Ionization Properties of Histidine Residues in the Lipid Bilayer Membrane Environment

Structural basis for calcium-stimulating pore formation of Vibrio α-hemolysin

Vibrio α-hemolysins (αHLs) are β-pore-forming toxins secreted by Vibrio pathogens, crucial for the facilitation of bacterial infections through host cell lysis. These toxins are produced as inactive precursors, requiring proteolytic maturation and membrane association for activation within host tissues. Here, we investigate Vibrio campbellii αHL (VcαHL), and establish that its hemolytic activity is significantly stimulated by calcium ions, with an EC50 that aligns with physiological calcium concentrations. Furthermore, we illustrate the vital contribution of calcium ions to the oligomerization of VcαHL on membranes. Using X-ray crystallography and cryo-electron microscopy, we decipher both the immature and assembled structures of VcαHL and elucidate the conformational changes corresponding to toxin assembly. We also identify a calcium-binding module that is integral for VcαHL's calcium-dependent activation. These findings provide insights into the regulatory mechanisms of VcαHL and have the potential to inform the development of targeted therapeutic strategies against Vibrio infections.

The membrane activity of the antimicrobial peptide caerin 1.1 is pH dependent

Antimicrobial peptides are an important class of membrane-active peptides that can provide alternatives or complements to classic antibiotics. Among the many classes of AMPs, the histidine-rich family is of particular interest since they may induce pH-sensitive interactions with cell membranes. The AMP caerin 1.1 (Cae-1), from Australian tree frogs, has three histidine residues, and thus we studied the pH dependence of its interactions with model cell membranes. Using NMR spectroscopy and molecular dynamics simulations, we showed that Cae-1 induced greater perturbation of the lipid dynamics and water penetrations within the membrane interior in an acidic environment compared with physiological conditions. Using 31P solid-state NMR, the packing, chemical environment, and dynamics of the lipid headgroup were monitored. 2H solid-state NMR showed that Cae-1 ordered the acyl chains of the hydrophobic core of the bilayer. These results supported the molecular dynamics data, which showed that Cae-1 was mainly inserted within the lipid bilayer for both neutral and negatively charged membranes, with the charged residues pulling the water and phosphate groups inward. This could be an early step in the mechanism of membrane disruption by histidine-rich antimicrobial peptides and indicated that Cae-1 acts via a transmembrane mechanism in bilayers of neutral and anionic phospholipid membranes, especially in acidic conditions.

Variations in exons 11 and 12 of the multi-pest resistance wheat gene Lr34 are independently additive for leaf rust resistance

INTRODUCTION

Characterization of germplasm collections for the wheat leaf rust gene Lr34 previously defined five haplotypes in spring wheat. All resistant lines had a 3-bp TTC deletion (null) in exon 11, resulting in the absence of a phenylalanine residue in the ABC transporter, as well as a single nucleotide C (Tyrosine in Lr34+) to T (Histidine in Lr34-) transition in exon 12. A rare haplotype present in Odesskaja 13 and Koktunkulskaja 332, both of intermediate rust resistance, had the 3-bp deletion typical of Lr34+ in exon 11 but the T nucleotide of Lr34- in exon 12.

METHODS

To quantify the role of each mutation in leaf rust resistance, Odesskaja 13 and Koktunkulskaja 332 were crossed to Thatcher and its near-isogenic line Thatcher-Lr34 (RL6058). Single seed descent populations were generated and evaluated for rust resistance in six different rust nurseries.

RESULTS

The Odesskaja 13 progeny with the TTC/T haplotype were susceptible with an average severity rating of 62.3%, the null/T haplotype progeny averaged 39.7% and the null/C haplotype was highly resistant, averaging 13.3% severity. The numbers for the Koktunkulskaja 332 crosses were similar with 63.5%, 43.5% and 23.7% severity ratings, respectively. Differences between all classes in all crosses were statistically significant, indicating that both mutations are independently additive for leaf rust resistance. The three-dimensional structural models of LR34 were used to analyze the locations and putative interference of both amino acids with the transport channel. Koktunkulskaja 332 also segregated for marker csLV46 which is linked to Lr46. Rust severity in lines with Lr34+ and csLV46+ had significantly lower rust severity ratings than those without, indicating the additivity of the two loci.

DISCUSSION

This has implications for the deployment of Lr34 in wheat cultivars and for the basic understanding of this important wheat multi-pest durable resistance gene.

Illuminating Disorder Induced by Glu in a Stable Arg-Anchored Transmembrane Helix

Membrane proteins are vital for biological function and are complex to study. Even in model peptide-lipid systems, the combined influence or interaction of pairs of chemical groups still is not well understood. Disordered proteins, whether in solution or near lipid membranes, are an emerging paradigm for the initiation and control of biological function. The disorder can involve molecular orientation as well as molecular folding. This paper reports an astonishing induction of disorder when one Glu residue is introduced into a highly stable 23-residue transmembrane helix. The parent helix is anchored by a single Arg residue, tilted at a well-defined angle with respect to the DOPC bilayer normal and undergoes rapid cone precession. When Glu is introduced two residues away from Arg, with 200° (or 160°) radial separation, the helix properties change radically to exhibit a multiplicity of three or more disordered states. The helix characteristics have been monitored by deuterium (²H) NMR spectroscopy as functions of the pH and lipid bilayer composition. The disordered multistate behavior of the (Glu, Arg)-containing helix varies with the lipid bilayer thickness and pH. The results highlight a fundamental induction of protein multistate properties by a single Glu residue in a lipid membrane environment.

Thermodynamics-Based Molecular Modeling of α-Helices in Membranes and Micelles

The Folding of Membrane-Associated Peptides (FMAP) method was developed for modeling α-helix formation by linear peptides in micelles and lipid bilayers. FMAP 2.0 identifies locations of α-helices in the amino acid sequence, generates their three-dimensional models in planar bilayers or spherical micelles, and estimates their thermodynamic stabilities and tilt angles, depending on temperature and pH. The method was tested for 723 peptides (926 data points) experimentally studied in different environments and for 170 single-pass transmembrane (TM) proteins with available crystal structures. FMAP 2.0 detected more than 95% of experimentally observed α-helices with an average error in helix end determination of around 2, 3, 4, and 5 residues per helix for peptides in water, micelles, bilayers, and TM proteins, respectively. Helical and nonhelical residue states were predicted with an accuracy from 0.86 to 0.96, and the Matthews correlation coefficient was from 0.64 to 0.88 depending on the environment. Experimental micelle- and membrane-binding energies and tilt angles of peptides were reproduced with a root-mean-square deviation of around 2 kcal/mol and 7°, respectively. The TM and non-TM states of hydrophobic and pH-triggered α-helical peptides in various lipid bilayers were reproduced in more than 95% of cases. The FMAP 2.0 web server (https://membranome.org/fmap) is publicly available to explore the structural polymorphism of antimicrobial, cell-penetrating, fusion, and other membrane-binding peptides, which is important for understanding the mechanisms of their biological activities.

Marker residue types at the structural regions of transmembrane alpha-helical and beta-barrel interfaces

Membrane proteins play a variety of biological functions to the survival of organisms and functionalities of these proteins are often due to their homo- or hetero-complexation. Encoded by ~30% of the genome in most organisms, they represent the target of over half of nowadays drugs. Spanning the entirety of the cell membrane, transmembrane proteins are the most common type of membrane proteins and can be classified by secondary structures: alpha-helical and beta-barrel structures. Protein-protein interaction (PPI) have been widely studied for globular proteins and many computational tools are available for predicting PPI sites and construct models of complexes. Here, the structural regions of a non-redundant set of 232 alpha-helical and 37 beta-barrel transmembrane complexes and their interfaces are analyzed. Using the residue composition, frequency and propensity, this study brings the light on the marker residue types located at the structural regions of alpha-helical and beta-barrel transmembrane homomeric protein complexes and of their interfaces. This study also shows the necessity to relate the frequency to the composition into a ratio for immediately figuring out residue types presenting high frequencies at the interface and/or at one of its structural regions despite being a minor contributor compared to other residue types to that location's residue composition.

Lipid-Dependent Titration of Glutamic Acid at a Bilayer Membrane Interface

The ionization properties of protein side chains in lipid-bilayer membranes will differ from the canonical values of side chains exposed to an aqueous solution. While the propensities of positively charged side chains of His, Lys, and Arg to release a proton in lipid membranes have been rather well characterized, the propensity for a negatively charged Glu side chain to receive a proton and achieve the neutral state in a bilayer membrane has been less well characterized. Indeed, the ionization of the glutamic acid side chain has been predicted to depend on its depth of burial in a lipid membrane but has been difficult to verify experimentally. To address the issue, we incorporated an interfacial Glu residue at position 4 of a distinct 23-residue transmembrane helix and used ²H NMR to examine the helix properties as a function of pH. We observe that the helix tilt and azimuthal rotation vary little with pH, but the extent of helix unraveling near residues 3 and 4 changes as the Glu residue E4 titrates. Remarkably, the ²H quadrupolar splitting for the side chain of alanine A3 responds to pH with an apparent pK _a of 4.8 in 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) and 6.3 in 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC), but is unchanged up to pH 8.0 in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) in the presence of residue E4. With bilayers composed of alkali-stable ether-linked lipids, the side chain of A3 responds to pH with an apparent pK _a of 11.0 in the ether analogue of DOPC. These results suggest that the depth dependence of Glu ionization in lipid-bilayer membranes may be steeper than previously predicted or envisioned.

Roles of histidine charge and cardiolipin in membrane disruption by antimicrobial peptides Gaduscidin-1 and Gaduscidin-2

Gad-1 and Gad-2 are helical, histidine-rich antimicrobial peptides (AMPs) from paralogous genes in cod. ¹⁵N and ²H solid state nuclear magnetic resonance (NMR) were used to characterize their lipid-bound structures and lipid interactions. Gad-1 was found to position in-plane in POPC: POPG bilayers. Gad-1 displayed greater effects than Gad-2 on lipid acyl chain order of POPE: POPG and POPE: POPG: CL bilayers, in keeping with its greater activity against E. coli. The effect of Gad-1 and Gad-2 on lipid bilayer order was only weakly affected by changes in pH, and hence changes in histidine charge. This was somewhat surprising for Gad-2 as this peptide's biological activity has been shown to be greater at low pH and thus the finding may point to the existence of functional interactions with non-lipid components of bacteria. The incorporation of cardiolipin into POPE: POPG bilayers in such a way as to preserve the overall charge of the bilayers did not alter Gad-1's effects on lipid acyl chain order parameters, which report on motions on the 10^-5 s timescale. When cardiolipin and Gad-1 were both present, there were subtle changes on membrane dynamics at other timescales.

Examination of pH dependency and orientation differences of membrane spanning alpha helices carrying a single or pair of buried histidine residues

We have employed the peptide framework of GWALP23 (acetyl-GGALWLALALALALALALWLAGA-amide) to examine the orientation, dynamics and pH dependence of peptides having buried single or pairs of histidine residues. When residue L8 is substituted to yield GWALP23-H⁸, acetyl-GGALWLAH⁸ALALALALALWLAGA-amide, the deuterium NMR spectra of ²H-labeled core alanine residues reveal a helix that occupies a single transmembrane orientation in DLPC, or in DMPC at low pH, yet shows multiple states at higher pH or in bilayers of DOPC. Moreover, a single histidine at position 8 or 16 in the GWALP23 framework is sensitive to pH. Titration points are observed near pH 3.5 for the deprotonation of H8 in lipid bilayers of DLPC or DMPC, and for H16 in DOPC. When residues L8 and L16 both are substituted to yield GWALP23-H^8,16, the ²H NMR spectra show, interestingly, no titration dependence from pH 2-8, yet bilayer thickness-dependent orientation differences. The helix with H8 and H16 is found to adopt a transmembrane orientation in thin bilayers of DLPC, a combination of transmembrane and surface orientations in DMPC, and then a complete transition to a surface bound orientation in the thicker DPoPC and DOPC lipid bilayers. In the surface orientations, alanine A7 no longer fits within the core helix. These results along with previous studies with different locations of histidine residues suggest that lipid hydrophobic thickness is a first determinant and pH a second determinant for the helical orientation, along with possible side-chain snorkeling, when the His residues are incorporated into the hydrophobic region of a lipid membrane-associated helix.

Induced fusion and aggregation of bacterial outer membrane vesicles: Experimental and theoretical analysis

Recombinantly engineered bacterial outer membrane vesicles (OMVs) are promising vaccine delivery vehicles. The diversity of exogenous antigens delivered by OMVs can be enhanced by induced fusion of OMV populations. To date there are no reports of induced fusion of bacterial OMVs. Here we measure the pH and salt-induced aggregation and fusion of OMVs and analyze the processes against the Derjaguin-Landau-Verwey-Overbeek (DLVO) colloidal stability model. Vesicle aggregation and fusion kinetics were investigated for OMVs isolated from native E. coli (Nissle 1917) and lipopolysaccharide (LPS) modified E. coli (ClearColi) strains to evaluate the effect of lipid type on vesicle aggregation and fusion. Electrolytes and low pHs induced OMV aggregation for both native and modified LPS constructs, approaching a calculated fusion efficiency of ~25% (i.e. ~1/4 of collision events lead to fusion). However, high fusion efficiency was achieved for Nissle OMVs solely with decreased pH as opposed to a combination of low pH and increased divalent counterion concentration for ClearColi OMVs. The lipid composition of the OMVs from Nissle negatively impacted fusion in the presence of electrolytes, causing higher deviations from DLVO-predicted critical coagulation concentrations with monovalent counterions. The outcome of the work is a defined set of conditions under which investigators can induce OMVs to fuse and make various combinations of vesicle compositions.

Comparing Interfacial Trp, Interfacial His and pH Dependence for the Anchoring of Tilted Transmembrane Helical Peptides

Charged and aromatic amino acid residues, being enriched toward the terminals of membrane-spanning helices in membrane proteins, help to stabilize particular transmembrane orientations. Among them, histidine is aromatic and can be positively charge at low pH. To enable investigations of the underlying protein-lipid interactions, we have examined the effects of single or pairs of interfacial histidine residues using the constructive low-dynamic GWALP23 (acetyl-GG²ALW⁵LALALALALALALW¹⁹LAG²²A-amide) peptide framework by incorporating individual or paired histidines at locations 2, 5, 19 or 22. Analysis of helix orientation by means of solid-state ²H NMR spectra of labeled alanine residues reveals marked differences with H^2,22 compared to W^2,22. Nevertheless, the properties of membrane-spanning H^2,22WALP23 helices show little pH dependence and are similar to those having Gly, Arg or Lys at positions 2 and 22. The presence of H5 or H19 influences the helix rotational preference but not the tilt magnitude. H5 affects the helical integrity, as residue 7 unwinds from the core helix; yet once again the helix orientation and dynamic properties show little sensitivity to pH. The overall results reveal that the detailed properties of transmembrane helices depend upon the precise locations of interfacial histidine residues.

Interaction of aliphatic amino acids with zwitterionic and charged lipid membranes: hydration and dehydration phenomena

Aliphatic amino acids interact differently in order to induce gelation or fluidization in zwitterionic and charged lipid membranes as a result of hydration or dehydration of the membrane surface.

Influence of Lipid Saturation, Hydrophobic Length and Cholesterol on Double-Arginine-Containing Helical Peptides in Bilayer Membranes

Membrane proteins are essential for many cell processes yet are more difficult to investigate than soluble proteins. Charged residues often contribute significantly to membrane protein function. Model peptides such as GWALP23 (acetyl-GGALW⁵ LAL⁸ LALALAL¹⁶ ALW¹⁹ LAGA-amide) can be used to characterize the influence of specific residues on transmembrane protein domains. We have substituted R8 and R16 in GWALP23 in place of L8 and L16, equidistant from the peptide center, and incorporated specific ² H-labeled alanine residues within the central sequence for detection by solid-state ² H NMR spectroscopy. The resulting pattern of [² H]Ala quadrupolar splitting (Δν_q ) magnitudes indicates the core helix for R^8,16 GWALP23 is significantly tilted to give a similar transmembrane orientation in thinner bilayers with either saturated C12:0 or C14:0 acyl chains (1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)) or unsaturated C16:1 Δ9 cis acyl chains. In bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC; C18:1 Δ9 cis) multiple orientations are indicated, whereas in longer, unsaturated 1,2-dieicosenoyl-sn-glycero-3-phosphocholine (DEiPC; C20:1 Δ11 cis) bilayers, the R^8,16 GWALP23 helix adopts primarily a surface orientation. The inclusion of 10-20 mol % cholesterol in DOPC bilayers drives more of the R^8,16 GWALP23 helix population to the membrane surface, thereby allowing both charged arginines access to the interfacial lipid head groups. The results suggest that hydrophobic thickness and cholesterol content are more important than lipid saturation for the arginine peptide dynamics and helix orientation in lipid membranes.

Breaking the Backbone: Central Arginine Residues Induce Membrane Exit and Helix Distortions within a Dynamic Membrane Peptide

Transmembrane domains of membrane proteins sometimes contain conserved charged or ionizable residues which may be essential for protein function and regulation. This work examines the molecular interactions of single Arg residues within a highly dynamic transmembrane peptide helix. To this end, we have modified the GW^4,20ALP23 (acetyl-GGAW⁴(AL)₇AW²⁰AGA-amide) model peptide framework to incorporate Arg residues near the center of the peptide. Peptide helix formation, orientation and dynamics were analyzed by means of solid-state NMR spectroscopy to monitor specific ²H- or ¹⁵N-labeled residues. GW^4,20ALP23 itself adopts a tilted orientation within lipid bilayer membranes. Nevertheless, the GW^4,20ALP23 helix exhibits moderate to high dynamic averaging of NMR observables, such as ²H quadrupolar splittings or ¹⁵N-¹H dipolar couplings, due to competition between the interfacial Trp residues on opposing helix faces. Here we examine how the helix dynamics are impacted by the introduction of a single Arg residue at position 12 or 14. Residue R14 restricts the helix to low dynamic averaging and a well-defined tilt that varies inversely with the lipid bilayer thickness. To compensate for the dominance of R14, the competing Trp residues cause partial unwinding of the helix at the C-terminal. By contrast, R¹²GW^4,20ALP23 exits the DOPC bilayer to an interfacial surface-bound location. Interestingly, multiple orientations are exhibited by a single residue, Ala-9. Quadrupolar splittings generated by ²H-labeled residues A3, A5, A7, and A9 do not fit to the α-helical quadrupolar wave plot defined by residues A11, A13, A15, A17, A19, and A21. The discontinuity at residue A9 implicates a helical swivel distortion and an apparent 3₁₀-helix involving the N-terminal residues preceding A11. These molecular features suggest that, while arginine residues are prominent factors controlling transmembrane helix dynamics, the influence of interfacial tryptophan residues cannot be ignored.

The Arginines in the N-Terminus of the Porcine Circovirus 2 Virus-like Particles Are Responsible for Disrupting the Membranes at Neutral and Acidic pH

Non-enveloped viruses that are endocytosed employ numerous mechanisms to disrupt endosomal membranes for escape into the cellular cytoplasm. These include the use of amphipathic helices or sheets, hydrophobic loops, myristoylated peptides, and proteins with phospholipase activity. Some mechanisms result in immediate deterioration of the endosome, while others form pores in the membrane causing osmolysis to disrupt the endosome and allow viral escape. We describe an additional mechanism by a non-enveloped virus to disrupt endosomal membranes. Porcine circovirus 2 (PCV2) possesses a 41-amino acid arginine-rich motif (ARM) at the N-terminus of its capsid protein that appears to be in the interior of the virus-like particle (VLP). Using in vitro membrane disruption assays, we demonstrate that PCV2 VLP, unassembled capsid, and ARM peptide possess the ability to disrupt endosomal-like membranes, whereas VLP lacking the ARM sequence does not possess this capability. Membrane disruption by VLP is insensitive to pH, but unassembled capsid protein and ARM peptide exhibit diminished activity at low pH. Our liposome disruption assays, circular dichroism, and intrinsic tryptophan fluorescence assays allow us to propose a model for PCV2-endosomal membrane interaction wherein the ARM peptide externalizes from the capsid, its C-terminus (amino acids 28-40) anchors into the membrane, and the arginine-rich N-terminus (amino acids 1-27) drives membrane disruption. To our knowledge, this is the first example of a non-enveloped virus using the arginines of an ARM to disrupt membranes. Also, this is the first example of such study for the Circoviridae family of viruses.

Structure and Function in Antimicrobial Piscidins: Histidine Position, Directionality of Membrane Insertion, and pH-Dependent Permeabilization

Piscidins are histidine-enriched antimicrobial peptides that interact with lipid bilayers as amphipathic α-helices. Their activity at acidic and basic pH in vivo makes them promising templates for biomedical applications. This study focuses on p1 and p3, both 22-residue-long piscidins with 68% sequence identity. They share three histidines (H3, H4, and H11), but p1, which is significantly more permeabilizing, has a fourth histidine (H17). This study investigates how variations in amphipathic character associated with histidines affect the permeabilization properties of p1 and p3. First, we show that the permeabilization ability of p3, but not p1, is strongly inhibited at pH 6.0 when the conserved histidines are partially charged and H17 is predominantly neutral. Second, our neutron diffraction measurements performed at low water content and neutral pH indicate that the average conformation of p1 is highly tilted, with its C-terminus extending into the opposite leaflet. In contrast, p3 is surface bound with its N-terminal end tilted toward the bilayer interior. The deeper membrane insertion of p1 correlates with its behavior at full hydration: an enhanced ability to tilt, bury its histidines and C-terminus, induce membrane thinning and defects, and alter membrane conductance and viscoelastic properties. Furthermore, its pH-resiliency relates to the neutral state favored by H17. Overall, these results provide mechanistic insights into how differences in the histidine content and amphipathicity of peptides can elicit different directionality of membrane insertion and pH-dependent permeabilization. This work features complementary methods, including dye leakage assays, NMR-monitored titrations, X-ray and neutron diffraction, oriented CD, molecular dynamics, electrochemical impedance spectroscopy, surface plasmon resonance, and quartz crystal microbalance with dissipation.

Activity and characterization of a pH-sensitive antimicrobial peptide

Antimicrobial peptides (AMPs) have been an area of great interest, due to the high selectivity of these molecules toward bacterial targets over host cells and the limited development of bacterial resistance to these molecules throughout evolution. Previous work showed that when Histidine was incorporated into the peptide C18G it lost antimicrobial activity. The role of pH on activity and biophysical properties of the peptide was investigated to explain this phenomenon. Minimal inhibitory concentration (MIC) results demonstrated that decreased media pH increased antimicrobial activity. Trichloroethanol (TCE) quenching and red-edge excitation spectroscopy (REES) showed a clear pH dependence on peptide aggregation in solution. Trp fluorescence was used to monitor binding to lipid vesicles and demonstrated the peptide binds to anionic bilayers at all pH values tested, however, binding to zwitterionic bilayers was enhanced at pH 7 and 8 (above the His pKa). Dual Quencher Analysis (DQA) confirmed the peptide inserted more deeply in PC:PG and PE:PG membranes, but could insert into PC bilayers at pH conditions above the His pKa. Bacterial membrane permeabilization assays which showed enhanced membrane permeabilization at pH 5 and 6 but vesicle leakage assays indicate enhanced permeabilization of PC and PC:PG bilayers at neutral pH. The results indicate the ionization of the His side chain affects the aggregation state of the peptide in solution and the conformation the peptide adopts when bound to bilayers, but there are likely more subtle influences of lipid composition and properties that impact the ability of the peptide to form pores in membranes.

Coarse-grained model of titrating peptides interacting with lipid bilayers

Molecular-level computer simulations of peptide aggregation, translocation, and protonation at and in biomembranes are impeded by the large time and length scales involved. We present a computationally efficient, coarse-grained, and solvent-free model for the interaction between lipid bilayers and peptides. The model combines an accurate description of mechanical membrane properties with a new granular representation of the dielectric mismatch between lipids and the aqueous phase. All-atom force fields can be easily mapped onto the coarse-grained model, and parameters for coarse-grained monopeptides accurately extrapolate to membrane permeation free energies for the corresponding dipeptides and tripeptides. Acid-base equilibria of titratable amino acid residues are further studied using a constant-pH ensemble, capturing protonation state changes upon membrane translocation. Important differences between histidine, lysine, and arginine are observed, which are in good agreement with experimental observations.

Salvaging the Thermodynamic Destabilization of Interface Histidine in Transmembrane β-Barrels

The ability of histidine to participate in a wide range of stabilizing polar interactions preferentially populates this residue in functionally important sites of proteins. Histidine possesses an amphiphilic and electrostatic nature that is essential for amino acids residing at membrane interfaces. However, the frequency of occurrence of histidine at membrane interfaces, particularly transmembrane β-barrels, is lower than those of other aromatic residues. Here, we carry out comprehensive energetic measurements using equilibrium folding of the outer membrane enzyme PagP to address the contribution of a C-terminal interface histidine to barrel stability. We show that placing histidine at the C-terminus universally destabilizes PagP by 4.0-8.0 kcal mol^-1 irrespective of the neighboring residue. Spectroscopic and electrophoretic measurements indicate that the altered stability may arise from a loss of barrel compaction. Isoleucine, methionine, and valine salvage this destabilization marginally (in addition to tyrosine, which shows an exceptionally high folding free energy value), when placed at the penultimate position, at the expense of an altered folding pathway. Double-mutant cycle analysis indicates that the coupling energy between the terminal and penultimate residues in PagP-X¹⁶⁰H¹⁶¹ increases when the level of intrinsic destabilization by the terminal H¹⁶¹ is high. Our observations that neighboring residues cannot salvage the energetic destabilization of histidine may explain why histidine is less abundant at membrane interfaces.

Helix formation and stability in membranes

In this article we review current understanding of basic principles for the folding of membrane proteins, focusing on the more abundant alpha-helical class. Membrane proteins, vital to many biological functions and implicated in numerous diseases, fold into their active conformations in the complex environment of the cell bilayer membrane. While many membrane proteins rely on the translocon and chaperone proteins to fold correctly, others can achieve their functional form in the absence of any translation apparatus or other aides. Nevertheless, the spontaneous folding process is not well understood at the molecular level. Recent findings suggest that helix fraying and loop formation may be important for overall structure, dynamics and regulation of function. Several types of membrane helices with ionizable amino acids change their topology with pH. Additionally we note that some peptides, including many that are rich in arginine, and a particular analogue of gramicidin, are able passively to translocate across cell membranes. The findings indicate that a final protein structure in a lipid-bilayer membrane is sequence-based, with lipids contributing to stability and regulation. While much progress has been made toward understanding the folding process for alpha-helical membrane proteins, it remains a work in progress. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Influence of glutamic acid residues and pH on the properties of transmembrane helices

Negatively charged side chains are important for the function of particular ion channels and certain other membrane proteins. To investigate the influence of single glutamic acid side chains on helices that span lipid-bilayer membranes, we have employed GWALP23 (acetyl-GGALW⁵LALALALALALALW¹⁹LAGA-amide) as a favorable host peptide framework. We substituted individual Leu residues with Glu residues (L12E or L14E or L16E) and incorporated specific ²H-labeled alanine residues within the core helical region or near the ends of the sequence. Solid-state ²H NMR spectra reveal little change for the core labels in GWALP23-E12, -E14 and -E16 over a pH range of 4 to 12.5, with the spectra being broader for samples in DOPC compared to DLPC bilayers. The spectra for samples with deuterium labels near the helix ends on alanines 3 and 21 show modest pH-dependent changes in the extent of unwinding of the helix terminals in DLPC and DOPC bilayers. The combined results indicate minor overall responses of these transmembrane helices to changes in pH, with the most buried residue E12 showing no pH dependence. While the Glu residues E14 and E16 may have high pK_a values in the lipid bilayer environment, it is also possible that a paucity of helix response is masking the pK_a values. Interestingly, when E16 is present, spectral changes at high pH report significant local unwinding of the core helix. Our results are consistent with the expectation that buried carboxyl groups aggressively hold their protons and/or waters of hydration.

Influence of High pH and Cholesterol on Single Arginine-Containing Transmembrane Peptide Helices

An essential component of mammalian cells, cholesterol exerts significant influence on the physical properties of the cell membrane and in turn its constituents, including membrane proteins. Although sparse, polar amino acid residues are highly conserved in membrane proteins and play pivotal roles in determining specific structural and functional properties. To improve our understanding of particular polar residues in the membrane environment, we have examined two specific “guest” Arg residues within a well-defined and deuterium-labeled “host” framework provided by the transmembrane helical peptide GWALP23 (acetyl-GGALWLALALALALALALWLAGA-amide). Solid-state ²H nuclear magnetic resonance (NMR) spectra from aligned bilayer membrane samples effectively report changes in the host helix properties because of the incorporation of the guest residues. The focus of this work is two-pronged. First, GWALP23-R14 was examined over a pH range of 2–13 in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) ester- or ether-linked bilayer membranes. Our findings indicate that the Arg guanidinium side chain remains charged over this entire range, in agreement with numerous molecular dynamics simulations. Second, GWALP23-R12 and GWALP23-R14 peptides were characterized in DOPC bilayers with varying cholesterol content. Our findings suggest that 10 or 20% cholesterol content has minimal impact on the orientation of the R14 peptide. Although the NMR signals are broader and weaker in the presence of 20% cholesterol, the deuterium quadrupolar splittings for [²H]Ala residues in GWALP23-R14 change very little. Conversely, cholesterol appears to modulate the multistate behavior of GWALP23-R12 and to favor a major interfacial state for the helix, bound at the bilayer surface. These results indicate a conditional sensitivity of a complex multistate transmembrane Arg-containing peptide helix to the presence of cholesterol.