Association Of Transmembrane Helices Research Articles

Effects of stand-alone polar residue on membrane protein stability and structure

Helical membrane proteins generally have a hydrophobic nature, with apolar side chains comprising the majority of the transmembrane (TM) helices. However, whenever polar side chains are present in the TM domain, they often exert a crucial role in structural interactions with other polar residues, such as TM helix associations and oligomerization. Moreover, polar residues in the TM region also often participate in protein functions, such as the Schiff base bonding between Lys residues and retinal in rhodopsin-like membrane proteins. Although many studies have focused on these functional polar residues, our understanding of stand-alone polar residues that are energetically unfavored in TM helixes is limited. Here, we adopted bacteriorhodopsin (bR) as a model system and systematically mutated 17 of its apolar Leu or Phe residues to polar Asn. Stability measurements of the resulting mutants revealed that all of these polar substitutions reduced bR stability to various extents, and the extent of destabilization of each mutant bR is also correlated to different structural factors, such as the relative accessible surface area and membrane depth of the mutation site. Structural analyses of these Asn residues revealed that they form sidechain-to-backbone hydrogen bonds that alleviate the unfavorable energetics in hydrophobic and apolar surroundings. Our results indicate that membrane proteins are able to accommodate certain stand-alone polar residues in the TM region without disrupting overall structures.

Probing the formation of a hetero-dimeric membrane transport complex with dual in vitro and in silico mutagenesis.

The reversible association of transmembrane helices is a fundamental mechanism in how living cells convey information and respond to physiological events. The cardiac calcium transport regulator phospholamban (PLN) is an example of a single-span transmembrane protein that populates a variety of reversible and competing oligomeric states. PLN primarily forms monomers and pentamers in the membrane, where the PLN pentamer is a storage form and the PLN monomer forms a hetero-dimeric inhibitory complex with SERCA. The binding affinity and free-energy of formation of the SERCA-PLN complex in a membrane have not been determined. As is the case for most transmembrane protein interactions, measuring these quantities experimentally is extremely challenging. In this study, we estimated binding affinities by employing in silico alchemical free-energy calculations for all PLN transmembrane alanine substitutions in a membrane bilayer. The binding affinities were calculated separately for the SERCA-PLN complex, a PLN monomer, and a PLN pentamer and compared to in vitro functional measurements of SERCA regulation by the PLN alanine substitutions. Initially, the changes in SERCA inhibition by PLN alanine substitutions were compared to the changes in free energy for the SERCA-PLN complex formed from the PLN monomer. However, the functional data for the PLN alanine substitutions were better explained by the formation of the SERCA-PLN complex directly from the PLN pentamer. This finding points to an inhibitory mechanism favoring conformational selection of SERCA and the interaction of a PLN pentamer with SERCA for 'delivery' of a PLN monomer to the inhibitory site. The implications of these findings suggest that the energetics of helix exchange between homo- and hetero-oligomeric signaling complexes is favored over an intermediate involving a free monomeric helix in the membrane bilayer.

Effects of Gly Residue and Cholesterol on the GXXXG-Mediated Parallel Association of Transmembrane Helices: A Single-Pair FRET Study.

Small residue-mediated interhelical packing is ubiquitous in helical membrane proteins: however, the lipid dependence of its stability remains unclear. We previously demonstrated that the introduction of a GXXXG sequence in the middle of de novo-designed (AALALAA)3 helices (AALALAA AGLALGA AALALAA) facilitated their dimerization, which was abolished by cholesterol. Here single-pair FRET measurements revealed that a longer GXXXGXXXG segment (AALALAA A GLALGA AAGALAA) promoted helix dimerization in POPC/cholesterol bilayers, but not without cholesterol. The predicted dimer structures and degrees of helix packing suggested that helix dimers with small (∼10°) and large (∼55°) crossing angles were only stabilized in POPC and POPC/cholesterol membranes, respectively. A steric hindrance in the dimer interface and the large flexibility of helices prevented the formation of stable dimers. Therefore, amino acid sequences and lipid compositions distinctively constrain stable dimer structures in membranes.

Effects of Membrane Cholesterol on Stability of Transmembrane Helix Associations.

Membrane cholesterol is an essential and abundant component of eukaryotic cell membranes. The unique chemical structure of cholesterol significantly influences the physicochemical properties of phospholipid bilayers, such as hydrophobic thickness and lateral pressure profile. However, the mechanisms by which these alterations regulate the balance of protein-lipid interactions in lipid bilayer environments remain unclear. To experimentally assess basic and common driving forces for helix associations in membranes, the self-associations of a de novo designed simple transmembrane helix (AALALAA)3 and its derivative helices were examined. Single-pair fluorescence resonance energy transfer (sp-FRET) experiments were performed to monitor the thermodynamic and kinetic stabilities of helix associations in single liposomes. The addition of cholesterol exerted both stabilizing and destabilizing effects on these associations, up to a change in ΔGa of approx. 10 kJ mol-1, and these effects were dependent on the association topology, amino acid sequence, and number of helices. These results demonstrate that cholesterol in the membrane regulates the stability of transmembrane proteins in a protein context-dependent manner through physicochemical mechanisms.

The delicate balance between apolar packing and hydrogen bonding determines membrane protein assembly and function

The apolar environs of the lipid membrane changes how the different forces that dictate assembly, such as the hydrophobic burial, apolar packing (e.g. van der Waals interactions), and polar interactions, contribute to the folding of membrane proteins. In the membrane, the hydrophobic burial contributes very little; instead, apolar packing and hydrogen-bonding play significant roles in the assembly, and, ultimately, the function of membrane proteins. In this work, we dissect the contributions of these two forces in mediating the association of transmembrane helices.

Lipid bilayer induces contraction of the denatured state ensemble of a helical-bundle membrane protein

Defining the denatured state ensemble (DSE) and disordered proteins is essential to understanding folding, chaperone action, degradation, and translocation. As compared with water-soluble proteins, the DSE of membrane proteins is much less characterized. Here, we measure the DSE of the helical membrane protein GlpG of Escherichia coli (E. coli) in native-like lipid bilayers. The DSE was obtained using our steric trapping method, which couples denaturation of doubly biotinylated GlpG to binding of two streptavidin molecules. The helices and loops are probed using limited proteolysis and mass spectrometry, while the dimensions are determined using our paramagnetic biotin derivative and double electron-electron resonance spectroscopy. These data, along with our Upside simulations, identify the DSE as being highly dynamic, involving the topology changes and unfolding of some of the transmembrane (TM) helices. The DSE is expanded relative to the native state but only to 15 to 75% of the fully expanded condition. The degree of expansion depends on the local protein packing and the lipid composition. E.coli's lipid bilayer promotes the association of TM helices in the DSE and, probably in general, facilitates interhelical interactions. This tendency may be the outcome of a general lipophobic effect of proteins within the cell membranes.

Atomistic mechanism of transmembrane helix association

Transmembrane helix association is a fundamental step in the folding of helical membrane proteins. The prototypical example of this association is formation of the glycophorin dimer. While its structure and stability have been well-characterized experimentally, the detailed assembly mechanism is harder to obtain. Here, we use all-atom simulations within phospholipid membrane to study glycophorin association. We find that initial association results in the formation of a non-native intermediate, separated by a significant free energy barrier from the dimer with a native binding interface. We have used transition-path sampling to determine the association mechanism. We find that the mechanism of the initial bimolecular association to form the intermediate state can be mediated by many possible contacts, but seems to be particularly favoured by formation of non-native contacts between the C-termini of the two helices. On the other hand, the contacts which are key to determining progression from the intermediate to the native state are those which define the native binding interface, reminiscent of the role played by native contacts in determining folding of globular proteins. As a check on the simulations, we have computed association and dissociation rates from the transition-path sampling. We obtain results in reasonable accord with available experimental data, after correcting for differences in native state stability. Our results yield an atomistic description of the mechanism for a simple prototype of helical membrane protein folding.

TMPfold, a Computational Method for Assessment of Stability of Transmembrane α-Helical Assemblies

A new physics-based computational method TMPfold has been developed and implemented in a public web server for assessment of stability of TM α-helical complexes (accessible at https://opm.phar.umich.edu/tmpfold_server). Using the available 3D structure of an arbitrary TM α-bundle formed by a single or multiple subunits as input, the method calculates the free energies of subunit-subunit and helix-helix association (ΔGasc), identifies stable two-helical units that can initiate folding, and predict the plausible folding pathways for multi-pass transmembrane (TM) proteins. The free energy components include van der Waals, hydrogen bonding and dipole interactions, side-chain conformational entropy, and solvation energy in the anisotropic lipid environment. The method was verified using the experimental ΔGasc values for 36 TM complexes, including dimers of 10 glycophorin A mutants. TMPfold was applied for evaluation of TM helix association energies in 554 PDB structures of 85 seven-helical TM proteins and identification of their stable two-helical folding intermediates. The proposed tentative paths of co-translational helix assembly of several polytopic proteins were qualitatively consistent with experimental studies of their folding. The assessment of free energy changes caused by mutations in TM helices demonstrated the good correlation between experimental ΔΔGfolding and calculated ΔΔGasc values for 42 mutants of bacteriorhodopsin (R2 of 0.41, RMSE of 0.81 kcal/mol) and 25 mutants of rhomboid protease (R2 of 0.43, RMSE of 0.40 kcal/mol).

TMPfold: A Web Tool for Predicting Stability of Transmembrane α-Helix Association

Estimating energies of transmembrane (TM) α-helix association is essential for understanding folding of membrane proteins and formation of their functional assemblies. A new physics-based method was developed and implemented in the TMPfold web server for the calculation of the free energy of TM helix association (ΔGasc) in TM α-bundles of known structure. The method was verified using the experimental ΔGasc values for 36 TM complexes, including dimers of 10 glycophorin A mutants. The calculated free energy changes (ΔΔGasc) caused by mutations in TM helices correlated with experimental changes in the stability of 42 mutants of bacteriorhodopsin and 25 mutants of rhomboid protease. TMPfold was applied for evaluation of ΔGasc in 554 PDB structures of 85 seven-helical TM proteins and identification of stable two-helical folding intermediates. The proposed tentative paths of cotranslational helix assembly of several polytopic proteins were consistent with experimental studies of their folding. TMPfold is accessible at (https://opm.phar.umich.edu/tmpfold_server).

G Protein-Coupled Receptors Contain Two Conserved Packing Clusters

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

Insights into the transmembrane helix associations of kit ligand by molecular dynamics simulation and TOXCAT.

Kit ligand (KITL) plays important roles in cell proliferation, differentiation, and survival via interaction with its receptor Kit. The previous studies demonstrated that KITL formed a noncovalent homodimer through transmembrane (TM) domain; however, the undergoing mechanism of transmembrane association that determines KITL TM dimerization is still not clear. Herein, molecular dynamics (MD) simulation strategy and TOXCAT assay were combined to characterize the dimerization interface and structure of KITL TM in details. KITL TM formed a more energetically favorable noncovalent dimer through a conserved SxxxGxxxG motif in the MD simulation. Furthermore, the TOXCAT results demonstrated that KITL TM self-associated strongly in the bilayer membrane environment. Mutating any one of the small residues Ser11, Gly15 or Gly19 to Ile disrupted KITL TM dimerization dramatically, which further validated our MD simulation results. In addition, our results showed that Tyr22 could help to stabilize the TM interactions via interacting with the phosphoric group in the bilayer membrane. Pro7 did not induce helix kinks or swivel angles in KITL TM, but it was related with the pitch of the turn around this residue so as to affect the dimer formation. Combining the results of computer modeling and experimental mutagenesis studies on the KITL TM provide new insights for the transmembrane helix association of KITL dimerization. Proteins 2017; 85:1362-1370. © 2017 Wiley Periodicals, Inc.

単一ペア蛍光測定と2次元IRによるGXXXG配列駆動の膜貫通ヘリックス二量体形成の解析

単一ペア蛍光測定と2次元IRによるGXXXG配列駆動の膜貫通ヘリックス二量体形成の解析

Observation of helix associations for insertion of a retinal molecule and distortions of helix structures in bacteriorhodopsin.

We applied a newly proposed prediction method for membrane protein structures to bacteriorhodopsin that has distorted transmembrane helices in the native structure. This method uses an implicit membrane model, which restricts sampling space during folding in a membrane region, and includes helix bending. Replica-exchange simulations were performed with seven transmembrane helices only without a retinal molecule. Obtained structures were classified into clusters of similar structures, which correspond to local-minimum free energy states. The two lowest free energy states corresponded to a native-like structure with the correct empty space for retinal and a structure with this empty space filled with a helix. Previous experiments of bacteriorhodopsin suggested that association of transmembrane helices enables them to make a room for insertion of a retinal. Our results are consistent with these results. Moreover, distortions of helices in the native-like structures were successfully reproduced. In the distortions, whereas the locations of kinks for all helices were similar to those of Protein Data Bank's data, the amount of bends was more similar for helices away from the retinal than for those close to the retinal in the native structure. This suggests a hypothesis that the amino-acid sequence specifies the location of kinks in transmembrane helices and that the amount of distortions depends on the interactions with the surrounding molecules such as neighboring helices, lipids, and retinal.

Adaptable Lipid Matrix Promotes Protein-Protein Association in Membranes.

The cell membrane is "stuffed" with proteins, whose transmembrane (TM) helical domains spontaneously associate to form functionally active complexes. For a number of membrane receptors, a modulation of TM domains' oligomerization has been shown to contribute to the development of severe pathological states, thus calling for detailed studies of the atomistic aspects of the process. Despite considerable progress achieved so far, several crucial questions still remain: How do the helices recognize each other in the membrane? What is the driving force of their association? Here, we assess the dimerization free energy of TM helices along with a careful consideration of the interplay between the structure and dynamics of protein and lipids using atomistic molecular dynamics simulations in the hydrated lipid bilayer for three different model systems - TM fragments of glycophorin A, polyalanine and polyleucine peptides. We observe that the membrane driven association of TM helices exhibits a prominent entropic character, which depends on the peptide sequence. Thus, a single TM peptide of a given composition induces strong and characteristic perturbations in the hydrophobic core of the bilayer, which may facilitate the initial "communication" between TM helices even at the distances of 20-30 Å. Upon tight helix-helix association, the immobilized lipids accommodate near the peripheral surfaces of the dimer, thus disturbing the packing of the surrounding. The dimerization free energy of the modeled peptides corresponds to the strength of their interactions with lipids inside the membrane being the lowest for glycophorin A and similarly higher for both homopolymers. We propose that the ability to accommodate lipid tails determines the dimerization strength of TM peptides and that the lipid matrix directly governs their association.

De novo design of transmembrane helix–helix interactions and measurement of stability in a biological membrane

Membrane proteins regulate a large number of cellular functions, and have great potential as tools for manipulation of biological systems. Developing these tools requires a robust and quantitative understanding of membrane protein folding and interactions within the bilayer. With this in mind, we have designed a series of proteins to probe the net thermodynamic contribution of well-known sequence motifs to transmembrane helix-helix association in a biological membrane. The proteins were designed from first principles (de novo) using current knowledge about membrane insertion and stabilizing interaction motifs. A simple poly-Leu “scaffold” was decorated with individual helix interaction motifs (G-XXX-G, polar residues, heptad repeat) to create transmembrane helix–helix interactions of increasing strength. The GALLEX assay, an in vivo assay for measurement of transmembrane helix self-association, was combined with computational methods to characterize the relative strength and mode of interaction for each sequence. In addition, the apparent free energy contribution (ΔΔGapp) of each motif to transmembrane helix self-association was measured in a biological membrane, results that are the first of their kind for these de novo designed sequences, and suggest that the free energy barrier to overcoming weak association is quite small (<1.4kcalmol−1) in a natural membrane. By quantifying and rationalizing the contribution of key motifs to transmembrane helix association, our work offers a route to direct the design of novel sequences for use in biotechnology or synthetic biology (e.g. molecular switches) and to predict the effects of sequence modification in known transmembrane domains (for control of cellular processes).

Dynamic Structural/Amphiphilic “Portrait” of Biomembranes as their Fundamental Property Relevant to Function: Results of Atomistic Simulations

Analysis of local dynamic heterogeneities and hydrophobic surface properties of lipid bilayers is an important step toward understanding of the physical processes underlying formation of lipid domains and molecular mechanisms of the so-called “membrane response” observed upon binding of external molecules to cell membranes. Here, we used atomistic molecular dynamics simulations to address these problems and study structural/amphiphilic “portrait” of a large series of model bilayer membranes (1,2). Rearrangements of membranes upon binding of antimicrobial peptides and association of transmembrane helices were investigated as well (3,4).It was shown that the water-lipid interface of biomembranes represents a highly dynamic and “mosaic” picture, whose parameters depend on the bilayer composition. Formation of such heterogeneities is caused by various intermolecular interactions between lipids, water and ions. Because clustering of the components is determined by a thin balance of differently directed factors, even their minor changes can induce serious rearrangements on the membrane surface. These issues still resist easy experimental characterization.Probably, these phenomena are indispensable for fast mass/energy transfer near the water/membrane boundary, thus regulating diffusion of molecules in this functionally important area: e.g., approaching of ligands to membrane receptors, delivery of proteins to their sites of insertion into membrane, and so on. Putative role of local heterogeneities and collective phenomena in lipid/water systems is discussed.

Thermodynamic and kinetic characterization of transmembrane helix association.

The transient dimerization of transmembrane proteins is an important event in several cellular processes and computational methods are being increasingly used to quantify their underlying energetics. Here, we probe the thermodynamics and kinetics of a simple transmembrane dimer to understand membrane protein association. A multi-step framework has been developed in which the dimerization profiles are calculated from coarse-grain molecular dynamics simulations, followed by meso-scale simulations using parameters calculated from the coarse-grain model. The calculated value of ΔGassoc is approx. -20 kJ mol(-1) and is consistent between three methods. Interestingly, the meso-scale stochastic model reveals low dimer percentages at physiologically-relevant concentrations, despite a favorable ΔGassoc. We identify generic driving forces arising from the protein backbone and lipid bilayer and complementary factors, such as protein density, that govern self-interactions in membranes. Our results provide an important contribution in understanding membrane protein organization and linking molecular, nano-scale computational studies to meso-scale experimental data.

A Gly-zipper motif mediates homodimerization of the transmembrane domain of the mitochondrial kinase ADCK3.

Interactions between α-helices within the hydrophobic environment of lipid bilayers are integral to the folding and function of transmembrane proteins; however, the major forces that mediate these interactions remain debated, and our ability to predict these interactions is still largely untested. We recently demonstrated that the frequent transmembrane association motif GASright, the GxxxG-containing fold of the glycophorin A dimer, is optimal for the formation of extended networks of Cα–H hydrogen bonds, supporting the hypothesis that these bonds are major contributors to association. We also found that optimization of Cα–H hydrogen bonding and interhelical packing is sufficient to computationally predict the structure of known GASright dimers at near atomic level. Here, we demonstrate that this computational method can be used to characterize the structure of a protein not previously known to dimerize, by predicting and validating the transmembrane dimer of ADCK3, a mitochondrial kinase. ADCK3 is involved in the biosynthesis of the redox active lipid, ubiquinone, and human ADCK3 mutations cause a cerebellar ataxia associated with ubiquinone deficiency, but the biochemical functions of ADCK3 remain largely undefined. Our experimental analyses show that the transmembrane helix of ADCK3 oligomerizes, with an interface based on an extended Gly-zipper motif, as predicted by our models. The data provide strong evidence for the hypothesis that optimization of Cα–H hydrogen bonding is an important factor in the association of transmembrane helices. This work also provides a structural foundation for investigating the role of transmembrane association in regulating the biological activity of ADCK3.

Characterization of Membrane Protein Interactions by Isothermal Titration Calorimetry

Understanding the structure, folding, and interaction of membrane proteins requires experimental tools to quantify the association of transmembrane (TM) helices. Here, we introduce isothermal titration calorimetry (ITC) to measure integrin αIIbβ3 TM complex affinity, to study the consequences of helix–helix preorientation in lipid bilayers, and to examine protein-induced lipid reorganization. Phospholipid bicelles served as membrane mimics. The association of αIIbβ3 proceeded with a free energy change of −4.61±0.04kcal/mol at bicelle conditions where the sampling of random helix–helix orientations leads to complex formation. At bicelle conditions that approach a true bilayer structure in effect, an entropy saving of >1kcal/mol was obtained from helix–helix preorientation. The magnitudes of enthalpy and entropy changes increased distinctly with bicelle dimensions, indicating long-range changes in bicelle lipid properties upon αIIbβ3 TM association. NMR spectroscopy confirmed ITC affinity measurements and revealed αIIbβ3 association and dissociation rates of 4500±100s−1 and 2.1±0.1s−1, respectively. Thus, ITC is able to provide comprehensive insight into the interaction of membrane proteins.

Interaction Profiles of Amino Acid Side-Chain Analog Pairs within Membrane Environments

The energetics of amino acid insertion into membranes has been studied extensively but relatively little is known about the association energetics of amino acids within biological membranes. Here, interactions between pairs of four amino acid analog (Asn, Ser, Phe, and Val) in the membrane environment are described from umbrella sampling molecular dynamics simulations. Association interaction free energy profiles as a function of bilayer insertion depth are presented and the physical characteristics of contact pairs are described. The results provide detailed insight into amino acid interactions within membrane environments with implications in the understanding of how membrane-bound peptides and proteins interact and for the association of transmembrane helices. Furthermore, the quantitative energy profiles presented here serve as a benchmark for the parameterization of more simplified models such as coarse-grained models or implicit models of membrane bilayers.