The N-Terminal End Truncated Mu-Opioid Receptor: from Expression to Circular Dichroism Analysis

Origin of Secondary Structure Transitions and Peptide Self-Assembly Propensity in Trifluoroethanol-Water Mixtures

Membrane-peptide interactions are key to the formation of helical intermediates in the early stages of amyloidogenesis. Aqueous solutions of 2,2,2-trifluoroethanol (TFE) provide a membrane-mimetic environment capable of promoting and stabilizing local peptide interactions. Uperin 3.5 (U3.5), a 17-residue and amidated antimicrobial peptide, is unstructured in water but self-assembles into fibrils in the presence of salt. Secondary structure transitions linked to U3.5 self-assembly were investigated in TFE/water mixtures, in both the absence and presence of salt, to assess the role of membrane-peptide interactions on peptide self-assembly and amyloid formation. A 5-to-7-fold increase in fibril yield of U3.5 was observed at low TFE concentrations (10% TFE/water v/v) compared with physiological buffer but only in the presence of salt. No aggregation was observed in salt-free TFE/water mixtures. Circular dichroism spectra showed that partial helical structures, initially stabilized by TFE, transitioned to β-sheet-rich aggregates in a saline buffer. Molecular dynamics simulations confirmed that TFE and salt act synergistically to enhance peptide-peptide interactions, resulting in β-sheet-rich U3.5 oligomers at low TFE concentrations. Specifically, TFE stabilized amphipathic, helical intermediates, leading to increased peptide-peptide attraction through hydrophobic interactions. The presence of salt further enhanced the peptide-peptide interactions by screening positively charged residues. Thus, the study revealed the role of a membrane mimic in stabilizing helical intermediates on the pathway to amyloid formation in the antimicrobial U3.5 peptide.

About TFE: Old and New Findings

The fluorinated alcohol 2,2,2-Trifluoroethanol (TFE) has been implemented for many decades now in conformational studies of proteins and peptides. In peptides, which are often disordered in aqueous solutions, TFE acts as secondary structure stabilizer and primarily induces an α -helical conformation. The exact mechanism through which TFE plays its stabilizing roles is still debated and direct and indirect routes, relying either on straight interaction between TFE and molecules or indirect pathways based on perturbation of solvation sphere, have been proposed. Another still unanswered question is the capacity of TFE to favor in peptides a bioactive or a native-like conformation rather than simply stimulate the raise of secondary structure elements that reflect only the inherent propensity of a specific amino-acid sequence. In protein studies, TFE destroys unique protein tertiary structure and often leads to the formation of non-native secondary structure elements, but, interestingly, gives some hints about early folding intermediates. In this review, we will summarize proposed mechanisms of TFE actions. We will also describe several examples, in which TFE has been successfully used to reveal structural properties of different molecular systems, including antimicrobial and aggregation-prone peptides, as well as globular folded and intrinsically disordered proteins.

Characterization of an engineered water-soluble variant of the full-length human mu opioid receptor

A water-soluble variant of the transmembrane domain of the human mu opioid receptor (wsMOR-TM) was previously characterized. This study explored whether the full-length version of the engineered water-soluble receptor, (wsMOR-FL), could be overexpressed in Escherichia coli and if it would retain water solubility, binding capability and thermostability. wsMOR was over-expressed and purified in E. coli BL21(DE3) cells (EMD/Novagen) as we reported previously for the wsMOR-TM. Both native N and C termini were added back to the highly engineered wsMOR-TM. Six His-tag was added in the N terminus for purification purposes. The wsMOR-FL was characterized using atomic force microscope for its monomeric state, circular dichroism for its secondary structure and thermostability. Its binding with naltrexone is also determined. Compared to the native human MOR, wsMOR-FL displays similar helical secondary structure content and comparable affinity (nM) for the antagonist naltrexone. The secondary structure of the receptor remains stable within a wide range of pH (6-9). In contrast to the transmembrane portion, the secondary structure of full-length receptor tolerated a wide range of temperature (10-90 °C). The receptor remains predominantly as a monomer in solution, as directly imaged using atomic force microscopy. This study demonstrated that functional full-length water-soluble variant of human mu receptor can be over-expressed and purified using an E. coli over-expression system. This provides a novel tool for the investigation of structural and functional properties of the human MOR. N- and C-termini strengthened the thermostability of the protein in this specific water soluble variant. Communicated by Ramaswamy H. Sarma.

Biosensor-based kinetic and thermodynamic characterization of opioids interaction with human μ-opioid receptor

Development of opioid analgesics with minimal side effects requires substantial knowledge on structure-kinetic and -thermodynamic relationship of opioid-receptor interactions. Here, combined kinetics and thermodynamics of opioid agonist binding to human μ-opioid receptor (h-μOR) was investigated using real-time label-free surface plasmon resonance (SPR)-based method. The N-terminal end truncated and C-terminal 6His-tagged h-μOR was constructed and expressed in E. coli. Receptor was purified, detergent-solubilized and characterized by circular dichroism. The uniform immobilization of h-μOR on Ni-NTA chips was achieved using hybrid capture-coupling approach followed by reconstitution in lipid bilayer. Thermodynamic equilibrium affinities of opioids were in narrow nanomolar range and in near quantitative agreement with their K_i values. However, they did not correlate with their in vitro EC₅₀ values, indicating that they might not have thermodynamic selectivity. Contrary, on and off rates exhibited much larger dispersion and well correlated with EC₅₀ values, indicating that opioids might exhibit kinetic-selectivity towards their target. Temperature-dependent SPR assays provided access to rate and equilibrium thermodynamic data, which demonstrated binding of morphine and naloxone to μOR was exothermic and essentially enthalpy driven. This work suggests that kinetic-based structure-activity of opioids in drug design and incorporation into the pharmacokinetics-pharmacodynamics predictions may have more value than thermodynamic equilibrium constants alone.

The antimicrobial peptides casocidins I and II: Solution structural studies in water and different membrane-mimetic environments

Antimicrobial peptides (AMPs) represent crucial components of the natural immune defense machinery of different organisms. Generally, they are short and positively charged, and bind to and destabilize bacterial cytoplasmic membranes, ultimately leading to cell death. Natural proteolytic cleavage of α_s2-casein in bovine milk generates the antimicrobial peptides casocidin I and II. In the current study, we report for the first time on a detailed structure characterization of casocidins in solution by means of Nuclear Magnetic Resonance spectroscopy (NMR). Structural studies were conducted in H₂O and different membrane mimetic environments, including 2,2,2-trifluoroethanol (TFE) and lipid anionic and zwitterionic vesicles. For both peptides, results indicate a mainly disordered conformation in H₂O, with a few residues in a partial helical structure. No wide increase of order occurs upon interaction with lipid vesicles. Conversely, peptide conformation becomes highly ordered in presence of TFE, with both casocidins presenting a large helical content. Our data point out a preference of casocidins to interact with model anionic membranes. These results are compatible with possible mechanisms of action underlying the antimicrobial activity of casocidins that ultimately may affect membrane bilayer stability.

A computationally designed water-soluble variant of a G-protein-coupled receptor: the human mu opioid receptor

G-protein-coupled receptors (GPCRs) play essential roles in various physiological processes, and are widely targeted by pharmaceutical drugs. Despite their importance, studying GPCRs has been problematic due to difficulties in isolating large quantities of these membrane proteins in forms that retain their ligand binding capabilities. Creating water-soluble variants of GPCRs by mutating the exterior, transmembrane residues provides a potential method to overcome these difficulties. Here we present the first study involving the computational design, expression and characterization of water-soluble variant of a human GPCR, the human mu opioid receptor (MUR), which is involved in pain and addiction. An atomistic structure of the transmembrane domain was built using comparative (homology) modeling and known GPCR structures. This structure was highly similar to the subsequently determined structure of the murine receptor and was used to computationally design 53 mutations of exterior residues in the transmembrane region, yielding a variant intended to be soluble in aqueous media. The designed variant expressed in high yield in Escherichia coli and was water soluble. The variant shared structural and functionally related features with the native human MUR, including helical secondary structure and comparable affinity for the antagonist naltrexone (K_d  = 65 nM). The roles of cholesterol and disulfide bonds on the stability of the receptor variant were also investigated. This study exemplifies the potential of the computational approach to produce water-soluble variants of GPCRs amenable for structural and functionally related characterization in aqueous solution.

Denatured G-protein coupled receptors as immunogens to generate highly specific antibodies

G-protein coupled receptors (GPCRs) play a major role in a number of physiological and pathological processes. Thus, GPCRs have become the most frequent targets for development of new therapeutic drugs. In this context, the availability of highly specific antibodies may be decisive to obtain reliable findings on localization, function and medical relevance of GPCRs. However, the rapid and easy generation of highly selective anti-GPCR antibodies is still a challenge. Herein, we report that highly specific antibodies suitable for detection of GPCRs in native and unfolded forms can be elicited by immunizing animals against purified full length denatured recombinant GPCRs. Contrasting with the currently admitted postulate, our study shows that an active and well-folded GPCR is not required for the production of specific anti-GPCR antibodies. This new immunizing strategy validated with three different human GPCR (μ-opioid, κ-opioid, neuropeptide FF2 receptors) might be generalized to other members of the GPCR family.

Structure and dynamics changes induced by 2,2,2-trifluoro-ethanol (TFE) on the N-terminal half of hepatitis C virus core protein

The Core protein of hepatitis C virus is involved in several interactions other than the encapsidation of viral RNA. We recently proposed that this is related to the fact that the N-terminal half of this protein (C82) is an intrinsically unstructured protein (IUP) domain. IUP domains can adopt a secondary structure when they are interacting with another molecule, such as a nucleic acid or a protein. It is also possible to mimic these conditions by modifying the environment of the protein. We investigated the propensity of this protein to fold as a function of salt concentration, detergent, pH, and 2,2,2-trifluoro-ethanol (TFE); only the addition of TFE resulted in a structural change. The effect of TFE addition was studied by circular dichroism, structural, and dynamic data obtained by NMR. The data indicate that C82 can adopt an alpha-helical structure; this conformation is likely relevant to one of the functional roles of the HCV Core protein.