Synthesis of functional bovine opsin in insect cells under control of the baculovirus polyhedrin promoter

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

Large scale expression and purification of mouse melanopsin-L in the baculovirus expression system

Melanopsin is the mammalian photopigment that primarily mediates non-visual photoregulated physiology. So far, this photopigment is poorly characterized with respect to structure and function. Here, we report large-scale production and purification of the intact long isoform of mouse melanopsin (melanopsin-L) using the baculovirus/insect cell expression system. Exploiting the baculoviral GP67 signal peptide, we obtained expression levels that varied between 10-30pmol/10(6)cells, equivalent to 2-5mg/L. This could be further enhanced using DMSO as a chemical chaperone. LC-MS analysis confirmed that full-length melanopsin-L was expressed and demonstrated that the majority of the expressed protein was N-glycosylated at Asn(30) and Asn(34). Other posttranslational modifications were not yet detected. Purification was achieved exploiting a C-terminal deca-histag, realizing a purification factor of several hundred-fold. The final recovery of purified melanopsin-L averaged 2.5% of the starting material. This was mainly due to low extraction yields, probably since most of the protein was present as the apoprotein. The spectral data we obtained agree with an absorbance maximum in the 460-500nm wavelength region and a significant red-shift upon illumination. This is the first report on expression and purification of full length melanopsin-L at a scale that can easily be further amplified.

Breaking the bottleneck: Eukaryotic membrane protein expression for high-resolution structural studies

The recombinant expression of eukaryotic membrane proteins has been a major stumbling block in efforts to determine their structures. In the last two years, however, five such proteins have yielded high-resolution X-ray or electron diffraction data, opening the prospect of increased throughput for eukaryotic membrane protein structure determination. Here, we summarize the major expression systems available, and highlight technical advances that should facilitate more systematic screening of expression conditions for this physiologically important class of targets.

Retinitis pigmentosa-associated rhodopsin mutations in three membrane-located cysteine residues present three different biochemical phenotypes

A large number of mutations in rhodopsin are associated with autosomal dominant retinitis pigmentosa (ADRP). We analyzed the biochemical phenotypes of the ADRP-associated cysteine mutants C167R, C222R, and C264del. C222R behaved as wild type in every aspect testable and is classified as a class I mutant. C167R produced intact protein but did not regenerate with 11-cis retinal and was not transported to the plasma membrane. We confirm its classification as a class IIa mutant. C264del represents a novel phenotype, which we propose to call class III. It produced a truncated protein of 27kDa that failed to regenerate with 11-cis retinal and was not targeted to the plasma membrane.

Development of stable cell lines expressing high levels of point mutants of human opsin for biochemical and biophysical studies

Stable HEK293S cell lines expressing high levels of normal and mutant human rod opsins were generated. Cellular expression is uniform across a population. Secondary overexpression of the same opsin transgene linked to a different drug selection marker (hygro(R)) yielded expression clones with increased opsin levels compared to the neo(R) parent strain. Wild-type and mutant human opsins regenerate with native chromophore and demonstrate spectroscopic properties consistent with previous reports of bovine opsin mutants. HEK293S cells can be grown in larger scale suspension culture (10(9) cells/liter) or in roller bottles (10(8) cells/bottle) to facilitate milligram-order preparations of purified pigments. These cell lines should be useful in any time-resolved spectroscopic or biophysical experiments that require either uniform cellular levels of opsin protein or regenerable pigment, or large amounts of purified visual pigment. They should also be useful in experiments where uniform constitutive levels of a given mutant human visual pigment are needed in each cell. These and similar types of constitutive or inducible cell lines may also be useful for studying mechanisms of human cell death that occur by mutations in the human rod opsin gene.

Heterologous expression of rat epitope-tagged histamine H2 receptors in insect Sf9 cells

1. Rat histamine H2 receptors were epitope-tagged with six histidine residues at the C-terminus to allow immunological detection of the receptor. Recombinant baculoviruses containing the epitope-tagged H2 receptor were prepared and were used to infect insect Sf9 cells. 2. The His-tagged H2 receptors expressed in insect Sf9 cells showed typical H2 receptor characteristics as determined with [125I]-aminopotentidine (APT) binding studies. 3. In Sf9 cells expressing the His-tagged H2 receptor histamine was able to stimulate cyclic AMP production 9 fold (EC50=2.1+/-0.1 microM) by use of the endogenous signalling pathway. The classical antagonists cimetidine, ranitidine and tiotidine inhibited histamine induced cyclic AMP production with Ki values of 0.60+/-0.43 microM, 0.25+/-0.15 microM and 28+/-7 nM, respectively (mean+/-s.e.mean, n=3). 4. The expression of the His-tagged H2 receptors in infected Sf9 cells reached functional levels of 6.6+/-0.6 pmol mg(-1) protein (mean+/-s.e.mean, n=3) after 3 days of infection. This represents about 2 x 10(6) copies of receptor/cell. Preincubation of the cells with 0.03 mM cholesterol-beta-cyclodextrin complex resulted in an increase of [125I]-APT binding up to 169+/-5% (mean+/-s.e.mean, n=3). 5. The addition of 0.03 mM cholesterol-beta-cyclodextrin complex did not affect histamine-induced cyclic AMP production. The EC50 value of histamine was 3.1+/-1.7 microM in the absence of cholesterol-beta-cyclodextrin complex and 11.1+/-5.5 microM in the presence of cholesterol-beta-cyclodextrin complex (mean+/-s.e.mean, n=3). Also, the amount of cyclic AMP produced in the presence of 100 microM histamine was identical, 85+/-18 pmol/10(6) cells in the absence and 81+/-11 pmol/10(6) cells in the presence of 0.03 mM cholesterol-beta-cyclodextrin complex (mean+/-s.e.mean, n=3). 6. Immunofluorescence studies with an antibody against the His-tag revealed that the majority of the His-tagged H2 receptors was localized inside the insect Sf9 cells, although plasma membrane labelling could be identified as well. 7. These experiments demonstrate the successful expression of His-tagged histamine H2 receptors in insect Sf9 cells. The H2 receptors couple functionally to the insect cell adenylate cyclase. However, our studies with cholesterol complementation and with immunofluorescent detection of the His-tag reveal that only a limited amount of H2 receptor protein is functional. These functional receptors are targeted to the plasma membrane.

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.

Point mutations in bovine opsin can be classified in four groups with respect to their effect on the biosynthetic pathway of opsin

Expression in vitro with the recombinant baculovirus expression system showed correct biosynthesis and post-translational processing of "wild-type' bovine opsin with regard to translocation, glycosylation, palmitoylation and targeting. However, several of these processes were severely affected by point mutations. From the overall results of 16 mutants reported here, four groups were distinguished. One group significantly affected neither biosynthesis nor folding of opsin (D83N, P291A, A299C-V300A-P303G). A second group produced a truncated protein (R69H, Y301F), suggesting that these positions are essential for a correct translational process. A third group affected membrane translocation as well as glycosylation, which can be interpreted as interference with the function of a transfer signal. Substitutions at positions Glu-113, Glu-122, Glu-134, Arg-135 and Lys-248 belong to this category. A fourth group induced structural changes in the protein that led to heterogeneous distribution in the plasma membrane (E113Q/D, W265F, Y268S). Taking any functional consequences of these mutations into consideration, it seems that point mutations can have mosaic effects and therefore should be examined at several levels (folding, targeting, functional parameters).

Functional expression of human cone pigments using recombinant baculovirus: compatibility with histidine tagging and evidence for N-glycosylation

Mammalian color vision is mediated by light-sensitive pigments in retinal cone cells. Biochemical studies on native mammalian cone visual pigments are seriously hampered by their low levels and instability. We describe a novel approach for their functional expression, employing the baculovirus system in combination with histidine tagging to allow future purification and structural analysis. The human red and green cone pigments are produced in relatively large amounts and can be detected by immunocytochemistry as well as by immunoblotting. Histidine tagging has no significant effect on the absorbance maxima. The first evidence is presented that these pigments are N-glycosylated.

Effect of carboxyl mutations on functional properties of bovine rhodopsin

Bovine rod rhodopsin and membrane-carboxyl group mutants are expressed using the recombinant baculovirus expression system. Biosynthesis of wild-type and the mutant D83N is normal. The mutations E122L and E134D/R affect glycosylation and translocation. After regeneration, purification and reconstitution in retina lipids a wild-type photosensitive pigment with spectral and photolytic properties identical to native bovine rod rhodopsin is generated. Only the mutations D83N and E122L affect the spectral properties and then only slightly. All mutations induce a shift in the Meta I<==>Meta II equilibrium towards Meta I (E134D/R) or Meta II (D83N, E122L). FT-IR analysis shows that the mutation E134D/R does not significantly affect the carboxyl-vibration region but, in particular in the case of E134R, affects secondary structural changes upon Meta II formation. E122L also has an effect on secondary structural changes and in addition eliminates a negative band at 1728 cm-1. The mutation D83N removes a pair of negative/positive bands from the carboxyl-vibration region, indicating that Asp83 stays protonated upon formation of Meta II but undergoes a change in hydrogen bonding.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

Histidine tagging both allows convenient single-step purification of bovine rhodopsin and exerts ionic strength-dependent effects on its photochemistry

For rapid single-step purification of recombinant rhodopsin, a baculovirus expression vector was constructed containing the bovine opsin coding sequence extended at the 3'-end by a short sequence encoding six histidine residues. Recombinant baculovirus-infected Spodoptera frugiperda cells produce bovine opsin carrying a C-terminal histidine tag (v-opshis6x). The presence of this tag was confirmed by immunoblot analysis. Incubation with 11-cis-retinal produced a photosensitive pigment (v-Rhohis6x) at a level of 15-20 pmol/10(6) cells. The histidine tag was exploited to purify v-Rhohis6x via immobilized metal affinity chromatography. Optimized immobilized metal affinity chromatography yielded a binding capacity of > or = 35 nmol of v-Rhohis6x per ml of resin and purification factors up to 500. Best samples were at least 85% pure, with an average purity of 70% (A280 nm/A500 nm = 2.5 +/- 0.4, n = 7). Remaining contamination was largely removed upon reconstitution into lipids, yielding rhodopsin proteoliposomes with a purity over 95%. Spectral analysis of v-Rhohis6x showed a small but significant red shift (501 +/- 1 nm) compared to wild type rhodopsin (498 +/- 1 nm). The pK alpha of the Meta I<==>Meta II equilibrium in v-Rhohis6x is down-shifted from 7.3 to 6.4 resulting in a significant shift at pH 6.5 toward the Meta I photointermediate. Both effects are reversed upon increasing the ionic strength. FT-IR analysis of the Rho-->Meta II transition shows that the corresponding structural changes are identical in wild type and v-Rhohis6x.

Molecular determinants of visual pigment function

Mutagenesis studies and comparisons of natural variants of rhodopsin and related visual pigments have led to new insights concerning photoreceptor function. The studies identify domains important for receptor folding, the residues that set the wavelength of absorption for the ligand 11-cis retinal, and residues, that when mutated, trigger the cell death of photoreceptors.

In vitro expression of bovine opsin using recombinant baculovirus: the role of glutamic acid (134) in opsin biosynthesis and glycosylation

Expression levels of functional bovine opsin in the insect cell line IPLB-Sf9 using recombinant baculovirus were shown not to depend on the use of novel transfer vectors (pAcRP23, pAcDZ1) that were reported to improve biosynthesis levels of other proteins in this system. A production of 5 micrograms opsin per 10(6) cells (approx. 1.5% of total cell protein) was achieved by batch fermentation of infected cells in spinner cultures. Infection of the cells in the presence of the glycosyltransferase inhibitor tunicamycin led to the synthesis of the complete protein, which, however, now migrated with a substantially lower Mr. This demonstrates that opsin in insect cells also undergoes N-linked glycosylation and allowed partial purification (10-fold) of the resulting rhodopsin by affinity chromatography over Concanavalin A-Sepharose. Through site-directed mutagenesis (rhod)opsin mutants have been obtained allowing dissection of functional domains of opsin. Amino acid substitutions that involved Glu-134 and/or Arg-135 affected the normal biosynthetic process leading in part to nonglycosylated, to a small extent even incomplete, protein. A number of mutations, that involve other charged residues within the second and third transmembrane domain of the protein, had no effect on the biosynthetic processing of the protein. We therefore suggest that the charge-pair Glu-134-Arg-135 is part of an important internal signal sequence and that alterations in this region may result in incorrect membrane translocation and/or folding of the protein.

Asp83, Glu113 and Glu134 are not specifically involved in Schiff base protonation or wavelength regulation in bovine rhodopsin

Site-specific mutagenesis was employed to investigate the proposed contribution of proton-donating residues (Glu, Asp) in the membrane domains of bovine rhodopsin to protonation of the Schiff base-linking protein and chromophore or to wavelength modulation of this visual pigment. Three point-mutations were introduced to replace the highly conserved residues Asp83 by Asn (D83N), Glu113 by Gln (E113 Q) or Glu134 by Asp (E134D), respectively. All 3 substitutions had only marginal effects on the spectral properties of the final pigment (less than or equal to 3 nm blue-shift relative to native rhodopsin). Hence, none of these residues by itself is specifically involved in Schiff base protonation or wavelength modulation of bovine rhodopsin.