained by a simple dilution of the cholestatrienol by the addition of cholesterol to the membrane. These data suggest an interaction between the sterol and the protein. The specificity of this interaction was explored by the addition of ergosterol, instead of cholesterol, to the disk membranes. Ergosterol was not able to inhibit the quenching of protein tryptophans beyond that due to dilution of the cholestatrienol by addition of ergosterol to the membrane. The ability of cholesterol to compete with cholestatrienol for that interaction suggests a ‘site’ at which cholesterol contacts rhodopsin. The inability of ergosterol to compete with cholestatrienol for this ‘site’ suggested that the site was specific for the structure of cholesterol.

Differential membrane protein phosphorylation in bovine retinal rod outer segment disk membranes as a function of disk age.

The outer segment portion of photoreceptor rod cells is composed of a stacked array of disk membranes. Newly formed disks are found at the base of the rod outer segment (ROS) and are relatively high in membrane cholesterol. Older disks are found at the apical tip of the ROS and are low in membrane cholesterol. Disk membranes were separated based on their membrane cholesterol content and the extent of membrane protein phosphorylation determined. Light induced phosphorylation of ROS disk membrane proteins was investigated using magic angle spinning 31P NMR. When intact rod outer segment preparations were stimulated by light, in the presence of endogenously available kinases, membrane proteins located in disks at the base of the ROS were more heavily phosphorylated than those at the tip. SDS-gel electrophoresis of the phosphorylated disk membranes subpopulations identified a phosphoprotein species with a molecular weight of approximately 68-72 kDa that was more heavily phosphorylated in newly formed disks than in old disks. The identity of this phosphoprotein is presently under investigation. When the phosphorylation reaction was carried out in isolated disk membrane preparations with exogenously added co-factors and kinases, there was no preferential protein phosphorylation. Taken collectively, these results suggest that within the ROS there is a protein phosphorylation gradient that maybe indicative of co-factor or kinase heterogeneity.

On the nature of linkage between retinal rod outer-segment disk and cellular membrane

In the retina of most vertebrates the cone outer segment disk are in continuity with the cell membrane, thus intradisk space is part of the extracellular space. The rod disk membranes, except for some basal ones, appear to be separated from the cell membrane. On the other hand it has been shown the existence of filament-like structures connecting retinal rod disks to one another and with the cell membrane. However due to the lack of observation of either electron-dense or fluorescent extracellular tracer penetration into rod outer segments, it is believed that rod intradisk space is isolated from the extracellular space. This investigation elucidates upon the nature of filament-like linkage between retinal rod outer segment disk and cell membrane. An incubation solution with 10 times lower than normal osmolarity was used to osmotically explode isolated dark adapted frog retina.Membranes under this condition were observed by transmission electron microscopy (TEM) using the freeze-fracture replica technique. Regularly spaced filaments were observed that were continuous between outer segment disks and the cell membrane in the retinal rods (Fig. 1). Probably these connections have a supporting function providing mechanical stability of the rod outer segment structure.

Lipid headgroup and acyl chain composition modulate the MI-MII equilibrium of rhodopsin in recombinant membranes

A current paradigm for visual function centers on the metarhodopsin I (MI) to metarhodopsin II (MII) conformational transition as the trigger for an intracellular enzyme cascade leading to excitation of the retinal rod. We investigated the influences of the membrane lipid composition on this key triggering event in visual signal transduction using flash photolysis techniques. Bovine rhodopsin was combined with various phospholipids to form membrane recombinants in which the lipid acyl chain composition was held constant at that of egg phosphatidylcholine (PC), while the identity of the lipid headgroups was varied. The ratio of MII/MI produced in these recombinants by an actinic flash at 28 degrees C was studied as a function of pH. The results were compared to the photochemical function observed for rhodopsin in native retinal rod outer segment (ROS) membranes, in total native ROS lipid recombinants, and in dimyristoylphosphatidylcholine (DMPC) recombinants. In membrane recombinants incorporating lipids derived from egg PC, as well as in the total ROS lipids control and the native ROS disk membranes, MI and MII were found to coexist in a pH-dependent, acid-base equilibrium on the millisecond time scale. The recombinants of rhodopsin with egg PC, either alone or in combination with egg PC-derived phosphatidylethanolamine (PE) or phosphatidylserine (PS), exhibited substantially reduced photochemical activity at pH 7.0. However, all recombinants comprising phospholipids with unsaturated acyl chains were capable of full native-like MII production at pH 5.0, confirming previous results [Gibson, N.J.. & Brown, M.F. (1990) Biochem. Biophys. Res. Commun. 169, 1028-1034]. It follows that energetic constraints on the MI and MII states imposed by egg PC-derived acyl chains can be offset by increased activity of H+ ions. The data reveal that the major effect of the membrane lipid composition is to alter the apparent pK for the MI-MII conformational equilibrium of rhodopsin [Gibson, N.J., & Brown, M.F. (1991) Biochem. Biophys. Res. Commun. 176, 915-921]. Recombinants containing only phosphocholine headgroups exhibited the lowest apparent pK values, whereas the presence of either 50 mol % PE or 15 mol % PS increased the apparent pK. The inability to obtain full native-like function in recombinants having egg PC-derived chains and a native-like headgroup composition indicates a significant role of the polyunsaturated docosahexaenoic acid (DHA) chains (22:6 omega 3) of the native retinal rod membrane lipids. Temperature studies of the MI-MII transition enabled an investigation of lipid influences on the thermodynamic parameters of a membrane protein conformational change linked directly to function.(ABSTRACT TRUNCATED AT 400 WORDS)

MEMBRANE LIPID INFLUENCES ON THE ENERGETICS OF THE METARHODOPSIN I and METARHODOPSIN II CONFORMATIONAL STATES OF RHODOPSIN PROBED BY FLASH PHOTOLYSIS*

We have investigated the relationship between rhodopsin photochemical function and the retinal rod outer segment (ROS) disk membrane lipid composition using flash photolysis techniques. Bovine rhodopsin was combined with various phospholipids to form recombinant membrane vesicles, in which the lipid acyl chain composition was maintained at that of egg phosphatidylcholine (PC), while the nature of the headgroups was varied. The ratio of metarhodopsin II (MII)/metarhodopsin I (MI) in these recombinants produced by an actinic flash was investigated as a function of pH, and compared with the photochemical activity observed for rhodopsin in native ROS membranes and dimyristoylphosphatidylcholine recombinants. In recombinants made with lipids derived from egg PC, as well as in native ROS membranes, MI and MII were found to be present in a pH-dependent, acid-base equilibrium on the millisecond timescale. The recombinants made with phospholipids containing unsaturated acyl chains were capable of full native-like MII production, but each demonstrated a titration curve with a different pK. In addition, some of the recombinants exhibited apparent deviations from the Henderson-Hasselbalch curve shape. The presence of either phosphatidylethanolamine (PE) or phosphatidylserine (PS) headgroups appeared to increase the amount of MII produced. This may result from alteration of the curvature free energy, in the case of PE, and from the influence of the membrane surface potential in the case of PS. An investigation of the effects of temperature on the MI-MII transition in native ROS membranes and the recombinants was also carried out.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional equivalence of metarhodopsin II and the Gt-activating form of photolyzed bovine rhodopsin

Absorption of a photon by the visual pigment rhodopsin leads to the formation of an activated conformational state, denoted rho*, which is capable of activating the visual G-protein, Gt. The bleaching of rhodopsin can be resolved into a series of spectrally distinct photointermediates. Previous studies suggest that the photointermediate metarhodopsin II (meta II, lambda max of 380 nm) corresponds to the physiologically active form rho*. In the studies reported herein, spectral and enzymological data were analyzed and compared so as to evaluate the temporal correspondence between meta II and rho*. This information was obtained by direct observation of the meta II and rho* decay times in parallel experiments utilizing identical preparations of urea-stripped, bovine retinal rod outer segment disk membranes at pH 8.0, 20 degrees C. Postflash spectra were deconvolved to resolve the meta II absorbance at 380 nm, and a decay time for the loss of meta II of 8.2 min (SD = 0.5 min) was obtained from fitting these data to a single-exponential decay process. The diminishing ability of bleached rhodopsin to activate Gt was measured by monitoring the level of catalyzed exchange of Gt-bound GDP for a nonhydrolyzable GTP analogue. Analysis of the decrease in the initial velocity of nucleotide exchange, measured at various postflash incubation times, yielded a rho* decay time of 7.7 min (SD = 0.5 min) when analyzed as a single-exponential process. The similarity of these decay times provides direct evidence that meta II and rho* are present over the same time regime, and further supports the equivalence of these two forms of photoactivated rhodopsin.(ABSTRACT TRUNCATED AT 250 WORDS)

Patterns of interdisk connections within the lamellar domains of retinal rod outer segment disks: Observations relevant to the axial propagation of incisures

In rod outer segments (ROSs) of the Congo eel salamander (Amphiuma), we have observed a system of cytoplasmic filaments that interconnect the lamellar domains of adjacent disks. In longitudinal sections, these filaments occur in pairs, spaced 18-20 nm apart. Each filament is 9-11 nm in length, and perpendicular to the disk membrane. Such filament pairs display extended axial alignment through the lamellar domains of successive disks, indicating the involvement of trans-membranous elements. In cross-sectional views, these filaments appear as a row of paired, punctate densities with a periodic spacing of 14-16 nm. Such views indicate that these filaments are organized on a two-dimensional crystalline lattice, oriented perpendicular to the disk lamellae. The linear interdisk densities of the terminal loop complex appear to be identical to these cytoplasmic filaments: both are equal in length, and both are organized as two-dimensional assemblies with equivalent orientations and lattice parameters. These shared features suggest a common subset of molecular components and lattice determinants. Where the paired filament assemblies intersect the disks, one observes a characteristic dilatation of the intradisk space. In these regions, the disk is 3-6 nm wider than adjacent lamellar regions, and tends to approximate the width of the terminal loop expansion along the disk perimeter. Within such dilatations, we have not been able to demonstrate crescentic densities corresponding to those found within the terminal loop region. Paired filament arrays are not randomly located within the lamellar domains, but lie parallel to the local disk perimeter. Both single and multiple arrays are commonly observed. Single arrays are recessed about 80 nm from the disk edge. When more than one array is present, the additional arrays lie parallel to the first and are spaced at intervals of about 70 nm. Thus, in three dimensions, the multiple arrays appear as regularly spaced sheets of filaments, parallel to the local disk perimeter and perpendicular to the lamellar domains. The characteristic interval between such filament arrays suggests the presence of 'spacer' molecules that organize the two-dimensional filament lattices in the third dimension. Such a spacing mechanism may also account for the minimum width observed for disk lobule segments defined by extended parallel incisures. The potential role of paired filament arrays in the axial propagation of incisures is discussed.