The conformation of the cytoplasmic helix 8 of the CB1 cannabinoid receptor using NMR and circular dichroism

Deciphering the Molecular Association of Human CRIP1a with an Agonist-Bound Cannabinoid Receptor 1

Cannabinoid receptor 1 (CB1) is a class A G-protein-coupled receptor that plays important roles in several physiological and pathophysiological processes. Therefore, targeted regulation of CB1 activity is a potential therapeutic strategy for several diseases, including neurological disorders. Apart from cannabinoid ligands, CB1 signaling can also be regulated by different CB1-associated proteins. In particular, the cannabinoid receptor interacting protein 1a (CRIP1a) associates with an activated CB1 receptor and alters the G-protein selectivity, thereby reducing the agonist-mediated signal transduction of the CB1 receptor. Experimental evidence suggests that two peptides corresponding to the distal and central C-terminal segments of CB1 could interact with CRIP1a. However, our knowledge of the molecular basis of CB1-CRIP1a recognition is still limited. In this work, we use an extensive combination of computational methods to build the first comprehensive atomistic model human CB1-CRIP1a complex. Our model provides novel structural insights into the interactions of CRIP1a with a membrane-embedded, complete, agonist-bound CB1 receptor in humans. Our results highlight the key residues that stabilize the CB1-CRIP1a complex, which will be useful to guide in vitro mutagenesis experiments. Furthermore, our human CB1-CRIP1a complex presents a model system for structure-based drug design to target this physiologically important complex for modulating CB1 activity.

Conformational dynamics of a G protein-coupled receptor helix 8 in lipid membranes

G protein-coupled receptors (GPCRs) are the largest and pharmaceutically most important class of membrane proteins encoded in the human genome, characterized by a seven-transmembrane helix architecture and a C-terminal amphipathic helix 8 (H8). In a minority of GPCR structures solved to date, H8 either is absent or adopts an unusual conformation. The controversial existence of H8 of the class A GPCR neurotensin receptor 1 (NTS1) has been examined here for the nonthermostabilized receptor in a functionally supporting membrane environment using electron paramagnetic resonance, molecular dynamics simulations, and circular dichroism. Lipid-protein interactions with phosphatidylserine and phosphatidylethanolamine lipids, in particular, stabilize the residues 374 to 390 of NTS1 into forming a helix. Furthermore, introduction of a helix-breaking proline residue in H8 elicited an increase in ß-arrestin-NTS1 interactions observed in pull-down assays, suggesting that the structure and/or dynamics of H8 might play an important role in GPCR signaling.

Cannabidiol binding and negative allosteric modulation at the cannabinoid type 1 receptor in the presence of delta-9-tetrahydrocannabinol: An In Silico study

Recent evidence has raised in discussion the possibility that cannabidiol can act as a negative allosteric modulator of the cannabinoid type 1 receptor. Here we have used computational methods to study the modulation exerted by cannabidiol on the effects of delta-9-tetrahydrocannabinol in the cannabinoid receptor type 1 and the possibility of direct receptor blockade. We propose a putative allosteric binding site that is located in the N-terminal region of receptor, partially overlapping the orthosteric binding site. Molecular dynamics simulations reveled a coordinated movement involving the outward rotation of helixes 1 and 2 and subsequent expansion of the orthosteric binding site upon cannabidiol binding. Finally, changes in the cytoplasmic region and high helix 8 mobility were related to impaired receptor internalization. Together, these results offer a possible explanation to how cannabidiol can directly modulate effects of delta-9-tetrahydrocannabinol on the cannabinoid receptor type 1.

CB1 cannabinoid receptor-phosphorylated fourth intracellular loop structure-function relationships

A peptide comprising the juxtamembrane C-terminal intracellular loop 4 (IL4) of the CB₁ cannabinoid receptor possesses three Serine residues (Ser402, Ser411 and Ser415). Here we report the effect of Ser phosphorylation on the CB₁ IL4 peptide conformation and cellular signaling functions using nuclear magnetic resonance spectroscopy, circular dichroism, G protein activation and cAMP production. Circular dichroism studies indicated that phosphorylation at various Ser residues induced helical structure in different environments. NMR data indicates that helical content varies in the order of IL4pSer411 > IL4pSer415 > IL4 > IL4pSer402. The efficacy of phosphorylated IL4 peptides in activating Go and Gi3 ([³⁵S]GTPγS binding) and inhibiting cAMP accumulation in N18TG2 cells were correlated with helicity changes. Treatment of cells with bradykinin, which activates PKC, augmented CB₁-mediated inhibition of cAMP accumulation, and this was reversed by a PKC inhibitor, suggesting that phosphorylation of serine might be a physiologically relevant modification in vivo. We conclude that phosphorylation-dependent alterations of helicity of CB₁ IL4 peptides can increase efficacy of G protein signaling.

Helix 8 and the i3 loop of the muscarinic M3 receptor are crucial sites for its regulation by the Gβ5-RGS7 complex

The muscarinic M3 receptor (M3R) is a Gq-coupled receptor and is known to interact with many intracellular regulatory proteins. One of these molecules is Gβ5-RGS7, the permanently associated heterodimer of G protein β-subunit Gβ5 and RGS7, a regulator of G protein signaling. Gβ5-RGS7 can attenuate M3R-stimulated release of Ca²⁺ from intracellular stores or enhance the influx of Ca²⁺ across the plasma membrane. Here we show that deletion of amino acids 304–345 from the central portion of the i3 loop renders M3R insensitive to regulation by Gβ5-RGS7. In addition to the i3 loop, interaction of M3R with Gβ5-RGS7 requires helix 8. According to circular dichroism spectroscopy, the peptide corresponding to amino acids 548–567 in the C-terminus of M3R assumes an α-helical conformation. Substitution of Thr553 and Leu558 with Pro residues disrupts this α-helix and abolished binding to Gβ5-RGS7. Introduction of the double Pro substitution into full-length M3R (M3R^TP/LP) prevents trafficking of the receptor to the cell surface. Using atropine or other antagonists as pharmacologic chaperones, we were able to increase the level of surface expression of the TP/LP mutant to levels comparable to that of wild-type M3R. However, M3R-stimulated calcium signaling is still severely compromised. These results show that the interaction of M3R with Gβ5-RGS7 requires helix 8 and the central portion of the i3 loop.

Structure of the C-terminal region of the Frizzled receptor 1 in detergent micelles

The C-terminal domains of the Frizzleds (FZDs) contain a short conserved motif (KTXXXW). It has been demonstrated that FZDs interacted with the PDZ domain of the cytoplasmic proteins such as Dishevelled through this motif and mutations in this motif disrupted Wnt/β-catenin signaling. We carried out structural studies for a peptide derived from the C-terminal domain of the FZD₁ in different solvents using circular dichroism and solution NMR spectroscopy. Our results showed that this domain was unstructured in an aqueous solution and formed a helical structure in detergent micelles. Fluorescence studies suggested that the tryptophan residue (W630) in the motif interacted with micelles. The solution structure of the peptide in sodium dodecyl sulfate micelles was determined and an amphipathic helix was identified. This helix may have similar function to the helix 8 of other G protein-coupled receptors.

Recombinant cannabinoid type 2 receptor in liposome model activates g protein in response to anionic lipid constituents

Human cannabinoid type 2 (CB(2)) receptor expressed in Escherichia coli was purified and successfully reconstituted in the functional form into lipid bilayers composed of POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS), and cholesteryl hemisuccinate (CHS). Reconstitution was performed by detergent removal from the protein/lipid/detergent mixed micelles either on an adsorbent column, or by rapid dilution to below the critical micelle concentration of detergent followed by removal of detergent monomers on a concentrator. Proteoliposomes prepared at a protein/phospholipid/CHS molar ratio of 1/620-650/210-220 are free of detergent as shown by (1)H NMR, have a homogeneous protein/lipid ratio shown by isopycnic gradient ultracentrifugation, and are small in size with a mean diameter of 150-200 nm as measured by dynamic light scattering. Functional integrity of the reconstituted receptor was confirmed by quantitative binding of (2)H-labeled agonist CP-55,940-d(6) measured by (2)H magic angle spinning NMR, as well as by activation of G protein. The efficiency of G protein activation by agonist-bound CB(2) receptor was affected by negative electric surface potentials of proteoliposomes controlled by the content of anionic CHS or POPS. The activation was highest at an anionic lipid content of about 50 mol %. There was no correlation between the efficiency of G protein activation and an increase of hydrocarbon chain order induced by CHS or cholesterol. The results suggest the importance of anionic lipids in regulating signal transduction by CB(2) receptor and other class A GPCR. The successful reconstitution of milligram quantities of pure, functional CB(2) receptor enables a wide variety of structural studies.

Binding between a distal C-terminus fragment of cannabinoid receptor 1 and arrestin-2

Internalization of G-protein-coupled receptors is mediated by phosphorylation of the C-terminus, followed by binding with the cytosolic protein arrestin. To explore structural factors that may play a role in internalization of cannabinoid receptor 1 (CB1), we utilize a phosphorylated peptide derived from the distal C-terminus of CB1 (CB1(5P)(454-473)). Complexes formed between the peptide and human arrestin-2 (wt-arr2(1-418)) were compared to those formed with a truncated arrestin-2 mutant (tr-arr2(1-382)) using isothermal titration calorimetry and nuclear magnetic resonance spectroscopy. The pentaphosphopeptide CB1(5P)(454-473) adopts a helix-loop conformation, whether binding to full-length arrestin-2 or its truncated mutant. This structure is similar to that of a heptaphosphopeptide, mimicking the distal segment of the rhodopsin C-tail (Rh(7P)(330-348)), binding to visual arrestin, suggesting that this adopted structure bears functional significance. Isothermal titration calorimetry (ITC) experiments show that the CB1(5P)(454-473) peptide binds to tr-arr2(1-382) with higher affinity than to the full-length wt-arr2(1-418). As the observed structure of the bound peptides is similar in either case, we attribute the increased affinity to a more exposed binding site on the N-domain of the truncated arrestin construct. The transferred NOE data from the bound phosphopeptides are used to predict a model describing the interaction with arrestin, using the data driven HADDOCK docking program. The truncation of arrestin-2 provides scope for positively charged residues in the polar core of the protein to interact with phosphates present in the loop of the CB1(5P)(454-473) peptide.

The cannabinoid type-1 receptor carboxyl-terminus, more than just a tail

The cannabinoid type-1 (CB(1)) receptor is a G protein-coupled receptor that binds the main active ingredient of marijuana, Δ(9)-tetrahydrocannabinol, and has been implicated in several disease states, including drug addiction, anxiety, depression, obesity, and chronic pain. In the two decades since the discovery of CB(1), studies at the molecular level have centered on the transmembrane core. This interest has now expanded as we discover that other regions of CB(1), including the CB(1) carboxyl-terminus, have critical structures that are important for CB(1) activity and regulation. Following the recent description of the three dimensional structure of the full-length CB(1) carboxyl-terminal tail [Biopolymers (2009) vol. 91, pp. 565-573], several residues and structural motifs including two α-helices (termed H8 and H9) have been postulated to interact with common G protein-coupled receptor accessory proteins, such as G-proteins and β-arrestins. This discourse will focus on the CB(1) carboxyl-terminus; our current understanding of the structural features of this region, evidence for its interaction with proteins, and the impact of structure on the binding and regulatory function of CB(1) accessory proteins. The involvement of the carboxyl-terminus in the receptor life cycle including activation, desensitization, and internalization will be highlighted.

Human cannabinoid 1 GPCR C-terminal domain interacts with bilayer phospholipids to modulate the structure of its membrane environment

G protein-coupled receptors (GPCRs) play critical physiological and therapeutic roles. The human cannabinoid 1 GPCR (hCB1) is a prime pharmacotherapeutic target for addiction and cardiometabolic disease. Our prior biophysical studies on the structural biology of a synthetic peptide representing the functionally significant hCB1 transmembrane helix 7 (TMH7) and its cytoplasmic extension, helix 8 (H8), [hCB1(TMH7/H8)] demonstrated that the helices are oriented virtually perpendicular to each other in membrane-mimetic environments. We identified several hCB1(TMH7/H8) structure-function determinants, including multiple electrostatic amino-acid interactions and a proline kink involving the highly conserved NPXXY motif. In phospholipid bicelles, TMH7 structure, orientation, and topology relative to H8 are dynamically modulated by the surrounding membrane phospholipid bilayer. These data provide a contextual basis for the present solid-state NMR study to investigate whether intermolecular interactions between hCB1(TMH7/H8) and its phospholipid environment may affect membrane-bilayer structure. For this purpose, we measured (1)H-(13)C heteronuclear dipolar couplings for the choline, glycerol, and acyl-chain regions of dimyristoylphosphocholine in a magnetically aligned hCB1(TMH7/H8) bicelle sample. The results identify discrete regional interactions between hCB1(TMH7/H8) and membrane lipid molecules that increase phospholipid motion and decrease phospholipid order, indicating that the peptide's partial traversal of the bilayer alters membrane structure. These data offer new insight into hCB1(TMH7/H8) properties and support the concept that the membrane bilayer itself may serve as a mechanochemical mediator of hCB1/GPCR signal transduction. Since interaction with its membrane environment has been implicated in hCB1 function and its modulation by small-molecule therapeutics, our work should help inform hCB1 pharmacology and the design of hCB1-targeted drugs.

CB(1) cannabinoid receptors and their associated proteins

CB1 receptors are G-protein coupled receptors (GPCRs) abundant in neurons, in which they modulate neurotransmission. The CB(1) receptor influence on memory and learning is well recognized, and disease states associated with CB(1) receptors are observed in addiction disorders, motor dysfunction, schizophrenia, and in bipolar, depression, and anxiety disorders. Beyond the brain, CB(1) receptors also function in liver and adipose tissues, vascular as well as cardiac tissue, reproductive tissues and bone. Signal transduction by CB(1) receptors occurs through interaction with Gi/o proteins to inhibit adenylyl cyclase, activate mitogen-activated protein kinases (MAPK), inhibit voltage-gated Ca(2+) channels, activate K(+) currents (K(ir)), and influence Nitric Oxide (NO) signaling. CB(1) receptors are observed in internal organelles as well as plasma membrane. beta-Arrestins, adaptor protein AP-3, and G-protein receptor-associated sorting protein 1 (GASP1) modulate cellular trafficking. Cannabinoid Receptor Interacting Protein1a (CRIP1a) is an accessory protein whose function has not been delineated. Factor Associated with Neutral sphingomyelinase (FAN) regulates ceramide signaling. Such diversity in cellular signaling and modulation by interacting proteins suggests that agonists and allosteric modulators could be developed to specifically regulate unique, cell type-specific responses.

Hydrophobic residues in helix 8 of cannabinoid receptor 1 are critical for structural and functional properties

In addition to the heptahelical transmembrane domain shared by all G protein-coupled receptors (GPCRs), many class A GPCRs adopt a helical domain, termed helix 8, in the membrane-proximal region of the C terminus. We investigated the role of residues in the hydrophobic and hydrophilic faces of amphiphilic helix 8 of human cannabinoid receptor 1 (CB1). To differentiate between a role for specific residues and global features, we made two key mutants: one involving replacement of the highly hydrophobic groups, Leu404, Phe408, and Phe412, all with alanine and the second involving substitution of the basic residues, Lys402, Arg405, and Arg409, all with the neutral glutamine. The former showed a very low B(max) based on binding isotherms, a minimal E(max) based on GTPgammaS binding analysis, and defective localization relative to the wild-type CB1 receptor as revealed by confocal microscopy. However, the latter mutant and the wild-type receptors were indistinguishable. Circular dichroism spectroscopy of purified peptides with corresponding sequences indicated that the highly hydrophobic residues are critical for maintaining a strong helical structure in detergent, whereas the positively charged residues are not. Further investigation of mutant receptors revealed that CB1 localization requires a threshold level of hydrophobicity but not specific amino acids. Moreover, mutant receptors carrying two- to six-residue insertions amino-terminal to helix 8 revealed a graded decrease in B(max) values. Our results identify the key helix 8 components (including hydrophobicity of specific residues, structure, and location relative to TM7) determinant for receptor localization leading to robust ligand binding and G protein activation.

Unraveling the structure and function of G protein-coupled receptors through NMR spectroscopy

G protein-coupled receptors (GPCRs) are a large superfamily of signaling proteins expressed on the plasma membrane. They are involved in a wide range of physiological processes and, therefore, are exploited as drug targets in a multitude of therapeutic areas. In this extent, knowledge of structural and functional properties of GPCRs may greatly facilitate rational design of modulator compounds. Solution and solid-state nuclear magnetic resonance (NMR) spectroscopy represents a powerful method to gather atomistic insights into protein structure and dynamics. In spite of the difficulties inherent the solution of the structure of membrane proteins through NMR, these methods have been successfully applied, sometimes in combination with molecular modeling, to the determination of the structure of GPCR fragments, the mapping of receptor-ligand interactions, and the study of the conformational changes associated with the activation of the receptors. In this review, we provide a summary of the NMR contributions to the study of the structure and function of GPCRs, also in light of the published crystal structures.

Membrane protein fragments reveal both secondary and tertiary structure of membrane proteins

Structural data on membrane proteins, while crucial to understanding cellular function, are scarce due to difficulties in applying to membrane proteins the common techniques of structural biology. Fragments of membrane proteins have been shown to reflect, in many cases, the secondary structure of the parent protein with fidelity and are more amenable to study. This chapter provides many examples of how the study of membrane protein fragments has provided new insight into the structure of the parent membrane protein.

Conformational and dynamics simulation study of antimicrobial peptide hedistin-heterogeneity of its helix-turn-helix motif

Hedistin is an antimicrobial peptide isolated from the coelomocytes of Nereis diversicolor, possessing activity against a large spectrum of bacteria including the methicillin resistant Staphylococcus aureus and Vibrio alginolyticus. The three-dimensional structure of hedistin in both aqueous solution and deuterated dodecylphosphocholine (DPC) micelles was examined using circular dichroism (CD) and nuclear magnetic resonance (NMR) techniques. And, the early events of the antibacterial process of hedistin were simulated using palmitoyl-oleoyl-phophatidylcholine (POPC) lipid bilayers and molecular dynamics (MD) simulation methods. Hedistin lacks secondary structure in aqueous solution, however, in DPC micelles, it features with a heterogeneous helix-turn-helix moiety and exhibits obvious amphipathic nature. The turn region (residues Val9-Thr12) in the moiety is a four-residue hinge, lying in between the first N-terminal alpha-helix (residues Leu5-Lys8) and the second alpha-helix (residues Val13-Ala17) regions and causing an approximately 120 degrees angle between the axes of the two helices. The segmental and nonlinear nature of hedistin structure is referred to as the heterogeneity of its helix-turn-helix motif which was found to be corresponding to a kind of discrete dynamics behavior, herein coined as its dynamical heterogeneity, at the early stage (0-50 ns) of the MD simulations. That is, the first helix segment, prior to (at 310 K) or following (at 363 K) the second helix, binds to the lipid head-group region and subsequently permeates into the hydrophobic lipid tail region, and the hinge is the last portion entering the lipid environment. This result implies that hedistin may adopt a "carpet" model action when disrupting bacterial membrane.

NMR solution structure of human cannabinoid receptor-1 helix 7/8 peptide: candidate electrostatic interactions and microdomain formation

We report the NMR solution structure of a synthetic 40-mer (T(377)-E(416)) that encompasses human cannabinoid receptor-1 (hCB1) transmembrane helix 7 (TMH7) and helix 8 (H8) [hCB1(TMH7/H8)] in 30% trifluoroethanol/H(2)O. Structural features include, from the peptide's amino terminus, a hydrophobic alpha-helix (TMH7); a loop-like, 11 residue segment featuring a pronounced Pro-kink within the conserved NPxxY motif; a short amphipathic alpha-helix (H8) orthogonal to TMH7 with cationic and hydrophobic amino-acid clusters; and an unstructured C-terminal end. The hCB1(TMH7/H8) NMR solution structure suggests multiple electrostatic amino-acid interactions, including an intrahelical H8 salt bridge and a hydrogen-bond network involving the peptide's loop-like region. Potential cation-pi and cation-phenolic OH interactions between Y(397) in the TMH7 NPxxY motif and R(405) in H8 are identified as candidate structural forces promoting interhelical microdomain formation. This microdomain may function as a flexible molecular hinge during ligand-induced hCB1 conformer transitions.

Structural analysis of the human cannabinoid receptor one carboxyl-terminus identifies two amphipathic helices

Recent research has implicated the C-terminus of G-protein coupled receptors in key events such as receptor activation and subsequent intracellular sorting, yet obtaining structural information of the entire C-tail has proven a formidable task. Here, a peptide corresponding to the full-length C-tail of the human CB1 receptor (residues 400-472) was expressed in E.coli and purified in a soluble form. Circular dichroism (CD) spectroscopy revealed that the peptide adopts an alpha-helical conformation in negatively charged and zwitterionic detergents (48-51% and 36-38%, respectively), whereas it exhibited the CD signature of unordered structure at low concentration in aqueous solution. Interestingly, 27% helicity was displayed at high peptide concentration suggesting that self-association induces helix formation in the absence of a membrane mimetic. NMR spectroscopy of the doubly labeled ((15)N- and (13)C-) C-terminus in dodecylphosphocholine (DPC) identified two amphipathic alpha-helical domains. The first domain, S401-F412, corresponds to the helix 8 common to G protein-coupled receptors while the second domain, A440-M461, is a newly identified structural motif in the distal region of the carboxyl-terminus of the receptor. Molecular modeling of the C-tail in DPC indicates that both helices lie parallel to the plane of the membrane with their hydrophobic and hydrophilic faces poised for critical interactions.

Dynamic conformational responses of a human cannabinoid receptor-1 helix domain to its membrane environment

The influence of membrane environment on human cannabinoid 1 (hCB(1)) receptor transmembrane helix (TMH) conformational dynamics was investigated by solid-state NMR and site-directed spin labeling/EPR with a synthetic peptide, hCB(1)(T377-E416), corresponding to the receptor's C-terminal component, i.e., TMH7 and its intracellular alpha-helical extension (H8) (TMH7/H8). Solid-state NMR experiments with mechanically aligned hCB(1)(T377-E416) specifically (2)H- or (15)N-labeled at Ala380 and reconstituted in membrane-mimetic dimyristoylphosphocholine (DMPC) or 1-palmitoyl-2-oleoyl-sn-glycerophosphocholine (POPC) bilayers demonstrate that the conformation of the TMH7/H8 peptide is more heterogeneous in the thinner DMPC bilayer than in the thicker POPC bilayer. As revealed by EPR studies on hCB(1)(T377-E416) spin-labeled at Cys382 and reconstituted into the phospholipid bilayers, the spin label partitions actively between hydrophobic and hydrophilic environments. In the DMPC bilayer, the hydrophobic component dominates, regardless of temperature. Mobility parameters (DeltaH(0)(-1)) are 0.3 and 0.73 G for the peptide in the DMPC or POPC bilayer environment, respectively. Interspin distances of doubly labeled hCB(1)(T377-E416) peptide reconstituted into a TFE/H(2)O mixture or a POPC or DMPC bilayer were estimated to be 10.6 +/- 0.5, 16.8 +/- 1, and 11.6 +/- 0.8 A, respectively. The extent of coupling (>or=50%) between spin labels located at i and i + 4 in a TFE/H(2)O mixture or a POPC bilayer is indicative of an alpha-helical TMH conformation, whereas the much lower coupling (14%) when the peptide is in a DMPC bilayer suggests a high degree of peptide conformational heterogeneity. These data demonstrate that hCB(1)(T377-E416) backbone dynamics as well as spin-label rotameric freedom are sensitive to and altered by the peptide's phospholipid bilayer environment, which exerts a dynamic influence on the conformation of a TMH critical to signal transmission by the hCB(1) receptor.

Beta-arrestin-based Bret2 screening assay for the "non"-beta-arrestin binding CB1 receptor

CB1 receptor (CB1R) antagonists have been demonstrated to be effective in treating obesity and related disorders. This study has been focused on establishing a beta-arrestin 2-based screening assay for the CB1R using BRET2 technology. When the existing BRET2 screening platform was applied to the CB1R, the authors discovered that the receptor interacted weakly with beta-arrestin 2, resulting in unsatisfactory assay performance. To enhance the beta-arrestin binding capacity, they replaced the C-terminal tail of the CB1R with tails from either the V2 or BRS3 receptors, both of which interact strongly with beta-arrestin 2. Using this chimeric approach, the authors screened a small compound library and identified 21 antagonist and inverse agonist hits with IC50 and EC50 values ranging from 0.3 nM to 7.5 microM. Both primary and secondary screening were performed with Z'>0.5, suggesting that the assay is a robust and cost-effective alternative to existing cell-based assays.

Modulation of molecular interactions and function by rhodopsin palmitylation

Rhodopsin is palmitylated at two cysteine residues in its carboxyl terminal region. We have looked at the effects of palmitylation on the molecular interactions formed by rhodopsin using single-molecule force spectroscopy and the function of rhodopsin using both in vitro and in vivo approaches. A knockin mouse model expressing palmitate-deficient rhodopsin was used for live animal in vivo studies and to obtain native tissue samples for in vitro assays. We specifically looked at the effects of palmitylation on the chromophore-binding pocket, interactions of rhodopsin with transducin, and molecular interactions stabilizing the receptor structure. The structure of rhodopsin is largely unperturbed by the absence of palmitate linkage. The binding pocket for the chromophore 11-cis-retinal is minimally altered as palmitate-deficient rhodopsin exhibited the same absorbance spectrum as wild-type rhodopsin. Similarly, the rate of release of all-trans-retinal after light activation was the same both in the presence and absence of palmitylation. Significant differences were observed in the rate of transducin activation by rhodopsin and in the force required to unfold the last stable structural segment in rhodopsin at its carboxyl terminal end. A 1.3-fold reduction in the rate of transducin activation by rhodopsin was observed in the absence of palmitylation. Single-molecule force spectroscopy revealed a 2.1-fold reduction in the normalized force required to unfold the carboxyl terminal end of rhodopsin. The absence of palmitylation in rhodopsin therefore destabilizes the molecular interactions formed in the carboxyl terminal end of the receptor, which appears to hinder the activation of transducin by light-activated rhodopsin.

Solid-state NMR and molecular dynamics characterization of cannabinoid receptor-1 (CB1) helix 7 conformational plasticity in model membranes

Little direct information is available regarding the influence of membrane environment on transmembrane (TM) G-protein-coupled receptor (GPCR) conformation and dynamics. The human CB1 cannabinoid receptor (hCB1) is a prominent GPCR pharmacotherapeutic target in which helix 7 appears critical to ligand recognition. We have chemically synthesized a hCB1 peptide corresponding to a segment of TM helix 7 and the entire contiguous helix 8 domain (fourth cytoplasmic loop) and reconstituted it in defined phospholipid-bilayer model membranes. Using an NMR-based strategy combined with molecular dynamics simulations, we provide the first direct experimental description of the orientation of hCB1 helix 7 in phospholipid membranes of varying thickness and the mechanism by which helix-7 conformation adjusts to avoid hydrophobic mismatch. Solid-state (15)N NMR data show that hCB1 helices 7 and 8 reconstituted into phospholipid bilayers are oriented in a TM and in-plane (i.e., parallel to the phospholipid membrane surface) fashion, respectively. TM helix orientation is influenced by the thickness of the hydrophobic membrane bilayer as well as the interaction of helix 8 with phospholipid polar headgroups. Molecular dynamics simulations show that a decrease in phospholipid chain-length induces a kink at P394 in TM helix 7 to avoid hydrophobic mismatch. Thus, the NP(X)nY motif found in hCB1 and highly conserved throughout the GPCR superfamily is important for flexing helix 7 to accommodate bilayer thickness. Dynamic modulation of hCB1-receptor TM helix conformation by its membrane environment may have general relevance to GPCR structure and function.

Functional and biophysical analysis of the C-terminus of the CGRP-receptor; a family B GPCR

G-protein coupled receptors (GPCRs) typically have a functionally important C-terminus which, in the largest subfamily (family A), includes a membrane-parallel eighth helix. Mutations of this region are associated with several diseases. There are few C-terminal studies on the family B GPCRs and no data supporting the existence of a similar eighth helix in this second major subfamily, which has little or no sequence homology to family A GPCRs. Here we show that the C-terminus of a family B GPCR (CLR) has a disparate region from N400 to C436 required for CGRP-mediated internalization, and a proximal region of twelve residues (from G388 to W399), in a similar position to the family A eighth helix, required for receptor localization at the cell surface. A combination of circular and linear dichroism, fluorescence and modified waterLOGSY NMR spectroscopy (SALMON) demonstrated that a peptide mimetic of this domain readily forms a membrane-parallel helix anchored to the liposome by an interfacial tryptophan residue. The study reveals two key functions held within the C-terminus of a family B GPCR and presents support for an eighth helical region with striking topological similarity to the nonhomologous family A receptor. This helix structure appears to be found in most other family B GPCRs.

Computational modeling study of functional microdomains in cannabinoid receptor type 1

The seven transmembrane helices (TMH) G-protein-coupled receptors (GPCRs) constitute one of the largest superfamily of signaling proteins found in mammals. Some of its members, in which the cannabinoid (CB) receptors are included, stand out because their functional states can be modulated by a broad spectrum of effector molecules. The relative ligand promiscuity exhibited by these receptors could be related with particular attributes conferred by their molecular architecture and represents a motivating issue to be explored. In this regard, this study represents an effort to investigate the cannabinoid receptor type 1 (CB1) ligand recognition plasticity, using comparative modeling, molecular dynamics (MD) simulations and docking. Our results suggest that a cooperative set of subtle structural rearrangements within the TMHs provide to the CB1 protein the plasticity to reach alternate configurations. These changes include the relaxation of intramolecular constraints, the rotations, translations and kinks of the majority of TMHs and the reorganization of the ligand binding cavities.

Solution NMR of signal peptidase, a membrane protein

Useful solution nuclear magnetic resonance (NMR) data can be obtained from full-length, enzymatically active type I signal peptidase (SPase I), an integral membrane protein, in detergent micelles. Signal peptidase has two transmembrane segments, a short cytoplasmic loop, and a 27-kD C-terminal catalytic domain. It is a critical component of protein transport systems, recognizing and cleaving amino-terminal signal peptides from preproteins during the final stage of their export. Its structure and interactions with the substrate are of considerable interest, but no three-dimensional structure of the whole protein has been reported. The structural analysis of intact membrane proteins has been challenging and only recently has significant progress been achieved using NMR to determine membrane protein structure. Here we employ NMR spectroscopy to study the structure of the full-length SPase I in dodecylphosphocholine detergent micelles. HSQC-TROSY spectra showed resonances corresponding to approximately 3/4 of the 324 residues in the protein. Some sequential assignments were obtained from the 3D HNCACB, 3D HNCA, and 3D HN(CO) TROSY spectra of uniformly 2H, 13C, 15N-labeled full-length SPase I. The assigned residues suggest that the observed spectrum is dominated by resonances arising from extramembraneous portions of the protein and that the transmembrane domain is largely absent from the spectra. Our work elucidates some of the challenges of solution NMR of large membrane proteins in detergent micelles as well as the future promise of these kinds of studies.

Interaction of a fragment of the cannabinoid CB1 receptor C-terminus with arrestin-2

Desensitization of the cannabinoid CB1 receptor is mediated by the interaction with arrestin. In this study, we report the structural changes of a synthetic diphosphorylated peptide corresponding to residues 419-439 of the CB1 C-terminus upon binding to arrestin-2. This segment is pivotal to the desensitization of CB1. Using high-resolution proton NMR, we observe two helical segments in the bound peptide that are separated by the presence a glycine residue. The binding we observe is with a diphoshorylated peptide, whereas a previous study reported binding of a highly phosphorylated rhodopsin fragment to visual arrestin. The arrestin bound conformations of the peptides are compared.

Determination of solution structure and lipid micelle location of an engineered membrane peptide by using one NMR experiment and one sample

Antimicrobial peptides are universal host defense membrane-targeting molecules in a variety of life forms. Structure elucidation provides important insight into the mechanism of action. Here we present the three-dimensional structure of a membrane peptide in complex with dioctanoyl phosphatidylglycerol (D8PG) micelles determined by solution NMR spectroscopy. The model peptide, derived from the key antibacterial region of human LL-37, adopted an amphipathic helical structure based on 182 NOE-generated distance restraints and 34 chemical shift-derived angle restraints. Using the same NOESY experiment, it is also possible to delineate in detail the location of this peptide in lipid micelles via one-dimensional slice analysis of the intermolecular NOE cross peaks between the peptide and lipid. Hydrophobic aromatic side chains gave medium to strong NOE cross peaks, backbone amide protons and interfacial arginine side chain HN protons showed weak cross peaks, and arginine side chains on the hydrophilic face yielded no cross peaks with D8PG. Such a peptide-lipid intermolecular NOE pattern indicates a surface location of the amphipathic helix on the lipid micelle. In contrast, the epsilon HN protons of the three arginine side chains showed more or less similar intermolecular NOE cross peaks with lipid acyl chains when the helical structure was disrupted by selective d-amino acid incorporation, providing the basis for the selective toxic effect of the peptide against bacteria but not human cells. The differences in the intermolecular NOE patterns indicate that these peptides interact with model membranes in different mechanisms. Major NMR experiments for detecting protein-lipid NOE cross peaks are discussed.

Helix 8 Leu in the CB1 cannabinoid receptor contributes to selective signal transduction mechanisms

The intracellular C-terminal helix 8 (H8) of the CB(1) cannabinoid receptor deviates from the highly conserved NPXXY(X)(5,6)F G-protein-coupled receptor motif, possessing a Leu instead of a Phe. We compared the signal transduction capabilities of CB(1) with those of an L7.60F mutation and an L7.60I mutation that mimics the CB(2) sequence. The two mutant receptors differed from wild type (WT) in their ability to regulate G-proteins in the [(35)S]guanosine 5'-3-O-(thio)triphosphate binding assay. The L7.60F receptor exhibited attenuated stimulation by agonists WIN-55,212-2 and CP-55,940 but not HU-210, whereas the L7.60I receptor exhibited impaired stimulation by all agonists tested as well as by the inverse agonist rimonabant. The mutants internalized more rapidly than WT receptors but could equally sequester G-proteins from the somatostatin receptor. Both the time course and maximal N-type Ca(2+) current inhibition by WIN-55,212-2 were reduced in the mutants. Reconstitution experiments with pertussis toxin-insensitive G-proteins revealed loss of coupling to Galpha(i3) but not Galpha(0A) in the L7.60I mutant, whereas the reduction in the time course for the L7.60F mutant was governed by Galpha(i3). Furthermore, Galpha(i3) but not Galpha(0A) enhanced basal facilitation ratio, suggesting that Galpha(i3) is responsible for CB(1) tonic activity. Co-immunoprecipitation studies revealed that both mutant receptors were associated with Galpha(i1) or Galpha(i2) but not with Galpha(i3). Molecular dynamics simulations of WT CB(1) receptor and each mutant in a 1-palmitoyl-2-oleoylphosphatidylcholine bilayer suggested that the packing of H8 is different in each. The hydrogen bonding patterns along the helix backbones of each H8 also are different, as are the geometries of the elbow region of H8 (R7.56(400)-K7.58(402)). This study demonstrates that the evolutionary modification to NPXXY(X)(5,6)L contributes to maximal activity of the CB(1) receptor and provides a molecular basis for the differential coupling observed with chemically different agonists.

Unique helical conformation of the fourth cytoplasmic loop of the CB1 cannabinoid receptor in a negatively charged environment

The proximal portion of the C-terminus of the CB(1) cannabinoid receptor is a primary determinant for G-protein activation. A 17 residue proximal C-terminal peptide (rodent CB1 401-417), the intracellular loop 4 (IL4) peptide, mimicked the receptor's G-protein activation domain. Because of the importance of the cationic amino acids to G-protein activation, the three-dimensional structure of the IL4 peptide in a negatively charged sodium dodecyl sulfate (SDS) micellar environment has been studied by two-dimensional proton nuclear magnetic resonance (2D (1)H NMR) spectroscopy and distance geometry calculations. Unambiguous proton NMR assignments were carried out with the aid of correlation spectroscopy (DQF-COSY and TOCSY) and nuclear Overhauser effect spectroscopy (NOESY and ROESY) experiments. The distance constraints were used in torsion angle dynamics algorithm for NMR applications (DYANA) to generate a family of structures which were refined using restrained energy minimization and dynamics. In water, the IL4 peptide prefers an extended conformation, whereas in SDS micelles, 3(10)-helical conformation is induced. The predominance of 3(10)-helical domain structure in SDS represents a unique difference compared with structure in alternative environments, which can significantly impact global electrostatic surface potential on the cytoplasmic surface of the CB(1) receptor and might influence the signal to the G-proteins.

G-protein coupled receptor structure

Because of their central role in regulation of cellular function, structure/function relationships for G-protein coupled receptors (GPCR) are of vital importance, yet only recently have sufficient data been obtained to begin mapping those relationships. GPCRs regulate a wide range of cellular processes, including the senses of taste, smell, and vision, and control a myriad of intracellular signaling systems in response to external stimuli. Many diseases are linked to GPCRs. A critical need exists for structural information to inform studies on mechanism of receptor action and regulation. X-ray crystal structures of only one GPCR, in an inactive state, have been obtained to date. However considerable structural information for a variety of GPCRs has been obtained using non-crystallographic approaches. This review begins with a review of the very earliest GPCR structural information, mostly derived from rhodopsin. Because of the difficulty in crystallizing GPCRs for X-ray crystallography, the extensive published work utilizing alternative approaches to GPCR structure is reviewed, including determination of three-dimensional structure from sparse constraints. The available X-ray crystallographic analyses on bovine rhodopsin are reviewed as the only available high-resolution structures for any GPCR. Structural information available on ligand binding to several receptors is included. The limited information on excited states of receptors is also reviewed. It is concluded that while considerable basic structural information has been obtained, more data are needed to describe the molecular mechanism of activation of a GPCR.

Solution NMR of membrane proteins: practice and challenges

This review focuses upon the application of solution NMR methods to multispan integral membrane proteins, particularly with respect to determination of global folds by this approach. Practical methods are described along with the special difficulties that confront the application of solution NMR to proteins that dwell in the netherworld of the lipid bilayer.

High-resolution NMR spectroscopy of a GPCR in aligned bicelles

Solid-state NMR spectra with single-site resolution of CXCR1, a G protein-coupled receptor (GPCR), were obtained in magnetically aligned phospholipid bicelles. These results demonstrate that GPCRs in phospholipid bilayers are suitable samples for structure determination by solid-state NMR. The spectra also enable studies of drug-receptor interactions.

Solution NMR structure of an immunodominant epitope of myelin basic protein. Conformational dependence on environment of an intrinsically unstructured protein

Using solution NMR spectroscopy, three-dimensional structures have been obtained for an 18-residue synthetic polypeptide fragment of 18.5 kDa myelin basic protein (MBP, human residues Q81-T98) under three conditions emulating the protein's natural environment in the myelin membrane to varying degrees: (a) an aqueous solution (100 mM KCl pH 6.5), (b) a mixture of trifluoroethanol (TFE-d2) and water (30 : 70% v/v), and (c) a dispersion of 100 mM dodecylphosphocholine (DPC-d38, 1 : 100 protein/lipid molar ratio) micelles. This polypeptide sequence is highly conserved in MBP from mammals, amphibians, and birds, and comprises a major immunodominant epitope (human residues N83-T92) in the autoimmune disease multiple sclerosis. In the polypeptide fragment, this epitope forms a stable, amphipathic, alpha helix under organic and membrane-mimetic conditions, but has only a partially helical conformation in aqueous solution. These results are consistent with recent molecular dynamics simulations that showed this segment to have a propensity to form a transient alpha helix in aqueous solution, and with electron paramagnetic resonance (EPR) experiments that suggested a alpha-helical structure when bound to a membrane [I. R. Bates, J. B. Feix, J. M. Boggs & G. Harauz (2004) J Biol Chem, 279, 5757-5764]. The high sensitivity of the epitope structure to its environment is characteristic of intrinsically unstructured proteins, like MBP, and reflects its association with diverse ligands such as lipids and other proteins.

The hydrophobic amino acid residues in the membrane-proximal C tail of the G protein-coupled vasopressin V2 receptor are necessary for transport-competent receptor folding

It is believed that the membrane-proximal C tail of the G protein-coupled receptors forms an additional alpha helix with amphipathic properties (helix 8). It was previously shown for the vasopressin V2 receptor (V2R) that a conserved dileucine motif (L(339), L(340)) in this putative helix 8 is necessary for endoplasmic reticulum (ER) to Golgi transfer of the receptor. Here, we demonstrate that the other hydrophobic residues forming the non-polar side of this helix (F(328), V(332) and L(336)) are also transport-relevant. In contrast, the multiple serine residues contributing to the more hydrophilic side (S(330), S(331), S(333), S(334), S(338)) do not influence receptor trafficking. In addition, we show unambiguously by the use of pharmacological chaperones that the hydrophobic residues of the putative helix 8 do not form a transport signal necessary for receptor sorting into ER to Golgi vesicles. Instead, they are necessary to establish a transport-competent folding state in the early secretory pathway.