Structural and functional role of helices I and II in rhodopsin. A novel interplay evidenced by mutations at Gly-51 and Gly-89 in the transmembrane domain

Differential Aggregation Properties of Mutant Human and Bovine Rhodopsin

Rhodopsin is the light receptor required for the function and health of photoreceptor cells. Mutations in rhodopsin can cause misfolding and aggregation of the receptor, which leads to retinal degeneration. Bovine rhodopsin is often used as a model to understand the effect of pathogenic mutations in rhodopsin due to the abundance of structural information on the bovine form of the receptor. It is unclear whether or not the bovine rhodopsin template is adequate in predicting the effect of these mutations occurring in human retinal disease or in predicting the efficacy of therapeutic strategies. To better understand the extent to which bovine rhodopsin can serve as a model, human and bovine P23H rhodopsin mutants expressed heterologously in cells were examined. The aggregation properties and cellular localization of the mutant receptors were determined by Förster resonance energy transfer and confocal microscopy. The potential therapeutic effects of the pharmacological compounds 9-cis retinal and metformin were also examined. Human and bovine P23H rhodopsin mutants exhibited different aggregation properties and responses to the pharmacological compounds tested. These observations would lead to different predictions on the severity of the phenotype and divergent predictions on the benefit of the therapeutic compounds tested. The bovine rhodopsin template does not appear to adequately model the effects of the P23H mutation in the human form of the receptor.

Characterizing variants of unknown significance in rhodopsin: A functional genomics approach

Characterizing the pathogenicity of DNA sequence variants of unknown significance (VUS) is a major bottleneck in human genetics, and is increasingly important in determining which patients with inherited retinal diseases could benefit from gene therapy. A library of 210 rhodopsin (RHO) variants from literature and in‐house genetic diagnostic testing were created to efficiently detect pathogenic RHO variants that fail to express on the cell surface. This study, while focused on RHO, demonstrates a streamlined, generalizable method for detecting pathogenic VUS. A relatively simple next‐generation sequencing‐based readout was developed so that a flow cytometry‐based assay could be performed simultaneously on all variants in a pooled format, without the need for barcodes or viral transduction. The resulting dataset characterized the surface expression of every RHO library variant with a high degree of reproducibility (r² = 0.92–0.95), recategorizing 37 variants. For example, three retinitis pigmentosa pedigrees were solved by identifying VUS which showed low expression levels (p.G18D, p.G101V, and p.P180T). Results were validated across multiple assays and correlated with clinical disease severity. This study presents a parallelized, higher‐throughput cell‐based assay for the functional characterization of VUS in RHO, and can be applied more broadly to other inherited retinal disease genes and other disorders.

Coupling of Human Rhodopsin to a Yeast Signaling Pathway Enables Characterization of Mutations Associated with Retinal Disease

G protein-coupled receptors (GPCRs) are crucial sensors of extracellular signals in eukaryotes, with multiple GPCR mutations linked to human diseases. With the growing number of sequenced human genomes, determining the pathogenicity of a mutation is challenging, but can be aided by a direct measurement of GPCR-mediated signaling. This is particularly difficult for the visual pigment rhodopsin-a GPCR activated by light-for which hundreds of mutations have been linked to inherited degenerative retinal diseases such as retinitis pigmentosa. In this study, we successfully engineered, for the first time, activation by human rhodopsin of the yeast mating pathway, resulting in signaling via a fluorescent reporter. We combine this novel assay for rhodopsin light-dependent activation with studies of subcellular localization, and the upregulation of the unfolded protein response in response to misfolded rhodopsin protein. We use these assays to characterize a panel of rhodopsin mutations with known molecular phenotypes, finding that rhodopsin maintains a similar molecular phenotype in yeast, with some interesting differences. Furthermore, we compare our assays in yeast with clinical phenotypes from patients with novel disease-linked mutations. We demonstrate that our engineered yeast strain can be useful in rhodopsin mutant classification, and in helping to determine the molecular mechanisms underlying their pathogenicity. This approach may also be applied to better understand the clinical relevance of other human GPCR mutations, furthering the use of yeast as a tool for investigating molecular mechanisms relevant to human disease.

G Protein-Coupled Receptors Contain Two Conserved Packing Clusters

G protein-coupled receptors (GPCRs) have evolved a seven-transmembrane helix framework that is responsive to a wide range of extracellular signals. An analysis of the interior packing of family A GPCR crystal structures reveals two clusters of highly packed residues that facilitate tight transmembrane helix association. These clusters are centered on amino acid positions 2.47 and 4.53, which are highly conserved as alanine and serine, respectively. Ala2.47 mediates the interaction between helices H1 and H2, while Ser4.53 mediates the interaction between helices H3 and H4. The helical interfaces outside of these clusters are lined with residues that are more loosely packed, a structural feature that facilitates motion of helices H5, H6, and H7, which is required for receptor activation. Mutation of the conserved small side chain at position 4.53 within packing cluster 2 is shown to disrupt the structure of the visual receptor rhodopsin, whereas sites in packing cluster 1 (e.g., positions 1.46 and 2.47) are more tolerant to mutation but affect the overall stability of the protein. These findings reveal a common structural scaffold of GPCRs that is important for receptor folding and activation.

An experimental comparison of human and bovine rhodopsin provides insight into the molecular basis of retinal disease

Rhodopsin is the visual pigment that mediates dim-light vision in vertebrates and is a model system for the study of retinal disease. The majority of rhodopsin experiments are performed using bovine rhodopsin; however, recent evidence suggests that significant functional differences exist among mammalian rhodopsins. In this study, we identify differences in both thermal decay and light-activated retinal release rates between bovine and human rhodopsin and perform mutagenesis studies to highlight two clusters of substitutions that contribute to these differences. We also demonstrate that the retinitis pigmentosa-associated mutation G51A behaves differently in human rhodopsin compared to bovine rhodopsin and determine that the thermal decay rate of an ancestrally reconstructed mammalian rhodopsin displays an intermediate phenotype compared to the two extant pigments.

The Activation Pathway of Human Rhodopsin in Comparison to Bovine Rhodopsin

Rhodopsin, the photoreceptor of rod cells, absorbs light to mediate the first step of vision by activating the G protein transducin (Gt). Several human diseases, such as retinitis pigmentosa or congenital night blindness, are linked to rhodopsin malfunctions. Most of the corresponding in vivo studies and structure-function analyses (e.g. based on protein x-ray crystallography or spectroscopy) have been carried out on murine or bovine rhodopsin. Because these rhodopsins differ at several amino acid positions from human rhodopsin, we conducted a comprehensive spectroscopic characterization of human rhodopsin in combination with molecular dynamics simulations. We show by FTIR and UV-visible difference spectroscopy that the light-induced transformations of the early photointermediates are very similar. Significant differences between the pigments appear with formation of the still inactive Meta I state and the transition to active Meta II. However, the conformation of Meta II and its activity toward the G protein are essentially the same, presumably reflecting the evolutionary pressure under which the active state has developed. Altogether, our results show that although the basic activation pathways of human and bovine rhodopsin are similar, structural deviations exist in the inactive conformation and during receptor activation, even between closely related rhodopsins. These differences between the well studied bovine or murine rhodopsins and human rhodopsin have to be taken into account when the influence of point mutations on the activation pathway of human rhodopsin are investigated using the bovine or murine rhodopsin template sequences.

Transmembrane helix: simple or complex

Transmembrane helical segments (TMs) can be classified into two groups of so-called ‘simple’ and ‘complex’ TMs. Whereas the first group represents mere hydrophobic anchors with an overrepresentation of aliphatic hydrophobic residues that are likely attributed to convergent evolution in many cases, the complex ones embody ancestral information and tend to have structural and functional roles beyond just membrane immersion. Hence, the sequence homology concept is not applicable on simple TMs. In practice, these simple TMs can attract statistically significant but evolutionarily unrelated hits during similarity searches (whether through BLAST- or HMM-based approaches). This is especially problematic for membrane proteins that contain both globular segments and TMs. As such, we have developed the transmembrane helix: simple or complex (TMSOC) webserver for the identification of simple and complex TMs. By masking simple TM segments in seed sequences prior to sequence similarity searches, the false-discovery rate decreases without sacrificing sensitivity. Therefore, TMSOC is a novel and necessary sequence analytic tool for both the experimentalists and the computational biology community working on membrane proteins. It is freely accessible at http://tmsoc.bii.a-star.edu.sg or available for download.

Tryptophan scanning mutagenesis of the first transmembrane domain of the innexin Shaking-B(Lethal)

The channel proteins of gap junctions are encoded by two distinct gene families, connexins, which are exclusive to chordates, and innexins/pannexins, which are found throughout the animal kingdom. Although the relationship between the primary structure and function of the vertebrate connexins has been relatively well studied, there are, to our knowledge, no structure-function analyses of invertebrate innexins. In the first such study, we have used tryptophan scanning to probe the first transmembrane domain (M1) of the Drosophila innexin Shaking-B(Lethal), which is a component of rectifying electrical synapses in the Giant Fiber escape neural circuit. Tryptophan was substituted sequentially for 16 amino acids within M1 of Shaking-B(Lethal). Tryptophan insertion at every fourth residue (H27, T31, L35, and S39) disrupted gap junction function. The distribution of these sites is consistent with helical secondary structure and identifies the face of M1 involved in helix-helix interactions. Tryptophan substitution at several sites in M1 altered channel properties in a variety of ways. Changes in sensitivity to transjunctional voltage (Vj) were common and one mutation (S39W) induced sensitivity to transmembrane voltage (Vm). In addition, several mutations induced hemichannel activity. These changes are similar to those observed after substitutions within the transmembrane domains of connexins.

Not all transmembrane helices are born equal: Towards the extension of the sequence homology concept to membrane proteins

BACKGROUND

Sequence homology considerations widely used to transfer functional annotation to uncharacterized protein sequences require special precautions in the case of non-globular sequence segments including membrane-spanning stretches composed of non-polar residues. Simple, quantitative criteria are desirable for identifying transmembrane helices (TMs) that must be included into or should be excluded from start sequence segments in similarity searches aimed at finding distant homologues.

RESULTS

We found that there are two types of TMs in membrane-associated proteins. On the one hand, there are so-called simple TMs with elevated hydrophobicity, low sequence complexity and extraordinary enrichment in long aliphatic residues. They merely serve as membrane-anchoring device. In contrast, so-called complex TMs have lower hydrophobicity, higher sequence complexity and some functional residues. These TMs have additional roles besides membrane anchoring such as intra-membrane complex formation, ligand binding or a catalytic role. Simple and complex TMs can occur both in single- and multi-membrane-spanning proteins essentially in any type of topology. Whereas simple TMs have the potential to confuse searches for sequence homologues and to generate unrelated hits with seemingly convincing statistical significance, complex TMs contain essential evolutionary information.

CONCLUSION

For extending the homology concept onto membrane proteins, we provide a necessary quantitative criterion to distinguish simple TMs (and a sufficient criterion for complex TMs) in query sequences prior to their usage in homology searches based on assessment of hydrophobicity and sequence complexity of the TM sequence segments.

Variables and strategies in development of therapeutic post-transcriptional gene silencing agents

Post-transcriptional gene silencing (PTGS) agents such as ribozymes, RNAi and antisense have substantial potential for gene therapy of human retinal degenerations. These technologies are used to knockdown a specific target RNA and its cognate protein. The disease target mRNA may be a mutant mRNA causing an autosomal dominant retinal degeneration or a normal mRNA that is overexpressed in certain diseases. All PTGS technologies depend upon the initial critical annealing event of the PTGS ligand to the target RNA. This event requires that the PTGS agent is in a conformational state able to support hybridization and that the target have a large and accessible single-stranded platform to allow rapid annealing, although such platforms are rare. We address the biocomplexity that currently limits PTGS therapeutic development with particular emphasis on biophysical variables that influence cellular performance. We address the different strategies that can be used for development of PTGS agents intended for therapeutic translation. These issues apply generally to the development of PTGS agents for retinal, ocular, or systemic diseases. This review should assist the interested reader to rapidly appreciate critical variables in PTGS development and facilitate initial design and testing of such agents against new targets of clinical interest.

Examining rhodopsin retention in endoplasmic reticulum and intracellular localization in vitro and in vivo by using truncated rhodopsin fragments

More than 100 mutations of rhodopsin have been identified to be associated with retinitis pigmentosa (RP), and mostly autosomal-dominant RP (ADRP). The majority of rhodopsin-associated ADRP is caused by protein misfolding and ER retention. In this study, we aimed to evaluate rhodopsin folding, exiting the ER and intracellular localization through expression of the rhodopsin fragments in COS-1 cells as well as in the transgenic zebrafish. We cloned human rhodopsin cDNA, which was then divided into the N-terminal domain, the C-terminal domain, and the fragment between the N- and C-terminal domains, and examine their intracellular expression in vitro and in vivo. We introduced a point mutation, either F45L or G51V, into this fragment and observed the intracellular localization of these mutants in COS-1 cells and in the zebrafish. The results revealed all of the truncated rhodopsin fragments except for the C-terminal domain and the full-length rhodopsin which had some plasma membrane localization, formed aggregates nearby or within the ER in COS-1 cells; however, the N-terminally truncated rhodopsin fragment, the C-terminal domain, and the full-length rhodopsin could traffic to the ROS in the zebrafish. Besides, the F45L mutation and the G51Vmutation in the rhodopsin fragment between the N- and C-terminal domains produced different effects on the aggresome formation and the intracellular distribution of the mutants both in vivo and in vitro. This current study provides new information about the mutant rhodopsin as well as in treatment of the RP in humans in the future.

A novel rhodopsin-like gene expressed in zebrafish retina

The visual pigment rhodopsin (rh1) constitutes the first step in the sensory transduction cascade in the rod photoreceptors of the vertebrate eye, forming the basis of vision at low light levels. In most vertebrates, rhodopsin is a single-copy gene whose function in rod photoreceptors is highly conserved. We found evidence for a second rhodopsin-like gene (rh1-2) in the zebrafish genome. This novel gene was not the product of a zebrafish-specific gene duplication event and contains a number of unique amino acid substitutions. Despite these differences, expression of rh1-2 in vitro yielded a protein that not only bound chromophore, producing an absorption spectrum in the visible range (λmax ≈ 500 nm), but also activated in response to light. Unlike rh1, rh1-2 is not expressed during the first 4 days of embryonic development; it is expressed in the retina of adult fish but not the brain or muscle. Similar rh1-2 sequences were found in two other Danio species, as well as a more distantly related cyprinid, Epalzeorhynchos bicolor. While sequences were only identified in cyprinid fish, phylogenetic analyses suggest an older origin for this gene family. Our study suggests that rh1-2 is a functional opsin gene that is expressed in the retina later in development. The discovery of a new previously uncharacterized opsin gene in zebrafish retina is surprising given its status as a model system for studies of vertebrate vision and visual development.

Structure and activation of the visual pigment rhodopsin

Rhodopsin is a specialized G protein-coupled receptor (GPCR) found in vertebrate rod cells. Absorption of light by its 11-cis retinal chromophore leads to rapid photochemical isomerization and receptor activation. Recent results from protein crystallography and NMR spectroscopy show how structural changes on the extracellular side of rhodopsin induced by retinal isomerization are coupled to the motion of membrane-spanning helices to create a G protein binding pocket on the intracellular side of the receptor. The signaling pathway provides a comprehensive explanation for the conservation of specific amino acids and structural motifs across the class A family of GPCRs, as well as for the conservation of selected residues within the visual receptor subfamily. The emerging model of activation indicates that, rather than being unique, the visual receptors provide a basis for understanding the common structural and dynamic elements in the class A GPCRs.

Rhodopsin: the functional significance of asn-linked glycosylation and other post-translational modifications

Rhodopsin, the G-protein coupled receptor in retinal rod photoreceptors, is a highly conserved protein that undergoes several types of post-translational modifications. These modifications are essential to maintain the protein's structure as well as its proper function in the visual transduction cycle. Rhodopsin is N-glycosylated at Asn-2 and Asn-15 in its extracellular N-terminal domain. Mutations within the glycosylation consensus sequences of rhodopsin cause autosomal dominant retinitis pigmentosa, a disease that leads to blindness. Several groups have studied the role of rhodopsin's N-linked glycan chains in protein structure and function using a variety of approaches. These include the generation of a transgenic mouse model, study of a naturally occurring mutant animal model, in vivo pharmacological inhibition of glycosylation, and in vitro analyses using transfected COS-1 cells. These studies have provided insights into the possible role of rhodopsin glycosylation, but have yielded conflicting results.

A methyl group at C7 of 11-cis-retinal allows chromophore formation but affects rhodopsin activation

The newly synthesized 11-cis-7-methylretinal can form an artificial visual pigment with kinetic and spectroscopic properties similar to the native pigment in the dark-state. However, its photobleaching behavior is altered, showing a Meta I-like photoproduct. This behavior reflects a steric constraint imposed by the 7-methyl group that affects the conformational change in the binding pocket as a result of retinal photoisomerization. Transducin activation is reduced, when compared to the native pigment with 11-cis-retinal. Molecular dynamics simulations suggest coupling of the C7 methyl group and the beta-ionone ring with Met207 in transmembrane helix 5 in agreement with recent experimental results.

Ca2+/recoverin dependent regulation of phosphorylation of the rhodopsin mutant R135L associated with retinitis pigmentosa

No single molecular mechanism accounts for the effect of mutations in rhodopsin associated with retinitis pigmentosa. Here we report on the specific effect of a Ca2+/recoverin upon phosphorylation of the autosomal dominant retinitis pigmentosa R135L rhodopsin mutant. This mutant shows specific features like impaired G-protein signaling but enhanced phosphorylation in the shut-off process. We now report that R135L hyperphosphorylation by rhodopsin kinase is less efficiently inhibited by Ca2+/recoverin than wild-type rhodopsin. This suggests an involvement of Ca2+/recoverin into the molecular pathogenic effect of the mutation in retinitis pigmentosa which is the cause of rod photoreceptor cell degeneration. This new proposed role of Ca2+/recoverin may be one of the specific features of the proposed new Type III class or rhodopsin mutations associated with retinitis pigmentosa.

Active and passive displacement of transmembrane domains both occur during opsin biogenesis at the Sec61 translocon

We used a site-specific crosslinking approach to study the membrane integration of the polytopic protein opsin at the endoplasmic reticulum. We show that transmembrane domain 1 occupies two distinct Sec61-based environments during its integration. However, transmembrane domains 2 and 3 exit the Sec61 translocon more rapidly in a process that suggests a displacement model for their integration where the biosynthesis of one transmembrane domain would facilitate the exit of another. In order to investigate this hypothesis further, we studied the integration of the first and third transmembrane domains of opsin in the absence of any additional C-terminal transmembrane domains. In the case of transmembrane domain 1, we found that its lateral exit from the translocon is clearly dependent upon the synthesis of subsequent transmembrane domains. By contrast, the lateral exit of the third transmembrane domain occurred independently of any such requirement. Thus, even within a single polypeptide chain, distinct transmembrane domains display different requirements for their integration through the endoplasmic reticulum translocon, and the displacement of one transmembrane domain by another is not a global requirement for membrane integration.

Comparison of the retinitis pigmentosa mutations in rhodopsin with a functional map of the C5a receptor

We compare the known retinitis pigmentosa (RP) mutations in rhodopsin with mutational data obtained for the complement factor 5a receptor (C5aR), a member of the rhodopsin-like family of G protein-coupled receptors (GPCRs). We have performed genetic analyses that define residues that are required for C5aR folding and function. The cognate residues in rhodopsin are not preferentially mutated in RP, suggesting that the predominant molecular defect in RP involves more than simple misfolding or inactivation. Energy calculations are performed to elucidate the structural effects of the RP mutations. Many of these mutations specifically disrupt the environment of the retinal prosthetic group of rhodopsin, and these do not correspond to essential residues in C5aR. This may be because a retinal group is present in rhodopsin but not in C5aR. Another subset of RP mutations is more generally important for receptor structure, and these mutations correlate with essential residues of C5aR.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

Systematic investigation of genetic variability in 111 human genes-implications for studying variable drug response

In order to identify single-nucleotide polymorphisms (SNPs) and analyze their characteristics in a set of 111 genes, we resequenced exons and flanking regions in an average of 170 chromosomes from individuals of European origin. Genetic variability was decreased in noncoding regions highly conserved between human and rodents, indicating functional relevance of these regions. Furthermore, diversity of coding nonsynonymous SNPs was found lower in regions encoding a known protein sequence motif. SNPs predicted to be of functional significance were more common amongst rare variants. Despite the significant recent growth of SNP numbers in public SNP databases, only a small fraction of these rare variants is represented. This may be relevant in the investigation of the genetic causes of severe side effects, for which rare variants are plausible candidates. Estimation of htSNPs reduces the genotyping effort required in capturing common haplotypes, for certain genes, however, this accounts for only a small fraction of haplotype diversity.

The spectrum of human rhodopsin disease mutations through the lens of interspecific variation

Mutations in rhodopsin, the visual pigment found in rod cells, account for a large fraction of genetic changes underlying the human retinal diseases, Retinitis Pigmentosa (RP). The availability of rhodopsin sequences from a large number of vertebrates has allowed us to investigate factors important in the development of RP by contrasting interspecific differences (long-term evolutionary patterns) with RP disease mutation data. We find that disease mutations in rhodopsin are overabundant in highly conserved sites and that amino acid positions with any potential of variability among vertebrates are likely to harbour disease mutations less frequently. At any amino acid position in rhodopsin, the set of disease-associated amino acids does not show any commonality with the set of amino acids present among species. The disease mutations are biochemically four times more radical than the interspecific (neutral) variation. This pattern is also observed when disease mutations are categorized based on clinical classifications that reflect biochemical, physiological and psychophysical traits such as protein folding, cone electroretinogram (ERG) amplitude, pattern of visual field loss, and equivalent field diameter. We also found that for artificial mutations (those not observed in nature interspecifically), there was a positive relationship between the biochemical distance and the magnitude of blue shift in the absorption spectrum maximum. We introduce the concept of the expected chemical severity based on the normal human codon at a position. Results reveal that the analysis of disease mutations in the context of the original codon is very important for the practical application of evolutionary principles when comparing original and disease amino acid mutations. We conclude that the analysis of rhodopsin data clearly demonstrates the usefulness of molecular evolutionary analyses for understanding patterns of clinical as well as artificial mutations and underscores the biomedical insights that can be gained by using simple measures of biochemical difference in the context of evolutionary divergence.

Helix packing moments reveal diversity and conservation in membrane protein structure

Helical membrane proteins are more tightly packed and the packing interactions are more diverse than those found in helical soluble proteins. Based on a linear correlation between amino acid packing values and interhelical propensity, we propose the concept of a helix packing moment to predict the orientation of helices in helical membrane proteins and membrane protein complexes. We show that the helix packing moment correlates with the helix interfaces of helix dimers of single pass membrane proteins of known structure. Helix packing moments are also shown to help identify the packing interfaces in membrane proteins with multiple transmembrane helices, where a single helix can have multiple contact surfaces. Analyses are described on class A G protein-coupled receptors (GPCRs) with seven transmembrane helices. We show that the helix packing moments are conserved across the class A family of GPCRs and correspond to key structural contacts in rhodopsin. These contacts are distinct from the highly conserved signature motifs of GPCRs and have not previously been recognized. The specific amino acid types involved in these contacts, however, are not necessarily conserved between subfamilies of GPCRs, indicating that the same protein architecture can be supported by a diverse set of interactions. In GPCRs, as well as membrane channels and transporters, amino acid residues with small side-chains (Gly, Ala, Ser, Cys) allow tight helix packing by mediating strong van der Waals interactions between helices. Closely packed helices, in turn, facilitate interhelical hydrogen bonding of both weakly polar (Ser, Thr, Cys) and strongly polar (Asn, Gln, Glu, Asp, His, Arg, Lys) amino acid residues. We propose the use of the helix packing moment as a complementary tool to the helical hydrophobic moment in the analysis of transmembrane sequences.

Effect of dodecyl maltoside detergent on rhodopsin stability and function

Detergent-solubilized bovine rhodopsin produces mixed detergent/lipid/protein micelles. The effect of dodecyl maltoside detergent on the thermal stability of dark-state rhodopsin, and upon formation of the different intermediates after rhodopsin photobleaching (metarhodopsin II and metarhodopsin III), and upon transducin activation has been studied. No significant effect is observed for the thermal stability of dark-state rhodopsin in the range of detergent concentrations studied, but a decrease in the stability of metarhodopsin II and an increase in metarhodopsin III formation is observed with decreasing detergent concentrations. The transducin activation process is also affected by the presence of detergent indicating that this process is dependent on the lipid micro-environment and membrane fluidity, and this stresses the importance of the native lipid environment in rhodopsin normal function.

Building a stage for interhelical play in rhodopsin

Biochemical data providing new insights into the packing of helices I and II in the transmembrane domain of rhodopsin reveals the existence of a specific set of size- and charge-sensitive interhelical interactions that influence protein tertiary structure. These findings have broad implications towards understanding the molecular consequences of naturally occurring mutations associated with the retinal degenerative disease autosomal dominant retinitis pigmentosa.