Glycoproteins specific for the retinal rod outer segment plasma membrane

Two ricin-specific glycoproteins have been identified on neuraminidase-treated rod outer segment plasma membranes of bovine retinal photoreceptor cells. Ricin-gold-dextran particles were observed by electron microscopy to densely label the surface of neuraminidase-treated rod outer segments. Western blotting of proteins separated by SDS-gel electrophoresis indicated that two ricin-binding glycoproteins of Mr 230,000 and 110,000 are specific for the plasma membrane and are not found in disk membranes. These glycoproteins can serve as specific probes for the purification of the rod outer segment plasma membrane.     

Glyceraldehyde-3-phosphate dehydrogenase is a major protein associated with the plasma membrane of retinal photoreceptor outer segments

A major 38-kDa protein associated with bovine rod outer segment plasma membranes, but not disk membranes, has been identified as glyceraldehyde-3-phosphate dehydrogenase on the basis of its N-terminal sequence and specific enzyme activity. This enzyme was extracted from lysed rod outer segments or isolated rod outer segment plasma membrane with 0.15 M NaCl and purified to homogeneity by affinity chromatography on a NAD(+)-agarose column. A specific activity of 90-100 units/mg of protein is within the range of activity obtained for glyceraldehyde-3-phosphate dehydrogenase isolated from other mammalian cells. Enzyme activity measurements indicate that this enzyme makes up approximately 2% of the total rod outer segment protein and over 11% of the plasma membrane protein. Protease digestion and binding studies on purified rod outer segment plasma and disk membranes suggest that glyceraldehyde-3-phosphate dehydrogenase reversibly interacts with a protease-sensitive plasma membrane-specific protein of rod outer segments. The finding that glyceraldehyde-3-phosphate dehydrogenase is present in large quantities in rod outer segments suggests that at least some of the energy required for the synthesis of ATP and GTP for phototransduction and other processes of the outer segment is derived from glycolysis which takes place within this organelle.     

Identification of the sodium-calcium exchanger as the major ricin-binding glycoprotein of bovine rod outer segments and its localization to the plasma membrane

After neuraminidase treatment the Na+/Ca2+ exchanger of bovine rod outer segments was found to specifically bind Ricinus communis agglutinin. SDS gel electrophoresis and Western blotting of ricin-binding proteins purified from rod outer segment membranes by lectin affinity chromatography revealed the existence of two major polypeptides of Mr 215K and 103K, the former of which was found to specifically react with PMe 1B3, a monoclonal antibody specific for the 230-kDa non-neuraminidase-treated Na+/Ca2+ exchanger. Reconstitution of the ricin affinity-purified exchanger into calcium-containing liposomes revealed that neuraminidase treatment had no significant effect on the kinetics of Na+/Ca2+ exchange activation by sodium. We further investigated the density of the Na+/Ca2+ exchanger in disk and plasma membrane preparations using Western blotting, radioimmunoassays, immunoelectron microscopy, and reconstitution procedures. The results indicate that the Na+/Ca2+ exchanger is localized in the rod photoreceptor plasma membrane and is absent or present in extremely low concentrations in disk membranes, as we have previously shown to be the case for the cGMP-gated cation channel. Previous reports describing the existence of Na+/Ca2+ exchange activity in rod outer segment disk membrane preparations may be due to the fusion of plasma membrane components and/or the presence of contaminating plasma membrane vesicles.     

Ultrastructural localization of rhodopsin in the vertebrate retina

Early work by Dewey and collaborators has shown the distribution of rhodopsin in the frog retina. We have repeated these experiments on cow and mouse eyes using antibodies specific to rhodopsin alone. Bovine rhodopsin in emulphogene was purified on an hydroxyapatite column. The purity of this reagent was established by spectrophotometric criteria, by sodium dodecyl sulfate (SDS) gel electrophoresis, and by isoelectric focusing. This rhodopsin was used as an immunoadsorbent to isolate specific antibodies from the antisera of rabbits immunized with bovine rod outer segments solubilized in 2% digitonin. The antibody so prepared was shown by immunoelectrophoresis to be in the IgG class and did not cross-react with lipid extracts of bovine rod outer segments. Papain-digested univalent antibodies (Fab) coupled with peroxidase were used to label rhodopsin in formaldehyde-fixed bovine and murine retinas. In addition to the disk membranes, the plasma membrane of the outer segment, the connecting cilium, and part of the rod inner segment membrane were labeled. We observed staining on both sides of the rod outer segment plasma membrane and the disk membrane. Discrepancies were observed between results of immunolabeling experiments and observations of membrane particles seen in freeze-cleaved specimens. Our experiments indicate that the distribution of membrane particles in freeze cleaving experiments reflects the distribution of membrane proteins. Immunolabeling, on the other hand, can introduce several different types of artifact, unless controlled with extreme care.     

In vitro translation and processing of rat kidney gamma-glutamyl transpeptidase

Rat kidney gamma-glutamyl transpeptidase is composed of two nonidentical glycosylated subunits. The enzyme is localized on the lumenal surface of the brush-border membranes of proximal tubule epithelial cells; it is attached to the membranes via an NH2-terminal segment of the larger of the two subunits. Tissue-labeling experiments followed by immunoprecipitation with antibodies directed against the enzyme and its two subunits demonstrate that a glycosylated single chain precursor (Mr = 78,000), containing the elements of both the subunits, is initially synthesized. Pulse-chase studies in the presence of pactamycin, and inhibitor of protein synthesis initiation, indicate that the larger of the two subunits is located at the NH2 terminus of the Mr = 78,000 precursor. The initial events in the biosynthesis and processing of gamma-glutamyl transpeptidase were investigated by in vitro translation of rat kidney mRNA. Such translation results in the synthesis of a Mr = 63,000 unglycosylated polypeptide which has been shown immunologically to contain the domains for both subunits. The Mr = 63,000 species is processed to a Mr = 78,000 core-glycosylated polypeptide when translation of mRNA is carried out in the presence of dog pancreas microsomes. This processing does not appear to be associated with cleavage of an NH2-terminal leader sequence. The Mr = 78,000 polypeptide is integrated into the microsomal membranes with an orientation that is analogous to that found on the brush-border membranes. Glycosylation and membrane integration of transpeptidase are cotranslational events. Upon longer incubation, the Mr = 78,000 species sequestered within the microsomal vesicles is cleaved to species corresponding in size to the two subunits of the kidney enzyme.     

Glycolytic enzymes and a GLUT-1 glucose transporter in the outer segments of rod and cone photoreceptor cells

The presence of glycolytic enzymes and a GLUT-1-type glucose transporter in rod and cone outer segments was determined by enzyme activity assays, glucose uptake measurements, Western blotting, and immunofluorescence microscopy. Enzyme activities of six glycolytic enzymes including hexokinase, phosphofructokinase, aldolase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, pyruvate kinase, and lactate dehydrogenase, were found to be present in purified rod outer segment (ROS) preparations. Immunofluorescence microscopy of bovine and chicken retina sections labeled with monoclonal antibodies against glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, and lactate dehydrogenase have confirmed that these enzymes are present in rod and cone outer segments and not simply contaminants from the inner segments or other cells. Rod outer segments were also found to contain glucose transport activity as detected by 3-O-[14C]methylglucose uptake and exchange. The glucose transporter had a Km of 6.3 mM and a Vmax of 0.15 nmol of 3-O-methylglucose/s/mg of ROS membrane protein for net uptake and a Km of 29 mM and a Vmax of 1.06 nmol of 3-O-methylglucose/s/mg of ROS membrane protein for equilibrium exchange. These Km values for net uptake and equilibrium exchange are similar to values obtained for human red blood cells and are characteristic of GLUT-1-type glucose transporter. The transport was inhibited by both cytochalasin B and phloretin. Western blot analysis and immunofluorescence microscopy using type-specific glucose transporter antibodies indicated that both rod and cone outer segment plasma membranes have a GLUT-1 glucose transporter of Mr 45K as found in red blood cells and brain microsomal membranes. Solid-phase radioimmune competitive inhibition studies indicated that rod outer segment plasma membranes contained 15% the number of glucose transporters found in human red blood cell membranes and had an estimated density of 400 glucose transporter per micron2 of plasma membrane. These studies support the view that outer segments can generate energy in the form of ATP and GTP by anaerobic glycolysis to supply at least some of the energy requirements for phototransduction and other metabolic processes.     

The cGMP-gated channel and related glutamic acid-rich proteins interact with peripherin-2 at the rim region of rod photoreceptor disc membranes

The rod cGMP-gated channel is localized in the plasma membrane of rod photoreceptor outer segments, where it plays a central role in phototransduction. It consists of alpha- and beta-subunits that assemble into a heterotetrameric protein. Each subunit contains structural features characteristic of nucleotide-gated channels, including a cGMP-binding domain, multiple membrane-spanning segments, and a pore region. In addition, the beta-subunit has a large glutamic acid- and proline-rich region called GARP that is also expressed as two soluble protein variants. Using monoclonal antibodies in conjunction with immunoprecipitation, cross-linking, and electrophoretic techniques, we show that the cGMP-gated channel associates with the Na/Ca-K exchanger in the rod outer segment plasma membrane. This complex and soluble GARP proteins also interact with peripherin-2 oligomers in the rim region of outer segment disc membranes. These results suggest that channel/peripherin protein interactions mediated by the GARP part of the channel beta-subunit play a role in connecting the rim region of discs to the plasma membrane and in anchoring the channel.exchanger complex in the rod outer segment plasma membrane.     

The cGMP-gated cation channel of bovine rod photoreceptor cells is associated with a 240-kDa protein exhibiting immunochemical cross-reactivity with spectrin.

A 240-kDa protein exhibiting immunochemical cross-reactivity with red blood cell spectrin has been shown to be directly associated with the 63-kDa cGMP-gated channel of bovine rod outer segments. When detergent-solubilized, chromatographically purified channel preparations were treated with Sepharose beads coupled to either an anti-240-kDa monoclonal antibody (PMs 4B2) or an anti-63-kDa channel monoclonal antibody (PMc 1D1), both the 240-kDa protein and the 63-kDa channel protein were concomitantly immunoprecipitated as analyzed by Western blotting of sodium dodecyl sulfate gels. Both of these antibody-Sepharose matrices also removed cGMP-gated channel activity as measured by functional reconstitution. In control studies anti-rhodopsin monoclonal antibody (Rho 1D4)-Sepharose beads removed residual rhodopsin, but not the 63/240-kDa complex or channel activity. Western blotting of purified rod outer segment disk and plasma membrane fractions and immunogold-dextran labeling of lysed rod outer segments indicated that the 240-kDa polypeptide, like the 63-kDa channel, is preferentially localized to the plasma membrane as visualized by electron microscopy. The 240-kDa protein does not appear to be directly involved in the cGMP-gated channel activity, but it may be part of a cytoskeletal system that serves to maintain the organization of the 63-kDa channel complex within the rod outer segment plasma membrane.     

Localization of peripherin/rds in the disk membranes of cone and rod photoreceptors: relationship to disk membrane morphogenesis and retinal degeneration

The outer segments of vertebrate rod photoreceptor cells consist of an ordered stack of membrane disks, which, except for a few nascent disks at the base of the outer segment, is surrounded by a separate plasma membrane. Previous studies indicate that the protein, peripherin or peripherin/rds, is localized along the rim of mature disks of rod outer segments. A mutation in the gene for this protein has been reported to be responsible for retinal degeneration in the rds mouse. In the present study, we have shown by immunogold labeling of rat and ground squirrel retinas that peripherin/rds is present in the disk rims of cone outer segments as well as rod outer segments. Additionally, in the basal regions of rod and cone outer segments, where disk morphogenesis occurs, we have found that the distribution of peripherin/rds is restricted to a region that is adjacent to the cilium. Extension of its distribution from the cilium coincides with the formation of the disk rim. These results support the model of disk membrane morphogenesis that predicts rim formation to be a second stage of growth, after the first stage in which the ciliary plasma membrane evaginates to form open nascent disks. The results also indicate how the proteins of the outer segment plasma membrane and the disk membranes are sorted into their separate domains: different sets of proteins may be incorporated into membrane outgrowths during different growth stages of disk morphogenesis. Finally, the presence of peripherin/rds protein in both cone and rod outer segment disks, together with the phenotype of the rds mouse, which is characterized by the failure of both rod and cone outer segment formation, suggest that the same rds gene is expressed in both types of photoreceptor cells.     

Solubilization, purification, and reconstitution of the sodium-calcium exchanger from bovine retinal rod outer segments

We have previously described a method for the solubilization and reconstitution of the cGMP-gated cation channel from the membranes of bovine rod outer segments (Cook, N. J., Zeilinger, C., Koch, K.-W., and Kaupp, U. B. (1986) J. Biol. Chem. 261, 17033-17039). Here we report that not only cGMP but also sodium is capable of releasing entrapped calcium from liposomes reconstituted with total rod outer segment membrane proteins. Other alkali cations tested were unable to induce calcium efflux; therefore, we concluded that the sodium-induced calcium efflux was due to the sodium-calcium exchanger. Sodium was found to activate calcium efflux from these liposomes with an EC50 of approximately equal to 35 mM, comparable to values reported for the sodium-calcium exchanger in native rod outer segment membranes. We found that reconstitution of the sodium-calcium exchanger is quantitative and used this method to assay the exchange protein during purification using conventional protein chromatographic techniques. In this way, we were able to purify and identify as the rod outer segment sodium-calcium exchanger a glycoprotein of apparent Mr = 220,000 to greater than 90% homogeneity. The specific activity of the purified protein at room temperature was 8.2 mumol of Ca2+ exchanged min-1 mg-1 of protein at 50 mM Na+, corresponding to a turnover number of approximately equal to 30 Ca2+ (or 90 Na+) s-1 exchanger-1. The Mr = 220,000 protein reported here appears to be distinct from another protein ("rim protein") with an identical Mr known to exist in these membranes.     

In vitro isoprenylation and membrane association of mouse rod photoreceptor cGMP phosphodiesterase alpha and beta subunits expressed in bacteria.

We investigated the specificity of CAAX box-related isoprenylation of rod photoreceptor cGMP phosphodiesterase (PDE) subunits expressed in bacteria and the consequences of this modification on rod disk membrane association. Full-length cDNA sequences of the alpha and beta subunits of mouse PDE, inserted into bacterial pET expression vectors, were overexpressed as fusion proteins containing 28 (bMP-alpha) and 26 (bMP-beta) additional amino acid residues at their N termini. Both fusion proteins were overexpressed and stored in inclusion bodies. Purified bMP-alpha and bMP-beta were recognized by bovine PDE-specific polyclonal antibodies, but did not associate with depleted rod disk membranes and were catalytically inactive. Using bovine brain or retina extracts as sources of protein prenyltransferases and tritiated farnesyl- or geranylgeranylpyrophosphate as donors, bMP-alpha (CAAX sequence CCIQ) was exclusively farnesylated, and bMP-beta (CAAX sequence CCIL) was exclusively geranylgeranylated. After isoprenylation, bMP-alpha and bMP-beta each associated with rod photoreceptor outer segment disk membranes under isotonic, but not under hypotonic, conditions. The results indicate that isoprenylated bMP-alpha and bMP-beta each interact independently with membranes and that isoprenylation is the key modification that facilitates membrane association.     

Immunocytochemical localization of opsin in outer segments and Golgi zones of frog photoreceptor cells. An electron microscope analysis of cross-linked albumin-embedded retinas

Adult vertebrate retinal cells (rod and cones) continuously synthesize membrane proteins and transport them to the organelle specialized for photon capture, the outer segment. The cell structures involved in the synthesis of opsin have been identified by means of immunocytochemistry at the electron microscope level. Two indirect detection systems were used: (a) rabbit antibodies to frog opsin were localized with ferritin conjugated F(ab')2 of sheep antibodies to rabbit F(ab')2 and (b) sheep antibodies to cattle opsin were coupled to biotin and visualized by means of avidin-ferritin conjugates (AvF). The reagents were applied directly to the surface of thin sections of frog retinal tissues embedded in glutaraldehyde cross-linked bovine serum albumin (BSA). Specific binding of anti-opsin antibodies indicates that opsin is localized in the disks of rod outer segments (ROS), as expected, and in the Golgi zone of the rod cell inner segments. In addition, we observed quantitatively different labeling patterns of outer segments of rods and cones with each of the sera employed. These reactions may indicate immunological homology of rod and cone photopigments. Because these quantitiative variations of labeling density extend along the entire length of the outer segment, they also serve to identify the cell which has shed its disks into adjacent pigment ipithelial cell phagosomes.     

Identification of frog photoreceptor plasma and disk membrane proteins by radioiodination

Several functions have been identified for the plasma membrane of the rod outer segment, including control of light-dependent changes in sodium conductance and a sodium-calcium exchange mechanism. However, little is known about its constituent proteins. Intact rod outer segments substantially free of contaminants were prepared in the dark and purified on a density gradient of Percoll. Surface proteins were then labeled by lactoperoxidase-catalyzed radioiodination, and intact rod outer segments were reisolated. Membrane proteins were identified by polyacrylamide gel electrophoresis and autoradiography. The surface proteins labeled included rhodopsin, the major membrane protein, and 12 other proteins. Several control experiments indicated that the labeled proteins are integral membrane proteins and that label is limited to the plasma membrane. To compare the protein composition of plasma membrane with that of the internal disk membrane, purified rod outer segments were lysed by hypotonic disruption or freeze-thawing, and plasma plus disk membranes were radioiodinated. In these membrane preparations, rhodopsin was the major iodinated constituent, with 12 other proteins also labeled. Autoradiographic evidence indicated some differences in protein composition between disk and plasma membranes. A quantitative comparison of the two samples showed that labeling of two proteins, 24 kilodaltons (kDa) and 13 kDa, was enriched in the plasma membrane, while labeling of a 220-kDa protein was enriched in the disk membrane. These plasma membrane proteins may be associated with important functions such as the light-sensitive conductance and the sodium-calcium exchanger.     

Identification and characterization of all-trans-retinol dehydrogenase from photoreceptor outer segments, the visual cycle enzyme that reduces all-trans-retinal to all-trans-retinol

Retinol dehydrogenase (RDH), the enzyme that catalyzes the reduction of all-trans-retinal to all-trans-retinol within the photoreceptor outer segment, was the first visual cycle enzymatic activity to be identified. Previous work has shown that this enzyme utilizes NADPH, shows a marked preference for all-trans-retinal over 11-cis-retinal, and is tightly associated with the outer segment membrane. This paper reports the identification of a novel member of the short chain dehydrogenase/reductase family, photoreceptor RDH (prRDH), using subtraction and normalization of retina cDNA, high throughput sequencing, and data base homology searches to detect retina-specific genes. Bovine and human prRDH are highly homologous and are most closely related to 17-beta-hydroxysteroid dehydrogenase 1. The enzymatic properties of recombinant bovine prRDH closely match those previously reported for RDH activity in crude bovine rod outer segment preparations. In situ hybridization and RNA blotting show that the PRRDH gene is expressed specifically in photoreceptor cells, and protein blotting and immunocytochemistry show that prRDH localizes exclusively to both rod and cone outer segments and that prRDH is tightly associated with outer segment membranes. Taken together, these data indicate that prRDH is the enzyme responsible for the reduction of all-trans-retinal to all-trans-retinol within the photoreceptor outer segment.     

In vitro biosynthesis, core glycosylation and membrane integration of opsin

A membrane-integrated , core-glycosylated form of bovine opsin was synthesized in vitro when bovine retina mRNA was translated in a wheat germ cell-free system supplemented with dog pancreas microsomal vesicles; glycosylation and integration of opsin into membranes were coupled to translation. Proteolysis with themolysin was used to probe the orientation of opsin within the dog pancreas microsomal membrane, and to compare it with that of opsin in rod cell disk membranes isolated from bovine retina. Intact microsomal or disk vesicles were required for production of discrete, membrane-associated thermolysin fragments of opsin; no discrete opsin fragments were detected when membranes were incubated with thermolysin in the presence of the nonionic detergent, Triton X-100. The major opsin fragments produced by themosylin treatment of intact microsomal vesicles resembled those from disk vesicles in their size, oligosaccharide content, and order of appearance. In each case, the first cleavage of opsin took place at the COOH-terminus, generating a glycosylated fragment, O’, which was only slightly smaller than intact opsin. Both the microsomal and disk membrane forms of O’ were next cleaved internally; glycosylated fragments of similar sizes in both cases were detected which were derived from the NH(2)-terminal portion of O’. Several smaller NH(2)-terminal fragments of opsin were detected only in thermolysin-treated microsomal membranes, and not in disk membranes. The data suggest that the topology of opsin integrated into dog pancreas microsomal vesicles is similar to that in rod cell disk vesicles, although not identical. In each case, the glycosylated NH(2)-terminal region of opsin is located within the lumen of the vesicle, while discrete COOH-terminal and internal segments of opsin apparently emerge at the outer, cytoplasmic face of the membrane. Thus, opsin in the heterologous microsomal membrane, like its counterpart in the native disk membrane, may cross the bilayer at least three times. The internal domain of the polypeptide that emerges at the outer membrane surface is apparently more highly exposed in the case of opsin in microsomal membranes, evidenced by the additional internal thermolysin cleavage sites detected.     

Oxidative damage of retinal rod outer segment membranes and the role of vitamin E

Highly purified bovine rod outer segment membranes show loss of structural integrity under an air atmosphere. Obvious ultrastructural changes are preceded by increases in absorbance below 400 nm. These changes are inhibited by Ar or N₂ atmospheres and appear to be due primarily to oxidative damage to the polyunsaturated fatty acids of the membrane lipids. Loss of polyunsaturated fatty acids, formation of malonaldehyde and fluorescent products characteristic of lipid oxidation accompany the spectral alterations. The elevated ultraviolet absorbance can largely be removed from the membranes by gentle extraction of the lipids using phospholipase C and hexane without changing the visible absorbance of rhodopsin.We have found a large seasonal variation in the endogenous level of α-tocopherol (vitamin E) in the bovine rod outer segment preparations. For much of the year we find that the rod outer segment membranes contain higher levels of α-tocopherol than have been previously reported in biological membranes. Rod outer segments which are low in endogenous tocopherol can be protected from oxygen damage by adding exogenous tocopherol. The rod outer segments are extremely susceptible to oxygen damage due to the unusually high content of polyunsaturated fatty acids in the membrane lipids. The presence of tocopherol inhibits oxygen damage but does not eliminate it. The tocopherol in the rod outer segments is consumed in air, thus complete protection from peroxidation in vitro requires an inert atmosphere as well as high levels of tocopherol.This work suggests that extensive precautions against oxidative degradation should also be employed in studies of other membrane systems where important deleterious effects of oxygen may be less obvious.     

Assembly of the sarcoplasmic reticulum. Cell-free synthesis of te Ca2+ + Mg2+-adenosine triphosphatase and calsequestrin

Polyadenylated RNA prepared from neonatal rat muscle was translated in a rabbit reticulocyte cell-free system. Two sarcoplasmic reticulum proteins, the Ca2+ + Mg2+-dependent adenosine triphosphatase (ATPase) and calsequestrin, were isolated from the translation mixture by immunoprecipitation, followed by electrophoresis in sodium dodecyl sulfate-polyacrylamide gels. The [35S]methionine-labeled translation products were characterized by molecular weight, peptide mapping, and NH2-terminal sequence analysis. The ATPase synthesized in the cell-free system was found to have the same molecular weight (Mr = 100,000) and [35S]-methionine-labeled peptide map as the mature ATPase. The methionine residue present at the NH2 terminus of the mature ATPase was donated by initiator methionyl-tRNArMet and it became acetylated during translation. These results suggest that the ATPase was synthesized without an NH2-terminal signal sequence. Calsequestrin (Mr - 63,000) was synthesized as a higher molecular weight precursor (Mr = 66,000) that contained an additional [35S]methionine-labeled peptide when compared to mature calsequestrin. The NH2-terminal sequence of the precursor was different from the mature protein. The precursor was processed to a polypeptide with a molecular weight identical with mature calsequestrin when microsomal membranes prepared from canine pancreas were included during translation. These results show that calsequestrin is synthesized with an NH2-terminal signal sequence that is removed during translation. These data add to the evidence that the ATPase and calsequestrin follow distinctly different biosynthetic pathways, even though, ultimately, they are both located in the same membrane.     

Immunocytochemical localization of opsin in the cell membrane of developing rat retinal photoreceptors

Mature retinal rod photoreceptors sequester opsin in the disk and plasma membranes of the rod outer segment (ROS). Opsin is synthesized in the inner segment and is transferred to the outer segment along the connecting cilium that joins the two compartments. We have investigated early stages of retinal development during which the polarized distribution of opsin is established in the rod photoreceptor cell. Retinas were isolated from newborn rats, 3-21 d old, and incubated with affinity purified biotinyl-sheep anti-bovine opsin followed by avidin- ferritin. At early postnatal ages prior to the development of the ROS, opsin is labeled by antiopsin on the inner segment plasma membrane. At the fifth postnatal day, as ROS formation begins opsin was detected on the connecting cilium plasma membrane. However, the labeling density of the ciliary plasma membrane was not uniform: the proximal cilium was relatively unlabeled in comparison with the distal cilium and the ROS plasma membrane. In nearly mature rat retinas, opsin was no longer detected on the inner segment plasma membrane. A similar polarized distribution of opsin was also observed in adult human rod photoreceptor cells labeled with the same antibodies. These results suggest that some component(s) of the connecting cilium and its plasma membrane may participate in establishing and maintaining the polarized distribution of opsin.     

Bovine angiotensin-converting enzyme: amino-terminal sequence analysis and preliminary characterization of a hybridization-selected primary translation product

Bovine lung angiotensin-converting enzyme was isolated in pure form and the sequence of the first twenty-two NH2-terminal amino acids determined. Oligonucleotides, complementary to a selected portion of the NH2-terminal amino acid sequence of the bovine glycoprotein (Mr 145,000), were synthesized and used for hybridization selection of angiotensin-converting enzyme mRNA. The hybridization-selected mRNA programmed the in vitro synthesis of a single polypeptide (Mr 130,000) that was specifically immunoadsorbed by anti-bovine enzyme antibodies. Preliminary sequence analysis of the primary translation product suggests that bovine angiotensin-converting enzyme is synthesized without a transient NH2-terminal signal sequence.     

A spectrin-like protein in retinal rod outer segments

Biochemical and immunochemical studies indicate that rod outer segments (ROS) of bovine photoreceptor cells contain a Mr 240,000 polypeptide related to the alpha-subunit of red blood cell (RBC) spectrin. With the use of sodium dodecyl sulfate gel electrophoresis in conjunction with the immunoblotting technique, monoclonal antibody 4B2 was found to bind to a Mr 240,000 polypeptide in ROS that is distinct from the prominent Mr 220,000 concanavalin A binding glycoprotein. The Mr 240,000 polypeptide is highly susceptible to degradation by endogenous proteases. It does not appear to be an integral membrane protein but is tightly membrane associated since it can be partially extracted from ROS membranes with urea in the absence of detergent. The 4B2 antibody cross-reacted with RBC ghosts and bovine brain microsomal membranes. Radioimmune assays and immunoblotting analysis of purified bovine RBC spectrin further revealed that the 4B2 antibody predominantly labeled the alpha-chain of RBC spectrin having an apparent molecular weight of 240,000. Polyclonal anti-spectrin antibody that bound to both the alpha- and beta-chain of RBC spectrin predominantly labeled a Mr 240,000 polypeptide of ROS membranes. Two faintly labeled bands in the molecular weight range of 210,000-220,000 were also observed. These components may represent variants of the beta-chain of spectrin that are weakly cross-reacting or present in smaller quantities than the alpha-chain.(ABSTRACT TRUNCATED AT 250 WORDS)