NMR analysis of rhodopsin–transducin interactions

Heterotrimeric G-protein activation by an agonist-stimulated G-protein coupled receptor (R*) requires the propagation of structural signals from the receptor interacting surfaces to the guanine nucleotide-binding pocket. Employing high-resolution NMR methods, we are probing heterotrimer-associated and rhodopsin-stimulated changes in an isotope-labeled G-protein alpha-subunit (G(alpha)). A key aspect of the work involves the trapping and interrogation of discrete R*-bound conformations of G(alpha). Our results demonstrate that functionally important changes in G(alpha) structure and dynamics can be detected and characterized by NMR, enabling the generation of robust models for the global and local structural changes accompanying signal transfer from R* to the G-protein.

Evidence for structural changes in carboxyl-terminal peptides of transducin alpha-subunit upon binding a soluble mimic of light-activated rhodopsin

Although a high-resolution crystal structure for the ground state of rhodopsin is now available, portions of the cytoplasmic surface are not well resolved, and the structural basis for the interaction of the cytoplasmic loops with the retinal G-protein transducin (G(t)) is still unknown. Previous efforts aimed at the design, construction, and functional characterization of soluble mimics for the light-activated state of rhodopsin have shown that grafting defined segments from the cytoplasmic region of bovine opsin onto a surface loop in a mutant form of thioredoxin (HPTRX) is sufficient to confer partial G(t) activating potential [Abdulaev et al. (2000) J. Biol. Chem. 275, 39354-39363]. To assess whether these designed mimics could provide a structural insight into the interaction between light-activated rhodopsin and G(t), the ability of an HPTRX fusion protein comprised of the second (CD) and third (EF) cytoplasmic loops (HPTRX/CDEF) to bind G(t) alpha-subunit (G(t)(alpha)) peptides was examined using nuclear magnetic resonance (NMR) spectroscopy. Transfer NOESY (TrNOESY) experiments show that an 11 amino acid peptide corresponding to the carboxyl terminus of G(t)(alpha) (GtP), as well as a "high-affinity" peptide analogue, HAP1, binds to HPTRX/CDEF in the fast-exchange regime and undergoes similar, subtle structural changes at the extreme carboxyl terminus. Observed TrNOEs suggest that both peptides when bound to HPTRX/CDEF adopt a reverse turn that is consistent with the C-cap structure that has been previously reported for the interaction of GtP with the light-activated signaling state, metarhodopsin II (MII). In contrast, TrNOESY spectra provide no evidence for structuring of the amino terminus of either GtP or HAP1 when bound to HPTRX/CDEF, nor do the spectra show any measurable changes in the CD and EF loop resonances of HPTRX/CDEF, which are conformationally dynamic and significantly exchange broadened. Taken together, the NMR observations indicate that HPTRX/CDEF, previously identified as a functional mimic of MII, is also an approximate structural mimic for this light-activated state of rhodopsin.

Functionally discrete mimics of light-activated rhodopsin identified through expression of soluble cytoplasmic domains

Numerous studies on the seven-helix receptor rhodopsin have implicated the cytoplasmic loops and carboxyl-terminal region in the binding and activation of proteins involved in visual transduction and desensitization. In our continuing studies on rhodopsin folding, assembly, and structure, we have attempted to reconstruct the interacting surface(s) for these proteins by inserting fragments corresponding to the cytoplasmic loops and/or the carboxyl-terminal tail of bovine opsin either singly, or in combination, onto a surface loop in thioredoxin. The purpose of the thioredoxin fusion is to provide a soluble scaffold for the cytoplasmic fragments thereby allowing them sufficient conformational freedom to fold to a structure that mimics the protein-binding sites on light-activated rhodopsin. All of the fusion proteins are expressed to relatively high levels in Escherichia coli and can be purified using a two- or three-step chromatography procedure. Biochemical studies show that some of the fusion proteins effectively mimic the activated conformation(s) of rhodopsin in stimulating G-protein or competing with the light-activated rhodopsin/G-protein interaction, in supporting phosphorylation of the carboxyl-terminal opsin fragment by rhodopsin kinase, and/or phosphopeptide-stimulated arrestin binding. These results suggest that specific segments of the cytoplasmic surface of rhodopsin can adopt functionally discrete conformations in the absence of the connecting transmembrane helices and retinal chromophore.

Folding and assembly in rhodopsin. Effect of mutations in the sixth transmembrane helix on the conformation of the third cytoplasmic loop

Previous studies on bovine opsin folding and assembly have identified an amino-terminal fragment, EF(1-232), which folds and inserts into a membrane only after coexpression with its complementary carboxyl-terminal fragment, EF(233-348). To further characterize this interaction, EF(1-232) production was examined upon coexpression with carboxyl-terminal fragments of varying length and/or amino acid composition. These included fragments with incremental deletions of the third cytoplasmic loop (TH(241-348) and EF(249-348)), a fragment composed of the third cytoplasmic loop and sixth transmembrane helix (HF(233-280)), a fragment composed of the sixth and seventh transmembrane helices (FG(249-312)), and EF(233-348) and TH(241-348) fragments with Pro-267 or Trp-265 mutations. Although EF(1-232) production was independent of the third cytoplasmic loop and carboxyl-terminal tail, both the sixth and seventh transmembrane helices were essential. The effects of mutations in the sixth transmembrane helix on EF(1-232) expression were dependent on the length of the third cytoplasmic loop. Although Pro-267 mutations in EF(233-348) failed to stabilize EF(1-232) expression, their introduction into TH(241-348) was without discernible effects. However, Trp-265 substitutions in the EF(233-348) and TH(241-348) fragments conferred significant EF(1-232) production. Therefore, key residues in the transmembrane helices may exert their effects on opsin folding, assembly, and/or function by influencing the conformation of the connecting loops.

[Biochemical characteristics of bovine retina nucleoside diphosphate kinase]

Nucleoside diphosphate kinase (NDP kinase; ATP: NDP phosphotransferase; EC 2.7.4.6) was purified from bovine retina. The molecular mass of the native enzyme was found to be 72 kDa, and those of its subunits were 17.5 and 18.5 kDa. Kinetic characteristics of the enzyme were determined. It was shown that NDP kinase exists in retina in both soluble and membrane-bound forms.

The three-dimensional structures of two isoforms of nucleoside diphosphate kinase from bovine retina

The crystal structures of two isoforms of nucleoside diphosphate kinase from bovine retina overexpressed in Escherischia coli have been determined to 2.4 A resolution. Both the isoforms, NBR-A and NBR-B, are hexameric and the fold of the monomer is in agreement with NDP-kinase structures from other biological sources. Although the polypeptide chains of the two isoforms differ by only two residues, they crystallize in different space groups. NBR-A crystallizes in space group P212121 with an entire hexamer in the asymmetric unit, while NBR-B crystallizes in space group P43212 with a trimer in the asymmetric unit. The highly conserved nucleotide-binding site observed in other nucleoside diphosphate kinase structures is also observed here. Both NBR-A and NBR-B were crystallized in the presence of cGMP. The nucleotide is bound with the base in the anti conformation. The NBR-A active site contained both cGMP and GDP each bound at half occupancy. Presumably, NBR-A had retained GDP (or GTP) from the purification process. The NBR-B active site contained only cGMP.

Light-induced exposure of the cytoplasmic end of transmembrane helix seven in rhodopsin

A key step in signal transduction in the visual cell is the light-induced conformational change of rhodopsin that triggers the binding and activation of the guanine nucleotide-binding protein. Site-directed mAbs against bovine rhodopsin were produced and used to detect and characterize these conformational changes upon light activation. Among several antibodies that bound exclusively to the light-activated state, an antibody (IgG subclass) with the highest affinity (Ka approximately 6 x 10(-9) M) was further purified and characterized. The epitope of this antibody was mapped to the amino acid sequence 304-311. This epitope extends from the central region to the cytoplasmic end of the seventh transmembrane helix and incorporates a part of a highly conserved NPXXY motif, a critical region for signaling and agonist-induced internalization of several biogenic amine and peptide receptors. In the dark state, no binding of the antibody to rhodopsin was detected. Accessibility of the epitope to the antibody correlated with formation of the metarhodopsin II photointermediate and was reduced significantly at the metarhodopsin III intermediate. Further, incubation of the antigen-antibody complex with 11-cis-retinal failed to regenerate the native rhodopsin chromophore. These results suggest significant and reversible conformational changes in close proximity to the cytoplasmic end of the seventh transmembrane helix of rhodopsin that might be important for folding and signaling.

Nucleoside diphosphate kinase from bovine retina: purification, subcellular localization, molecular cloning, and three-dimensional structure

The biochemical and structural properties of bovine retinal nucleoside diphosphate kinase were investigated. The enzyme showed two polypeptides of approximately 17.5 and 18.5 kDa on SDS-PAGE, while isoelectric focusing revealed seven to eight proteins with a pI range of 7.4-8.2. Sedimentation equilibrium yielded a molecular mass of 96 +/- 2 kDa for the enzyme. Carbohydrate analysis revealed that both polypeptides contained Gal, Man, GlcNAc, Fuc, and GalNac saccharides. Like other nucleoside diphosphate kinases, the retinal enzyme showed substantial differences in the Km values for various di- and triphosphate nucleotides. Immunogold labeling of bovine retina revealed that the enzyme is localized on both the membranes and in the cytoplasm. Screening of a retinal cDNA library yielded full-length clones encoding two distinct isoforms (NBR-A and NBR-B). Both isoforms were overexpressed in Escherichia coli and their biochemical properties compared with retinal NDP-kinase. The structures of NBR-A and NBR-B were determined by X-ray crystallography in the presence of guanine nucleotide(s). Both isoforms are hexameric, and the fold of the monomer is similar to other nucleoside diphosphate kinase structures. The NBR-A active site contained both a cGMP and a GDP molecule each bound at half occupancy while the NBR-B active site contained only cGMP.

[Functional expression of bovine retina adenylate cyclase b fragment cDNA in Escherichia Coli cells]

A 1488-bp fragment of bovine retina guanylate cyclase B gene encoding the catalytic and dimerizing domains as well as part of the protein kinase domain was expressed in Escherichia coli cells. The expression product was obtained as inclusion bodies and solubilized in 6 M guanidine hydrochloride. The fragment of guanylate cyclase B is a dimer close in catalytic activity to the native enzyme.

Functional expression of bovine opsin in the methylotrophic yeast Pichia pastoris

The methylotrophic yeast Pichia pastoris was examined for functional expression of bovine opsin. An expression plasmid was constructed where the bovine opsin gene was placed downstream from the P. pastoris alcohol oxidase 1 gene promoter and fused at its amino-terminus to the acid phosphatase secretion signal. Quantitative-competitive PCR analysis of a stable yeast transformant showed that one copy of the opsin gene was integrated into the yeast genome. The expression level in this transformant corresponded to approximately 0.3 mg of opsin per liter of cell culture (A600 = 1.0). Sucrose density sedimentation analysis indicated that the opsin was associated exclusively with the membrane fraction. Similar to retinal opsin, P. pastoris-expressed opsin migrated as a single band of approximately 37 kDa on SDS-PAGE and showed high mannose N-glycosylation. A portion of the expressed opsin (approximately 4-15%) reacted with 11-cis-retinal to form the rhodopsin chromophore (lambda max 500 nm), and after purification showed ground and excited state spectral characteristics indistinguishable from those of the native pigment. Further, the metarhodopsin-II-mediated G-protein-activating potential of yeast expressed rhodopsin was similar to that of native rhodopsin. These results show that P. pastoris cells have the capacity to functionally express bovine opsin.

[Nucleoside diphosphate kinase from bovine retina is a glycoprotein]

Nucleoside diphosphate (NDP) kinase from bovine retina was found to contain carbohydrates. The subunits of NDP kinase were separated by SDS-PAGE, blotted onto an Immobilon-P membrane, and their carbohydrate content was determined. Both subunits contained equal amounts of Gal, Man, Fuc, Gal-NAc, and Glc-NAc. The total carbohydrate content was 2 to 3% of the protein weight.

Examining rhodopsin folding and assembly through expression of polypeptide fragments

Previous work on the expression of bovine opsin fragments separated in the cytoplasmic region has allowed the identification of specific polypeptide segments that contain sufficient information to fold independently, insert into a membrane, and assemble to form a functional photoreceptor. To further examine the contributions of these and other polypeptide segments to the mechanism of opsin folding and assembly, we have constructed 20 additional opsin gene fragments where the points of separation occur in the intradiscal, transmembrane, and cytoplasmic regions. Nineteen of the fragments were stably expressed in COS-1 cells. A five-helix fragment was stably produced only after coexpression with its complementary two-helix fragment. Two fragments composed of the amino-terminal region and the first transmembrane helix were not N-glycosylated and were only partially membrane integrated. One of the singly expressed fragments, which is truncated after the retinal attachment site, bound 11-cis-retinal. Of the coexpressed complementary fragments, only those separated in the second intradiscal and third cytoplasmic regions formed noncovalently linked rhodopsin. Both of the pigments showed reduced transducin activation. Therefore, while many opsin fragments contain enough information to fold and insert into a membrane, only those separated at specific locations assemble to a retinal-binding opsin.

Conformation of surface exposed N-terminus part of bacteriorhodopsin studied by transferred NOE technique

Interaction of the monoclonal antibody A5 raised against native bacteriorhodopsin (BR) with the synthetic peptide pGlu1-Ala-Gln-Ile-Thr-Gly-Arg7-NH2, corresponding to the amino acid sequence 1-7 was studied by transferred nuclear Overhauser effect (TRNOE) spectroscopy. The denaturing reagents and the specially designed pulse sequences which eliminate broad signals from the TRNOE spectra were used to favour evaluation of the TRNOE peaks. On the basis of the data obtained, the conformation of peptide bound with A5 was calculated. A model of the mutual arrangement of bacteriorhodopsin N-terminus and the first transmembrane alpha-helical segment 8-32 was proposed.

[Guanylate kinase from bovine retina: isolation, primary structure, and expression in E. coli]

Guanylate kinase (EC 2.7.4.8), catalysing the reaction GMP+ATP = GDP+ADP, was purified to homogeneity from bovine retina. Primary structure of the enzyme was determined by parallel analyses of amino acid sequences of its peptides and nucleotide sequence of the corresponding cDNA. It is shown that the bovine retinal guanylate kinase like the analogous enzyme from yeast Saccharomyces cerevisiae contains a characteristic glycine-rich motif, involved in ATP binding. All of the amino acids, involved in GMP binding in the yeast enzyme, are conserved or conservatively substituted in the bovine retinal guanylate kinase. The bovine retinal enzyme was expressed in E. coli as a fusion protein. Data are presented on the purification of the fusion protein, its digestion by enteropeptidase, purification of the recombinant enzyme and its functional characteristics.

Enzymes of the cyclic GMP metabolism in bovine retina. I. Cloning and expression of the gene for guanylate kinase

Guanylate kinase (EC 2.7.4.8) catalyzing the reaction GMP + ATP = GDP + ADP, was purified to homogeneity from bovine retina. Using oligonucleotides based on the amino acid sequence of this enzyme, the cDNA encoding guanylate kinase (GK) was isolated and its nucleotide sequence was determined. Expression of the GK cDNA in E. coli, and the purification and functional characterization of the expressed enzyme are presented. It is shown that bovine retinal GK, like its yeast counterpart, contains the characteristic glycine-rich motif and all the amino acids involved in GMP binding. Bovine retinal enzyme is extended for several amino acid residues both at the N- and C-termini, compared to the yeast enzyme.

[Detection of expression of a membrane form of the guanylate cyclase type of GC-B in cattle retina]

cDNA clones encoding the central and C-terminal parts of a membrane-bound guanylate cyclase (GC) were isolated from the lambda ZAP bovine retinal library. All of the analysed recombinants appeared to carry inserts encoding the guanylate cyclase GC-B. Analysis of the determined nucleotide and deduced amino acid sequences showed extremely high level of homology to the sequences of known GC-B. The results indicate that a mRNA for GC-B is expressed in the bovine retina.

[p26--a calcium binding protein from photoreceptor cells in the bovine retina: primary structure and expression in E. coli]

The primary structure of the bovine retinal calcium binding protein P26 has been determined by the parallel analysis of the protein and the corresponding cDNA. This protein is identical to recovering and shares 59% homology with visinin, a cone specific calcium binding protein from chicken retina. P26 was expressed in E. coli as a fusion protein and, after purification by affinity chromatography on IgG-Sepharose 6, cleaved off with enteropeptidase.

P26-calcium binding protein from bovine retinal photoreceptor cells

The primary structure of bovine retinal calcium binding protein P26 has been determined by parallel analysis of protein and corresponding cDNA. This protein is identical to recovering and shares 59% homology with visinin, a cone specific calcium binding protein from chicken retina. Preliminary data are presented on expression of P26 as a fusion protein in E. coli.

Functioning of quinone acceptors in the reaction center of the green photosynthetic bacterium Chloroflexus aurantiacus

The photosynthetic reaction centers (RC) of the green bacterium Chloroflexus aurantiacus have been investigated by spectral and electrometrical methods. In these reaction centers, the secondary quinone was found to be reconstituted by the addition of ubiquinone-10. The equilibrium constant of electron transfer between primary (QA) and secondary (QB) quinones was much higher than that in RC of purple bacteria. The QB binding to the protein decreased under alkalinization with apparent pK 8.8. The single flash-induced electric responses were about 200 mV. An additional electrogenic phase due to the QB protonation was observed after the second flash in the presence of exogenous electron donors. The magnitude of this phase was 18% of that related to the primary dipole (P+QA-) formation. Since the C. aurantiacus RC lacks H-subunit, this subunit was not an obligatory component for electrogenic QB protonation.

[Study of the structure of the photosynthetic reaction center of the green thermophilic bacteria Chloroflexus aurantiacus]

Analysis of the Chloroflexus aurantiacus reaction centre (RC) using both protein and recombinant DNA techniques resulted in determination of its polypeptide composition and the primary structures of its two subunits. A model of the polypeptide chains' folding in the membrane is suggested based on: i) homology between L- and M-subunits of Chloroflexus aurantiacus RC and their counterparts in purple bacteria; ii) comparison of their hydropathy plots, and iii) data on the tertiary structures of purple bacteria RCs. The role of a number of functionally important amino acid residues in the RC electron transport activity is discussed. Limited proteolysis of the RC under non-denaturing conditions was used to determine the contribution of the N-terminal regions to its thermal stability.

Two-dimensional crystallization of reaction centers from Chloroflexus aurantiacus

Two-dimensional crystals of photosynthetic reaction centers from Chloroflexus aurantiacus were obtained from protein-lipid-detergent micelles by detergent dialysis. The size of crystals was up to 2 microns. Some of them were multilayered crystals. However, other crystal forms were also observed. Preliminary image processing analysis showed that crystals of one crystal form referred to two-sided plane group p2 and had the following unit cell parameters: a = 17.6 nm, b = 18.0 nm, gamma = 84 degrees. The contour map of the crystal stain-excluding region was calculated by the Fourier-filtering procedure at about 2 nm resolution.

[Photosystem II of rye. Nucleotide sequence of genes psbE, psbF, psbL and OPC40 of chloroplast DNA]

The primary structure of the 1084 bp AccI-BamHI fragment of rye chloroplast DNA containing psbE, psbF and psbL genes and ORF40 with their flanking and intergenic regions is elucidated.

[Photosystem II of rye. Nucleotide sequence of psbB and psbH genes, coding 47-kDa of chlorophyll(a)-binding and 10-kDa phosphorylated subunits]

Chloroplast DNA was isolated from rye seedlings by the non-aqueous method. The region of rye ctDNA which comprises two genes psbB and psbH encoding polypeptide subunits of photosystem II (47 kappa l) Chl alpha -binding protein (CP alpha -1) and 10 kD phosphoprotein, respectively) and two ORFs in the opposite strands in the psbB--psbH spacer region encoding hydrophobic peptides with strongly charged C-terminal segments was sequenced. The deduced amino acid sequences of polypeptide products of the genes were compared with those of different plant species (in case of the psbB product also with sequence of a cyano-bacterium Synechocystis) and revealed some highly conservative amino acid residues and regions of polypeptide chains, which apparently play essential role in the interaction with other PS II subunits and in the binding of chlorophyll molecules. Some speculations are made on the possible function of the peptides encoded by the two ORFs.

Photosynthetic reaction centre of Chloroflexus aurantiacus. Primary structure of M-subunit

The M-subunit primary structure of the reaction centre (RC) from Chloroflexus aurantiacus composed of 306 amino acid residues has been determined by parallel analysis of the protein and corresponding DNA. The blocked N-terminus as well as replacement of the essential histidine liganding Mg of an accessory bacteriochlorophyll in purple bacteria by leucine distinguishes the M-subunit of Chloroflexus RC from that of purple bacteria.

Octopus rhodopsin. Amino acid sequence deduced from cDNA

The primary structure of rhodopsin from the octopus Paroctopus defleini has been determined by parallel analysis of the protein and corresponding cDNA. The amino acid sequence is most similar to the recently cloned Drosophila opsins. Similarities to bovine and human opsins are also evident. The transmembrane topology of octopus rhodopsin is discussed.

Photosynthetic reaction centre of Chloroflexus aurantiacus. I. Primary structure of L-subunit

The L-subunit primary structure of the reaction centre from Chloroflexus aurantiacus composed of 310 amino acid residues has been determined by parallel analysis of the protein and corresponding DNA. Significant homology between this protein and L-subunits from reaction centres of purple bacteria is observed. This implies close similarity in the tertiary structure of these proteins.

Two adjacent cysteine residues in the C-terminal cytoplasmic fragment of bovine rhodopsin are palmitylated

Covalent coupling of bovine rhodopsin to CPG-thiol glass was used for separation of CNBr peptides. It is shown that cysteine residues 322 and 323 in the C-terminal cytoplasmic fragment of rhodopsin are modified with palmitic acid.

The bacteriorhodopsin proton pump: effect of crosslinkings of lysine residues

All six available lysine residues in bacteriorhodopsin were amidinated with dimethyl-3,3'-dithiobispropionimidate, which is a crosslinking agent. The photocycle was studied by measuring light absorption and electric signals. The data show an essential change in the photocycle: instead of single components, the rise of the signal due to the M intermediate can be decomposed into two components, and the decay into three. The life-times and the intensities of these components and in general the proton pumping activity of bacteriorhodopsin depend only negligibly upon pH. Changes upon removing the crosslinks are not significantly different from those in the crosslinked samples. The lysine residues therefore may not be considered of primary importance in proton translocation.

Effect of cross linkers on the bacteriorhodopsin photocycle

A general behavior of bacteriorhodopsin in purple membranes from Halobacterium halobium has been observed upon modification resulting in cross-linking of carboxyl and lysine groups. The rise of the M-intermediate contained two components with approximately 50-50% intensity; its decay showed three components with approximately 25-50-25% intensity respectively in a pH range of 5-9. The significance of these remarkably similar data with respect to the proton translocation mechanism in bacteriorhodopsin is that chemical modification allows us to conclude that disturbing parts of the hypothetical "proton conducting chain" does not inhibit proton translocation.

Retinal Schiff base position relative to the surfaces of photoreceptor disk

Surface-enhanced Raman spectra of native and photobleached bovine rod outer segment disks as well as inside-out (inverted) photoreceptor disks adsorbed on silver hydrosol have been analyzed. Surface-enhanced spectra of inverted disks and disk-monoclonal antibody complexes reveal the short-range mechanism of enhancement. The distance between retinal Schiff base and the cytoplasmic side of native disk has been shown to be 5-10 A.

Group-directed modification of bacteriorhodopsin by arylisothiocyanates. Labeling, identification of the binding site and topology

Group-directed hydrophobic modification of membrane-integrated protein segments by arylisothiocyanates is applied to bacteriorhodopsin. Labeling of purple membrane with phenylisothiocyanate and 4-N,N'-dimethylamino-azobenzene-4'-isothiocyanate results in covalent modification of a unique lysine epsilon-amino group of bacteriorhodopsin. Lysine residue 41, located in the amino-terminal chymotryptic fragment, has been identified as the arylisothiocyanate binding site by established sequencing techniques. The phenylisothiocyanate binding site is not accessible for the aqueously soluble analog p-sulfophenylisothiocyanate. Furthermore, the acid-induced bathochromic shift of the bound chromophore reagent is not observed following acidification of 4-N,N'-dimethylamino-azobenzene-4'-isothiocyanate-labeled purple membrane. The modification thus occurs in the hydrophobic membrane domain, providing further evidence for intramembraneous disposition of the modified protein segment. Light-induced proton translocation is preserved in reconstituted vesicles containing either phenylisothiocyanate-modified or 4-N,N'-dimethylamino-azobenzene-4'-isothiocyanate-modified bacteriorhodopsin.

[Visual rhodopsin. III. Complete amino acid sequence and topography in a membrane]

Tryptic hydrolysis of apomembranes, BNPS-skatole cleavage of carboxymethylated rhodopsin and thermolytic digestion of native membranes were carried out to obtain the peptides necessary for the polypeptide chain reconstruction. Gel-filtration on Bio-Gel P-30 in 80% formic acid, ion-exchange and reversed-phase high performance liquid chromatography were used for the peptide isolation. A comparison of rhodopsin hydrophobicity profile with the accessibility of the polypeptide chain in native photoreceptor membranes for proteases allowed to distinguish seven alpha-helical segments and propose a model for arrangement of the protein molecule in the membrane.