Methods to study membrane protein structure in solution

Solution structure of micelle-bound H5 peptide (427-452): a primary structure corresponding to the pore forming region of the voltage dependent potassium channel

A 26-mer peptide with the sequence of the pore forming region (residues 427-452) of the Shaker K(+) channel (H5 region) was chemically synthesized. Analyses by CD and two-dimensional 1H NMR spectroscopy were used to investigate the structure of the peptide bound to SDS micelles in solution, which are commonly used in biophysical studies. The tertiary structure of the peptide as a monomer was composed of an alpha-helix (431-438), a turn (439-442), and random coils (427-430, 443-452), and was very similar to that of the pore forming region of the native K(+) channel from Streptomyces lividans determined by X-ray analysis. This result suggests that even an isolated peptide forms a native-like conformation for residues from 431 to 442, depending on its intrinsic amino acid sequence and the surrounding environment.

Structure and dynamics of micelle-bound neuropeptide Y: comparison with unligated NPY and implications for receptor selection

The biological importance of the neuropeptide Y (NPY) has steered a number of investigations about its solution structure over the last 20 years. Here, we focus on the comparison of the structure and dynamics of NPY free in solution to when bound to a membrane mimetic, dodecylphosphocholine (DPC) micelles, as studied by 2D (1)H NMR spectroscopy. Both, free in solution and in the micelle-bound form, the N-terminal segment (Tyr1-Glu15) is shown to extend like a flexible tail in solution. This is not compatible with the PP-fold model for NPY that postulates backfolding of the flexible N terminus onto the C-terminal helix. The correlation time (tau(c)) of NPY in aqueous solution, 5.5 (+/-1.0) ns at 32 degrees C, is only consistent with its existence in a dimeric form. Exchange contributions especially enhancing transverse relaxation rates (R(2)) of residues located on one side of the C-terminal helix of the molecule are supposed to originate from dimerization of the NPY molecule. The dimerization interface was directly probed by looking at (15)N-labeled NPY/spin-labeled [TOAC34]-[(14)N]-NPY heterodimers and revealed both parallel and anti-parallel alignment of the helices. The NMR-derived three-dimensional structure of micelle-bound NPY at 37 degrees C and pH 6.0 is similar but not identical to that free in solution. The final set of 17 lowest-energy DYANA structures is particularly well defined in the region of residues 21-31, with a mean pairwise RMSD of 0.23 A for the backbone heavy atoms and 0.85 A for all heavy atoms. The combination of NMR relaxation data and CD measurements clearly demonstrates that the alpha-helical region Ala18-Thr32 is more stable, and the C-terminal tetrapeptide becomes structured only in the presence of the phosphocholine micelles. The position of NPY relative to the DPC micelle surface was probed by adding micelle integrating spin labels. Together with information from (1)H,(2)H exchange rates, we conclude that the interaction of NPY with the micelle is promoted by the amphiphilic alpha-helical segment of residues Tyr21-Thr32. NPY is located at the lipid-water interface with its C-terminal helix parallel to the membrane surface and penetrates the hydrophobic interior only via insertions of a few long aliphatic or aromatic side-chains. From these data we can demonstrate that the dimer interface of neuropeptide Y is similar to the interface of the monomer binding to DPC-micelles. We speculate that binding of the NPY monomer to the membrane is an essential key step preceeding receptor binding, thereby pre-orientating the C-terminal tetrapeptide and possibly inducing the bio-active conformation.

Secondary structure of human apolipoprotein A-I(1-186) in lipid-mimetic solution

The solution structure of an apoA-I deletion mutant, apoA-I(1-186) was determined by the chemical shift index (CSI) method and the torsion angle likelihood obtained from shift and sequence similarity (TALOS) method, using heteronuclear multidimensional NMR spectra of [u-(13)C, u-(15)N, u-50% (2)H]apoA-I(1-186) in the presence of sodium dodecyl sulfate (SDS). The backbone resonances were assigned from a combination of triple-resonance data (HNCO, HNCA, HN(CO)CA, HN(CA)CO and HN(COCA)HA), and intraresidue and sequential NOEs (three-dimensional (3D) and four-dimensional (4D) 13C- and 15N-edited NOESY). Analysis of the NOEs, H(alpha), C(alpha) and C' chemical shifts shows that apoA-I(1-186) in lipid-mimetic solution is composed of alpha-helices (which include the residues 8-32, 45-64, 67-77, 83-87, 90-97, 100-140, 146-162, and 166-181), interrupted by short irregular segments. There is one relatively long, irregular and mostly flexible region (residues 33-44), that separates the N-terminal domain (residues 1-32) from the main body of protein. In addition, we report, for the first time, the structure of the N-terminal domain of apoA-I in a lipid-mimetic environment. Its structure (alpha-helix 8-32 and flexible linker 33-44) would suggest that this domain is structurally, and possibly functionally, separated from the other part of the molecule.

Structure determination of membrane-associated proteins from nuclear magnetic resonance data

This Review covers the delineation and optimization of protein-lipid systems for study using solution-state NMR spectroscopy. The first half presents the necessary background for a membrane protein biochemist to initiate collaboration with an NMR spectroscopist. The second half provides guidelines for the spectroscopist on data collection, analysis for obtaining conformational information, and structure generation and assessment. Although the emphasis is on the study of peptides in detergent micelles, methods are outlined for larger membrane-associated proteins and for use of other solubilizing agents.

Structural characterization of a low density lipoprotein receptor-active apolipoprotein E peptide, ApoE3-(126-183)

Apolipoprotein E (apoE) plays a critical role in lipoprotein particle clearance from blood plasma through its interaction with the low density lipoprotein (LDL) receptor and other related receptors. Here, we studied a 58-residue peptide encompassing the receptor binding region of apoE. ApoE3-(126-183) was generated by cyanogen bromide cleavage of recombinant apoE3-(1-183), purified by reversed-phase high pressure liquid chromatography, and characterized by mass spectrometry. Far UV CD spectroscopy of the peptide showed that it is unstructured in aqueous solution. The addition of trifluoroethanol or dodecylphosphocholine induces the peptide to adopt an alpha-helical conformation. ApoE3-(126-183) efficiently transforms dimyristoylphosphatidylglycerol (DMPG) vesicles into peptide-lipid complexes. Analysis of apoE3-(126-183). DMPG complexes by electron microscopy revealed disc-shaped particles with an average diameter of 13 +/- 3 nm. Flotation equilibrium analysis yielded a particle molecular mass of 252 kDa. Far UV CD analysis of apoE3-(126-183).DMPG discs provided evidence that the peptide adopts a helical conformation. Competition binding experiments with (125)I-labeled low density lipoprotein (LDL) were conducted to assess the ability of apoE3-(126-183).DMPG complexes to bind to the LDL receptor. Both N-terminal apoE and the peptide, when complexed with DMPG, competed with (125)I-LDL for binding sites on the surface of cultured human skin fibroblasts. Under the conditions employed, apoE3-(126-183).DMPG complexes were similar to apoE3-(1-183).DMPG discs in their ability to bind to the receptor, demonstrating that the peptide represents a good model to study the interaction between apoE and the LDL receptor. Preliminary NMR results indicated that a high resolution structure of the apoE3-(126-183) peptide is obtainable.

Requirement of specific intrahelical interactions for stabilizing the inactive conformation of glycoprotein hormone receptors

Systematic analysis of structural changes induced by activating mutations has been frequently utilized to study activation mechanisms of G-protein-coupled receptors (GPCRs). In the thyrotropin receptor and the lutropin receptor (LHR), a large number of naturally occurring mutations leading to constitutive receptor activation were identified. Saturating mutagenesis studies of a highly conserved Asp in the junction of the third intracellular loop and transmembrane domain 6 suggested a participation of this anionic residue in a salt bridge stabilizing the inactive receptor conformation. However, substitution of all conserved cationic residues at the cytoplasmic receptor surface did not support this hypothesis. Asp/Glu residues are a common motif at the N-terminal ends of alpha-helices terminating and stabilizing the helical structure (helix capping). Since Asp/Glu residues in the third intracellular loop/transmembrane domain 6 junction are not only preserved in glycoprotein hormone receptors but also in other GPCRs we speculated that this residue probably participates in an N-terminal helix-capping structure. Poly-Ala stretches are known to form and stabilize alpha-helices. Herein, we show that the function of the highly conserved Asp can be mimicked by poly-Ala substitutions in the LHR and thyrotropin receptor. CD and NMR studies of peptides derived from the juxtamembrane portion of the LHR confirmed the helix extension by the poly-Ala substitution and provided further evidence for an involvement of Asp in a helix-capping structure. Our data implicate that in addition to well established interhelical interactions the inactive conformation of GPCRs is also stabilized by specific intrahelical structures.

Customizing model membranes and samples for NMR spectroscopic studies of complex membrane proteins

Both solution and solid state nuclear magnetic resonance (NMR) techniques for structural determination are advancing rapidly such that it is possible to contemplate bringing these techniques to bear upon integral membrane proteins having multiple transmembrane segments. This review outlines existing and emerging options for model membrane media for use in such studies and surveys the special considerations which must be taken into account when preparing larger membrane proteins for NMR spectroscopic studies.

Characterization of human anti-acetylcholine receptor monoclonal autoantibodies from the peripheral blood of a myasthenia gravis patient using combinatorial libraries

Myasthenia gravis (MG) is an autoimmune disease caused by autoantibodies against the nicotinic acetylcholine receptor (AChR). Using phage-display technology we have characterized the largest panel of anti-AChR monoclonal antibodies thus far isolated from a single patient. Despite having been isolated with either Torpedo AChR or a human peptide, the recombinant antibodies shared with the donor's serum the ability to recognize human AChR expressed in its native configuration on the surface of TE671 cells. Their specificity for the main immunogenic region (MIR) of the AChR was demonstrated using a synthetic peptide corresponding to the region 67-76 of the human AChR alpha subunit and by inhibition of a highly pathogenic rat anti-MIR monoclonal antibody (mAb35). This work demonstrates the value of combinatorial libraries in isolating pathogenic autoantibodies from peripheral blood lymphocytes. Future genetic, structural, and functional analyses of the monoclonal antibodies reported herein should enhance our understanding of the pathogenesis of MG.

Structural studies of a baboon (Papio sp.) plasma protein inhibitor of cholesteryl ester transferase

A 38-residue protein associated with cholesteryl ester transfer inhibition has been identified in baboons (Papio sp.). The cholesteryl ester transfer inhibitor protein (CETIP) corresponds to the N-terminus of baboon apoC-I. Relative to CETIP, baboon apoC-I is a weak inhibitor of baboon cholesteryl ester transferase (CET). To study the structural features responsible for CET inhibition, CETIP was synthesized by solid-phase methods. Using sodium dodecyl sulfate (SDS) to model the lipoprotein environment, the solution structure of CETIP was probed by optical and 1H NMR spectroscopy. Circular dichroism data show that the protein lacks a well-defined structure in water but, upon the addition of SDS, becomes helical (56%). A small blue shift of 8 nm was observed in the intrinsic tryptophan fluorescence of CETIP in the presence of saturating amounts of SDS, suggesting that tryptophan-23 is not buried deeply in the lipid environment. The helical nature of CETIP in the presence of SDS was confirmed by upfield 1Halpha secondary shifts and an average solution structure determined by distance geometry/simulated annealing calculations using 476 NOE-based distance restraints. The backbone (N-Calpha-C=O) root-mean-square deviation of an ensemble of 17 out of 25 calculated structures superimposed on the average structure was 1.06+0.30 A using residues V4-P35 and 0.51+/-0.17 A using residues A7-S32. Although the side-chain orientations fit the basic description of a class A amphipathic helix, both intramolecular salt bridge formation and "snorkeling" of basic side chains toward the polar face play minor, if any, roles in stabilizing the lipid-bound amphipathic structure. Conformational features of the calculated structures for CETIP are discussed relative to models of CETIP inhibition of cholesteryl ester transferase.

Structure of a biologically active fragment of human serum apolipoprotein C-II in the presence of sodium dodecyl sulfate and dodecylphosphocholine

We have studied the three-dimensional structure of a biologically active peptide of apolipoprotein C-II (apoC-II) in the presence of lipid mimetics by CD and NMR spectroscopy. This peptide, corresponding to residues 44-79 of apoC-II, has been shown to reverse the symptoms of genetic apoC-II deficiency in a human subject. A comparison of alpha-proton secondary shifts and CD spectroscopic data indicates that the structure of apoC-II(44-79) is similar in the presence of dodecylphosphocholine and sodium dodecyl sulfate. The three-dimensional structure of apoC-II(44-79) in the presence of sodium dodecyl sulfate, determined by relaxation matrix calculations, contains two amphipathic helical domains formed by residues 50-58 and 67-75, separated by a non-helical linker centered at Tyr63. The C-terminal helix is terminated by a loop formed by residues 76-79. The C-terminal helix is better defined and has a larger hydrophobic face than the N-terminal helix, which leads us to propose that the C-terminal helix together with the non-helical Ile66 constitute the primary lipid binding domain of apoC-II(44-79). Based on our structure we suggest a new mechanism of lipoprotein lipase activation in which both helices of apoC-II(44-79) remain lipid bound, while the seven-residue interhelical linker extends away from the lipid surface in order to project Tyr63 into the apoC-II binding site of lipoprotein lipase.

The NMR solution structure of the ion channel peptaibol chrysospermin C bound to dodecylphosphocholine micelles

Chrysospermin C is a 19-residue peptaibol capable of forming transmembrane ion channels in phospholipid bilayers. The conformation of chrysospermin C bound to dodecylphosphocholine micelles has been solved using heteronuclear NMR spectroscopy. Selective 15N-labeling and 13C-labeling of specific alpha-aminoisobutyric acid residues was used to obtain complete stereospecific assignments for all eight alpha-aminoisobutyric acid residues. Structures were calculated using 339 distance constraints and 40 angle constraints obtained from NMR data. The NMR structures superimpose with mean global rmsd values to the mean structure of 0. 27 A (backbone heavy atoms) and 0.42 A (all heavy atoms). Chrysospermin C bound to decylphosphocholine micelles displays two well-defined helices at the N-terminus (residues Phe1-Aib9) and C-terminus (Aib13-Trp-ol19). A slight bend preceding Pro14, i.e. encompassing residues 10-12, results in an angle of approximately 38 degrees between the mean axes of the two helical regions. The bend structure observed for chrysospermin C is compatible with the sequences of all 18 long peptaibols and may represent a common 'active' conformation. The structure of chrysospermin C shows clear hydrophobic and hydrophilic surfaces which would be appropriate for the formation of oligomeric ion channels.

A structural basis for integrin activation by the cytoplasmic tail of the alpha IIb-subunit

A key step in the activation of heterodimeric integrin adhesion receptors is the transmission of an agonist-induced cellular signal from the short alpha- and/or beta-cytoplasmic tails to the extracellular domains of the receptor. The structural details of how the cytoplasmic tails mediate such an inside-out signaling process remain unclear. We report herein the NMR structures of a membrane-anchored cytoplasmic tail of the alpha(IIb)-subunit and of a mutant alpha(IIb)-cytoplasmic tail that renders platelet integrin alpha(IIb)beta(3) constitutively active. The structure of the wild-type alpha(IIb)-cytoplasmic tail reveals a "closed" conformation where the highly conserved N-terminal membrane-proximal region forms an alpha-helix followed by a turn, and the acidic C-terminal loop interacts with the N-terminal helix. The structure of the active mutant is significantly different, having an "open" conformation where the interactions between the N-terminal helix and C-terminal region are abolished. Consistent with these structural differences, the two peptides differ in function: the wild-type peptide suppressed alpha(IIb)beta(3) activation, whereas the mutant peptide did not. These results provide an atomic explanation for extensive biochemical/mutational data and support a conformation-based "on/off switch" model for integrin activation.

Alkylation of cysteine-containing peptides to mimic palmitoylation

Numerous proteins that are involved in cell signaling and viral replication require post-translational modification by palmitoylation to function properly. The molecular details by which this palmitoyl modification affects protein function remain poorly understood. To facilitate in vitro biochemical and structural studies of the role of palmitoylation on protein function, a method was developed for alkylating peptides with saturated C16 groups at cysteine residues and demonstrated using peptides derived from the palmitoylated region of Sindbis virus E2 glycoprotein. The synthetic approach takes advantage of disulfide chemistry to specifically modify only the cysteine residues within peptides and covalently links C16 groups via disulfide bridges using a new thioalkylating reagent, hexyldexyldithiopyridine. The chemistry presented here takes place in solution under mild conditions without the need for protection of the peptide functional groups. A method for purifying these modified peptides is also described. This protocol can be of general use to investigators studying the role of palmitoylation in biological systems.

NMR-Based amide hydrogen-deuterium exchange measurements for complex membrane proteins: development and critical evaluation

A method for measuring site-specific amide hydrogen-deuterium exchange rates for membrane proteins in bilayers is reported and evaluated. This method represents an adaptation and extension of the approach of Dempsey and co-workers (Biophys. J. 70, 1777-1788 (1996)) and is based on reconstituting (15)N-labeled membrane proteins into phospholipid bilayers, followed by lyophilization and rehydration with D(2)O or H(2)O (control). Following incubation for a time t under hydrated conditions, samples are again lyophilized and then solubilized in an organic solvent system, where (1)H-(15)N HSQC spectra are recorded. Comparison of spectra from D(2)O-exposed samples to spectra from control samples yields the extent of the H-D exchange which occurred in the bilayers during time t. Measurements are site specific if specific (15)N labeling is used. The first part of this paper deals with the search for a suitable solvent system in which to solubilize complex membrane proteins in an amide "exchange-trapped" form for NMR quantitation of amide peak intensities. The second portion of the paper documents application of the overall procedure to measuring site-specific amide exchange rates in diacylglycerol kinase, a representative integral membrane protein. Both the potential usefulness and the significant limitations of the new method are documented.

Diversity of antimicrobial peptides and their mechanisms of action

Antimicrobial peptides encompass a wide variety of structural motifs. Many peptides have alpha-helical structures. The majority of these peptides are cationic and amphipathic but there are also hydrophobic alpha-helical peptides which possess antimicrobial activity. In addition, some beta-sheet peptides have antimicrobial activity and even antimicrobial alpha-helical peptides which have been modified to possess a beta-structure retain part of their antimicrobial activity. There are also antimicrobial peptides which are rich in a certain specific amino acid such as Trp or His. In addition, antimicrobial peptides exist with thio-ether rings, which are lipopeptides or which have macrocyclic Cys knots. In spite of the structural diversity, a common feature of the cationic antimicrobial peptides is that they all have an amphipathic structure which allows them to bind to the membrane interface. Indeed, most antimicrobial peptides interact with membranes and may be cytotoxic as a result of disturbance of the bacterial inner or outer membranes. Alternatively, a necessary but not sufficient property of these peptides may be to be able to pass through the membrane to reach a target inside the cell. The interaction of these peptides with biological membranes is not just a function of the peptide but is also modulated by the lipid components of the membrane. It is not likely that this diverse group of peptides has a single mechanism of action, but interaction of the peptides with membranes is an important requirement for most, if not all, antimicrobial peptides.

Probing membrane surfaces and the location of membrane-embedded peptides by (13)C MAS NMR using lanthanide ions

A simple but efficient (13)C MAS NMR method is presented for the determination of the location of embedded molecules such as peptides relative to biological membrane surfaces by exploiting the interaction with paramagnetic lanthanide ions. Using various aqueous Dy(3+) concentrations a distance-dependent differential paramagnetic quenching of NMR lipid resonance intensities for specific carbon sites was observed, with residues at the bilayer surface quenched effectively and hydrophobic sites unaffected by Dy(3+). Tested on the membrane-embedded 50 residue long M13 coat protein, (13)C labeled at its Val-29 and Val-31 residues, no paramagnetic quenching was observed for the peptide resonances by Dy(3+), suggesting that Val-29 and Val-31 are not in close proximity to the bilayer interface, but buried deeply inside the hydrophobic region of the lipid bilayer.

Conjugation of polyethylene glycol via a disulfide bond confers water solubility upon a peptide model of a protein transmembrane segment

The aqueous insolubility of hydrophobic peptides has presented a barrier to the structural characterization of membrane protein transmembrane domains. Since the conjugation of polyethylene glycol is known to modulate the solubility of certain proteins and peptides, we have prepared PEG-a-Cys reagent, a polyethylene glycol derivative which reacts spontaneously with Cys residues to attach polyethylene glycol to polypeptides via a mixed disulfide bond. When desired, the PEG moiety can be readily removed by reduction with tricarboxyethylphosphine. The aqueous solubilizing power of PEG-a-Cys reagent is confirmed with a synthetic hydrophobic peptide model of a generic transmembrane segment-soluble carrier fusion protein.

NMR studies of the low-density lipoprotein receptor-binding peptide of apolipoprotein E bound to dodecylphosphocholine micelles

Circular dichroism and NMR spectroscopy have been used to determine the structure of the low-density lipoprotein (LDL) receptor-binding peptide, comprising residues 130-152, of the human apolipoprotein E. This peptide has little persistent three-dimensional structure in solution, but when bound to micelles of dodecylphosphocholine (DPC) it adopts a predominantly alpha-helical structure. The three-dimensional structure of the DPC-bound peptide has been determined by using 1H-NMR spectroscopy: the structure derived from NOE-based distance constraints and restrained molecular dynamics is largely helical. The derived phi and psi angle order parameters show that the helical structure is well defined but with some flexibility that causes the structures not to be superimposable over the full peptide length. Deuterium exchange experiments suggest that many peptide amide groups are readily accessible to the solvent, but those associated with hydrophobic residues exchange more slowly, and this helix is thus likely to be positioned on the surface of the DPC micelles. In this conformation the peptide has one hydrophobic face and two that are rich in basic amino acid side chains. The solvent-exposed face of the peptide contains residues previously shown to be involved in binding to the LDL receptor.

13C-13C rotational resonance in a transmembrane peptide: a comparison of the fluid and gel phases

A comparative study of two doubly 13C labeled amphiphilic transmembrane peptides was undertaken to determine the potential of rotational resonance for measuring internuclear distances through the direct dipolar coupling in the presence of motion. The two peptides, having the sequence acetyl-K2-G-L16-K2-A-amide, differed only in the position of 13C labels. The first peptide, [1-13C]leu(11):[alpha-13C]leu(12), had labels on adjacent residues, at the carbonyl of leu(11) and the alpha carbon of leu(12). The second, [1-13C]leu(8):[alpha-(13)/C]leu(11), was labeled on consecutive turns of the alpha-helical peptide. The internuclear distance between labeled positions of the first peptide, which for an ideal alpha helix has a value of 2.48 A, is relatively independent of internal flexibility or peptide conformational change. The dipolar coupling between these two nuclei is sensitive to motional averaging by molecular reorientation, however, making this peptide ideal for investigating these motions. The internuclear distance between labels on the second peptide has an expected static ideal alpha-helix value of 4.6 A, but this is sensitive to internal flexibility. In addition, the dipolar coupling between these two nuclei is much weaker because of their larger separation, making this peptide a much more difficult test of the rotational resonance technique. The dipolar couplings between the labeled nuclei of these two peptides were measured by rotational resonance in the dry peptide powders and in multilamellar dispersions with dimyristoylphosphatidylcholine in the gel phase, at -10 degrees C, and in the fluid phase, at 40 degrees C. The results for the peptide having adjacent labels can be readily interpreted in terms of a simple model for the peptide motion. The results for the second peptide show that, in the fluid phase, the motionally averaged dipolar coupling is too small to be measured by rotational resonance. Rotational resonance, rotational echo double resonance, and related techniques can be used to obtain reliable and valuable dipolar couplings in static solid and membrane systems. The interpretation of these couplings in terms of internuclear distances is straightforward in the absence of molecular motion. These techniques hold considerable promise for membrane protein structural studies under conditions, such as at low temperatures, where molecular motion does not modulate the dipolar couplings. However, a typical membrane at physiological temperatures exhibits complex molecular motions. In the absence of an accurate and detailed description of both internal and whole body molecular motions, it is unlikely that techniques of this type, which are based on extracting distances from direct internuclear dipolar couplings, can be used to study molecular structure under these conditions. Furthermore, the reduction in the strengths of the dipolar couplings by these motions dramatically reduces the useful range of distances which can be measured.

A light-harvesting antenna protein retains its folded conformation in the absence of protein-lipid and protein-pigment interactions

The first study by nmr of the integral membrane protein, the bacterial light-harvesting (LH) antenna protein LH1 beta, is reported. The photosynthetic apparatus of purple bacteria contains two different kinds of antenna complexes (LH1 and LH2), which consist of two small integral membrane proteins alpha and beta, each of approximately 6 kDa, and bacteriochlorophyll and carotenoid pigments. We have purified the antenna polypeptide LH1 beta from Rhodobacter sphaeroides, and have recorded CD spectra and a series of two-dimensional nmr spectra. A comparison of CD spectra of LH1 beta observed in organic solvents and detergent micelles shows that the helical character of the peptide does not change appreciably between the two milieus. A significantly high-field shifted methyl signal was observed both in organic solvents and in detergent micelles, implying that a similar three-dimensional structure is present in each case. However, the 1H-nmr signals observed in organic solvents had a narrower line width and better resolution, and it is shown that in this case organic solvents provide a better medium for nmr studies than detergent micelles. A sequential assignment has been carried out on the C-terminal transmembrane region, which is the region in which the pigment is bound. The region is shown to have a helical structure by the chemical shift values of the alpha-CH protons and the presence of nuclear Overhauser effects characteristic of helices. An analysis of the amide proton chemical shifts of the residues surrounding the histidine chlorophyll ligand suggests that the local structure is well ordered even in the absence of protein-lipid and protein-pigment interactions. Its structure was determined from 348 nmr-derived constraints by using distance geometry calculations. The polypeptide contains an alpha-helix extending from Leu19 (position of cytoplasmic surface) to Trp44 (position of periplasmic surface). The helix is bent, as expected from the amide proton chemical shifts, and it is similar to the polypeptide fold of the previously determined crystal structure of Rhodopseudomonas acidophila Ac10050 LH2 beta (S. M. Prince et al., Journal of Molecular Biology, 1997, Vol. 268, pp. 412-423). It is concluded that the polypeptide conformation of this region may facilitate assembly of the LH complex.

NMR studies of the C-terminal secretion signal of the haem-binding protein, HasA

HasA is a haem-binding protein which is secreted under iron-deficiency conditions by the gram-negative bacterium Serratia marcescens. It is a monomer of 19 kDa (187 residues) able to bind free haem as well as to capture it from haemoglobin. HasA delivers haem to a specific outer-membrane receptor HasR and allows the bacteria to grow in the absence of any other source of iron. It is secreted by a signal peptide-independent pathway which involves a C-terminal secretion signal and an ABC (ATP-binding cassette) transporter. The C-terminal region of the secretion signal containing the essential secretion motif is cleaved during or after the secretion process by proteases secreted by the bacteria. In this work, we study by 1H NMR the conformation of the C-terminal extremity of HasA in the whole protein and that of the isolated secretion signal peptide in a zwitterionic micelle complex that mimicks the membrane environment. We identify a helical region followed by a random-coil C-terminus in the peptide-micelle complex and we show that in both the whole protein and the complex, the last 15 residues containing the motif essential for secretion are highly flexible and unstructured. This flexibility may be a prerequisite to the recognition of HasA by its ABC transporter. We determine the cleavage site of the C-terminal extremity of the protein and analyse the effect of the cleavage on the haem acquisition process.

The structure of the antimicrobial active center of lactoferricin B bound to sodium dodecyl sulfate micelles

Lactoferricin B (LfcinB) is a 25-residue antimicrobial peptide released from bovine lactoferrin upon pepsin digestion. The antimicrobial center of LfcinB consists of six residues (RRWQWR-NH2), and it possesses similar bactericidal activity to LfcinB. The structure of the six-residue peptide bound to sodium dodecyl sulfate (SDS) micelles has been determined by NMR spectroscopy and molecular dynamics refinement. The peptide adopts a well defined amphipathic structure when bound to SDS micelles with the Trp sidechains separated from the Arg residues. Additional evidence demonstrates that the peptide is oriented in the micelle such that the Trp residues are more deeply buried in the micelle than the Arg and Gln residues.

Solution structure and membrane interactions of the C2 domain of cytosolic phospholipase A2

The amino-terminal, 138 amino acid C2 domain of cytosolic phospholipase A2 (cPLA2-C2) mediates an initial step in the production of lipid mediators of inflammation: the Ca2+-dependent translocation of the enzyme to intracellular membranes with subsequent liberation of arachidonic acid. The high resolution solution structure of this Ca2+-dependent, lipid-binding domain (CaLB) has been determined using heteronuclear three-dimensional NMR spectroscopy. Secondary structure analysis, derived from several sets of spectroscopic data, shows that the domain is composed of eight antiparallel beta-strands with six interconnecting loops that fits the "type II" topology for C2 domains. Using a total of 2370 distance and torsional restraints, the structure was found to be a beta-sandwich in the "Greek key" motif. The solution structure of cPLA2-C2 domain is very similar to the X-ray crystal structure of the C2 domain of phospholipase-C-delta and phylogenetic analysis clarifies the structural role of highly conserved residues. Calorimetric studies further demonstrate that cPLA2-C2 binds two Ca2+ with observed Kds of approximately 2 microM in an entropically assisted process. Moreover, regions on cPLA2-C2 interacting with membranes were identified by 15N-HSQC-spectroscopy of cPLA2-C2 in the presence of low molecular weight lipid micelles. An extended binding site was identified that binds the phosphocholine headgroup in a Ca2+-dependent manner and also interacts with proximal regions of the membrane surface. Based upon these results, a structural model is presented for the mechanism of association of cPLA2 with its membrane substrate.

Solution and solid state conformation of the human EGF receptor transmembrane region

The epidermal growth factor receptor (EGFR) is a member of the tyrosine kinase family of signalling cell surface molecules. Signalling by this protein is mediated through binding of epidermal growth factor to its extracellular region ultimately leading to phosphorylation of several residues on the intracellular portion of the receptor. The only means of communication between the intracellular and extracellular domains is via the transmembrane region of the protein. In this work we describe the first structural studies of a 34-residue synthetic peptide (hEGFRp), representative of the human EGFR transmembrane region, using two-dimensional and 2H wideline NMR and CD spectroscopies. In water the peptide demonstrated a lack of regular secondary structure and existed as oligomers. Addition of the lipomimetic solvent, trifluoroethanol (TFE), led to the production of monomeric structured species. Analysis of NMR spectra of the hEGFRp indicated that an alpha-helix was present between residues M626 and R647. This observation was reinforced by solid state 2H NMR studies in lipid bilayers which showed typical 'Pake' spectra indicating axially symmetric motion. The helical region in hEGFRp commences four residues later than predicted via hydrophobicity profiles, and extends to include several charged arginine residues which would lie on the cytosolic side of the membrane. These observations provide the first evidence that the transmembrane alpha-helical region in EGFR may not only traverse the membrane but may continue to the cytosolic region near T654, an important phosphorylation site.

The use of sodium dodecyl sulfate to model the apolipoprotein environment. Evidence for peptide-SDS complexes using pulsed-field-gradient NMR spectroscopy

Pulsed-field-gradient NMR spectroscopy was used to measure translational diffusion coefficients (Ds) for a peptide corresponding to a proposed lipid-binding domain of human apolipoprotein C-I, residues 7-24 (apoC-I(7-24)). Diffusion coefficients for apoC-I(7-24) were determined directly by following the decay of the resonance intensity of selected peptide protons at various concentrations of sodium dodecyl sulfate (SDS), a detergent increasingly being used to model the apolipoprotein environment. Previously, diffusion coefficients of peptides in the presence of SDS have been determined indirectly by monitoring the SDS diffusion coefficient. The direct measurement of the diffusion coefficient of the peptide enables one to distinguish whether SDS simply coats the peptide's surface to produce a uniformly charged 'rod' or if the peptide associates with a micelle. Using the direct method, at SDS concentrations above 5 mM (which is below the SDS critical micelle concentration (8.1 mM)), apoC-I(7-24) exhibited diffusion coefficients consistent with the formation of a large-molecular-weight complex. Based on the ratio of the diffusion coefficients for free- and SDS-associated peptide, the molecular weight of the peptide-SDS complex was much larger than a factor of 1. 4, the increase in molecular weight of the free peptide predicted if apoC-I(7-24) was uniformly surface coated with SDS.

Solid-state NMR for the study of membrane systems: the use of anisotropic interactions

The use of solid-state nuclear magnetic resonance (NMR) as a tool to determine the structure of membrane molecules is reviewed with a particular emphasis on techniques that provide information on orientation or order. Experiments reported here have been performed in membranes, rather than in micelles or organic solvents. Several ways to prepare and handle the samples are discussed, like sample orientation and magic-angle spinning (MAS). Results concerning lipids, membrane peptides and proteins are included, as well as a discussion regarding the potential of such methods and their pitfalls.

Solubilization of hydrophobic peptides by reversible cysteine PEGylation

"PEG-a-Cys" reagent, synthesized by the esterification of monomethoxy-poly(ethylene glycol) (avg. MW = 5 kDa) to Ellman's reagent [5,5'-dithiobis(2-nitrobenzoic acid)], is shown to "PEGylate" reversibly the cysteine residue of a 25-residue synthetic hydrophobic peptide (H2N-REAAALAAAAALAAWAALCPARRRR-CO2H) designed to model a transmembrane segment of a membrane protein. A mixed disulfide bond was formed between the reagent and the peptide that was readily cleaved with the mild reducing agent tricarboxyethylphosphine hydrochloride (TCEP.HCl). Carboxypeptidase B digestion of the charged carboxyl terminus of the peptide through to the Ala residue--which mimics the enzymatic cleavage of a TM segment from a fusion protein--releases a highly hydrophobic peptide. A time-dependent decrease in the amplitude of the digested peptide circular dichroism (CD) spectra was attributed to the aggregation and/or precipitation of the peptide. While PEGylation of the peptide with PEG-a-Cys had a negligible effect on conformation, it inhibited the loss of CD amplitude in both intact and digested peptides, suggesting that it was effective in solubilization of hydrophobic peptides.

Interhelical contacts are required for the helix bundle fold of apolipophorin III and its ability to interact with lipoproteins

Apolipophorin-III (apoLp-III) from the insect, Manduca sexta, is a 166-residue exchangeable apolipoprotein that plays a critical role in the dynamics of plasma lipoprotein interconversions. Our previous work indicated that a 36-residue C-terminal peptide fragment, generated by cyanogen bromide digestion of apoLp-III, was unable to bind to lipid surfaces (Narayanaswami V, Kay CM, Oikawa K, Ryan RO, 1994, Biochemistry 33:13312-13320), and showed no secondary structure in aqueous solution. In this paper, we have performed structural studies of this peptide (E131-Q166) complexed with SDS detergent micelles, or in the presence of the helix-inducing solvent trifluoroethanol (TFE), by two-dimensional 1H NMR spectroscopy. The peptide adopts an alpha-helical structure in the presence of both SDS and 50% TFE. The lipid-bound structure of the peptide, generated from the NMR NOE data, showed an elongated, slightly curved alpha-helix. Despite its high alpha-helix forming propensity, the peptide requires alpha helix-promoting environment to adopt an alpha-helical structure. This indicates the importance of the surrounding chemical environment and implies that, in the absence of lipid, tertiary contacts in the folded protein play a role in maintaining its structural integrity. Furthermore, the data suggest that the amphipathic helix bundle organization serves as a prerequisite structural motif for the reversible lipoprotein-binding activity of M. sexta apoLp-III.

Secondary structure analysis of individual transmembrane segments of the nicotinic acetylcholine receptor by circular dichroism and Fourier transform infrared spectroscopy

Circular dichroism (CD) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy are used to establish the secondary structure of peptides containing one or more transmembrane segments (M1-M4) of the Torpedo californica nicotinic acetylcholine receptor (AChR). Peptides containing the M2-M3 and M1-M2-M3 transmembrane segments of the AChR beta-subunit and the M4 segment of the alpha- and gamma-subunits were isolated from proteolytic digests of receptor subunits, purified, and reconstituted into lipid vesicles. For each peptide, an amide I vibrational frequency centered between 1650 and 1656 cm-1 and negative CD absorption bands at 208 and 222 nm indicate that the peptide is largely alpha-helical. In addition, the CD spectrum of a tryptic peptide of the alpha-subunit containing the M1 segment is also consistent with a largely alpha-helical structure. However, secondary structure analysis of the alpha-M1 CD spectrum indicates the presence of other structures, suggesting that the M1 segment may represent either a distorted alpha-helix, likely the consequence of several proline residues, or may not be entirely alpha-helical. Overall, these findings are consistent with studies that indicate that the transmembrane region of the AChR comprises predominantly, if not exclusively, membrane-spanning alpha-helices.

Conformational studies of the N-terminal lipid-associating domain of human apolipoprotein C-I by CD and 1H NMR spectroscopy

A peptide comprising the N-terminal 38 residues of human apolipoprotein C-I (apoC-I(1-38)) was synthesized using solid-phase methods and its solution conformation studied by CD and 1H NMR spectroscopy. The CD data indicate that apoC-I(1-38) has a similar helical content (55%) in the presence of saturating amounts of SDS or egg yolk lysophosphatidylcholine. A structural ensemble of SDS-bound apoC-I(1-38) was calculated from 464 NOE-based distance restraints using distance geometry methods. ApoC-I(1-38) adopts a helical structure between residues V4 and K30 and an extended C-terminus from Q31 when associated with SDS. The region K12-G15 undergoes slow conformational exchange as indicated by above-average amide resonance linewidths, large temperature coefficients, and fast exchange (< 2 h) of backbone amide protons with deuterium. The mobility of K12-G15 is reflected in the poorly defined dihedral angles of K12 and E13 in the calculated ensemble of structures. The average structure of apoC-I(1-38) is curved toward its hydrophobic face with bends of 125 degrees, centered at K12/E13, and 150 degrees, centered at K21. This curvature appears to be driven by the interaction of two hydrophobic clusters, one formed by residues L8, L11, F14, and L18, and the other by L25, I26, and I29, with the amphiphile SDS. Based on our present structural definition of apoC-I(1-38) and the previously obtained structure of the fragment apoC-I(35-53), we propose the secondary structure of intact apolipoprotein C-I.

Escherichia coli diacylglycerol kinase: a case study in the application of solution NMR methods to an integral membrane protein

Diacylglycerol kinase (DAGK) is a 13-kDa integral membrane protein that spans the lipid bilayer three times and which is active in some micellar systems. In this work DAGK was purified using metal ion chelate chromatography, and its structural properties in micelles and organic solvent mixtures studies were examined, primarily to address the question of whether the structure of DAGK can be determined using solution NMR methods. Cross-linking studies established that DAGK is homotrimeric in decyl maltoside (DM) micelles and mixed micelles. The aggregate detergent-protein molecular mass of DAGK in both octyl glucoside and DM micelles was determined to be in the range of 100-110 kDa-much larger than the sum of the molecular weights of the DAGK trimers and the protein-free micelles. In acidic organic solvent mixtures, DAGK-DM complexes were highly soluble and yielded relatively well-resolved NMR spectra. NMR and circular dichroism studies indicated that in these mixtures the enzyme adopts a kinetically trapped monomeric structure in which it irreversibly binds several detergent molecules and is primarily alpha-helical, but in which its tertiary structure is largely disordered. Although these results provide new information regarding the native oligomeric state of DAGK and the structural properties of complex membrane proteins in micelles and organic solvent mixtures, the results discourage the notion that the structure of DAGK can be readily determined at high resolution with solution NMR methods.

Role of tryptophan-14 in the interaction of dynorphin A(1-17) with micelles

Fluorescence spectroscopy has been used to examine the interaction between the opioid peptide dynorphin A(1-17) (dynorphin) and dodecylphosphocholine (DPC) micelles. Fluorescence emission spectra as a function of added lipid indicate insertion of the Trp14 side chain into the hydrophobic portion of the micelle, supporting NMR results from this laboratory. A model of interaction with micelles consistent with the fluorescence results and earlier NMR results is proposed. The critical micelle concentration in the presence of peptide was also determined, and is discussed in the context of relevance to both NMR spectroscopy and peptide-lipid interactions.

Magic angle spinning NMR spectroscopy of membrane proteins

The passage of molecules and information across cell membranes is mediated largely by membrane-spanning proteins acting as channels, pumps, receptors and enzymes. These proteins perform many tasks: they control electrochemical gradients across the membrane, receive signals from the environment or from other cells, convert light energy into chemical signals, transport small molecules into and out of cells, and harness proton gradients to generate the energy consumed in metabolism. Indeed, of the estimated 50000–100000 genes in the human genome, fully 20–40 % are thought to encode integral membrane proteins. If one also includes membrane-associated proteins, which are attached to the membrane surface through fatty acyl chains or electrostatic interactions, this percentage is likely to be much higher.

High-resolution mono- and multidimensional magic angle spinning 1H nuclear magnetic resonance of membrane peptides in nondeuterated lipid membranes and H2O

High-speed (14 kHz) solid-state magic angle spinning (MAS) 1H NMR has been applied to several membrane peptides incorporated into nondeuterated dilauroyl or dimyristoylphosphatidylcholine membranes suspended in H2O. It is shown that solvent suppression methods derived from solution NMR, such as presaturation or jump-return, can be used to reduce water resonance, even at relatively high water content. In addition, regioselective excitation of 1H peptide resonances promotes an efficient suppression of lipid resonances, even in cases where these are initially two orders of magnitude more intense. As a consequence, 1H MAS spectra of the peptide low-field region are obtained without interference from water and lipid signals. These display resonances from amide and other exchangeable 1H as well as from aromatic nonexchangeable 1H. The spectral resolution depends on the specific types of resonance and membrane peptide. For small amphiphilic or hydrophobic oligopeptides, resolution of most individual amide resonance is achieved, whereas for the transmembrane peptide gramicidin A, an unresolved amide spectrum is obtained. Partial resolution of aromatic 1H occurs in all cases. Multidimensional 1H-MAS spectra of membrane peptides can also be obtained by using water suppression and regioselective excitation. For gramicidin A, F2-regioselective 2D nuclear Overhauser effect spectroscopy (NOESY) spectra are dominated by intermolecular through-space connectivities between peptide aromatic or formyl 1H and lipid 1H. These appear to be compatible with the known structure and topography of the gramicidin pore. On the other hand, for the amphiphilic peptide leucine-enkephalin, F2-regioselective NOESY spectra mostly display cross-peaks originating from though-space proximities of amide or aromatic 1H with themselves and with aliphatic 1H. F3-regioselective 3D NOESY-NOESY spectra can be used to obtain through-space correlations within aliphatic 1H. Such intrapeptide proximities should allow determination of the conformation of the peptide in membranes. It is suggested that high-speed MAS multidimensional 1H NMR of peptides in nondeuterated membranes and in H2O can be used for studies of both peptide structure and lipid-peptide interactions.

Conformational studies of an amphipathic peptide corresponding to human apolipoprotein A-II residues 18-30 with a C-terminal lipid binding motif EWLNS

A peptide was designed and synthesized to enhance the lipid binding properties of a 13-residue fragment of apolipoprotein A-II. The peptide, VTDYGKDLMEKVKEWLNS [apoA-II(18-30)+], contains a five-residue amphipathic motif, EWLNS, at the C-terminus of apolipoprotein A-II residues 18-30. The lipid binding properties of apoA-II(18-30)+ were assessed using optical spectroscopy in the presence of sodium dodecyl sulfate (SDS), dodecylphosphocholine (DPC), tetradecyltrimethyl ammonium chloride (TMA) and dimyristoylphosphatidylcholine (DMPC). The fluorescence emission spectra and the circular dichroism data suggested that apoA-II(18-30)+ interacted most strongly with SDS and most weakly with DMPC. An ensemble of structures for apoA-II(18-30)+ in aqueous solution containing SDS was calculated using distance geometry/simulated annealing methods from 308 NOE-based distance restraints. The backbone (N-C-C = O) RMSD from the average structure of an ensemble of 15 out of 20 calculated structures was 0.54 +/- 0.16 A. Apart from some dynamic fraying at both termini, the distance geometry and simulated annealing calculations showed that apoA-II(18-30)+ adopted a well defined amphipathic helix with distinct hydrophobic and hydrophilic faces.

Conformation of human serum apolipoprotein A-I(166-185) in the presence of sodium dodecyl sulfate or dodecylphosphocholine by 1H-NMR and CD. Evidence for specific peptide-SDS interactions

The segment, YSDELRQRLAARLEALKENG, corresponding to residues 166 to 185 of human serum apolipoprotein A-I, was studied by circular dichroism and NMR spectroscopy in sodium dodecyl sulfate and dodecylphosphocholine micelles. 2-Dimensional NOESY, TOCSY and DQF-COSY spectra of apoA-I(166-185) in perdeuterated sodium dodecyl sulfate (SDS-d25) and dodecylphosphocholine (DPC-d38) micelles were collected at a peptide/SDS (DPC) ratio of 1:40. Similar CD spectra and NOE connectivity patterns were observed for apoA-I(166-185) in SDS and DPC, indicating a similar helical conformation in both. Conformations of apoA-I(166-185) in DPC-d38 micelles, and in SDS-d25 micelles at two pH values, 6.6 and 3.7, were determined using distance geometry calculations. Backbone superposition (N,C alpha,C = O) for an ensemble of twenty-nine structures in DPC at pH 6.0 gave a RMSD of 0.45 +/- 0.09 A for the region D168 to K182, while for all atoms it was 1.60 +/- 0.17 A. In SDS, the ensemble of nineteen structures each at pH 6.6 and 3.7 gave RMSDs of 0.28 +/- 0.07 A and 0.35 +/- 0.10 A, respectively, for the region D168 to K182. RMSD for superposition of all atoms was 1.36 +/- 0.10 A and 1.38 +/- 0.21 A at the respective pH values. In all cases a highly defined class A amphipathic helical structure was found for the region R171 to K182. Since the same structure occurs in micelles with either negatively charged or zwitterionic head groups it strongly suggests a dominant role for hydrophobic interactions in stabilizing the complex. The Y166 aromatic ring is bent back upon the helix axis at the lower pH. NMR determination of pKa values for D168, E169, E179 and E183 in the presence of SDS or DPC indicated a micro-pH at the micellar surface approximately one pH unit higher than the normal residue pKa. SDS interactions with the peptide were examined by collecting 1H NOESY spectra in the presence of protiated SDS. Residues R171, R173, R177, as well as the aromatic ring of Y166, were shown by intermolecular NOE measurements to interact with SDS, hence a key interaction in stabilizing the complex appears to be between interfacial basic side-chains and SDS alkyl chains.

An NMR, CD, molecular dynamics, and fluorometric study of the conformation of the bradykinin antagonist B-9340 in water and in aqueous micellar solutions

A detailed NMR, CD, fluorometry, and molecular modeling study of a novel bradykinin antagonist B-9340, containing a novel amino acid D-Igl (alpha-(2-indanyl)glycine) at position 7, was carried out. The sequence of B-9340 is D-Arg0-Arg1-Pro2-Hyp3-Gly4-Thi5-Ser6-D- Igl7-Oic8-Arg9, where Hyp is hydroxyproline, Thi is beta-(2-thienyl)alanine, and Oic is (3aS,7aS)-octahydroindole-2-carboxylic acid. The CD results exhibit a striking effect of SDS on the spectrum of the BK antagonist, indicating that interaction with the surfactant induces a folded peptide structure. The interaction of this antagonist with phosphatidylinositol was monitored by fluorometry, indicating that the interaction of the peptide with the lipid is cooperative, and gives a Hill coefficient of 2.3. The two-dimensional proton NMR measurements indicate that B-9340 has no stable secondary structure in water solution and contains about 10-15% cis peptide bonds arising from Pro2, Hyp3, and Oic8. In SDS micelles, NMR reveals the existence of two beta-turns based on a number of medium-range connectivities that were useful for molecular modeling. The actual molecular modeling and dynamic runs were performed on B-9340 in an environment consisting of a layer of octyl sulfate anions and water. Ther results indicate that the structure of B-9340 in a micellar environment is characterized by a nonideal betaII-turn comprising residues Pro2 to Thi5, a nonideal betaII'-turn comprising residues Ser6-Arg9, and broad folding in the middle part of the molecule. The structure is stabilized by several hydrogen bonds and by a salt bridge between the guanidine moiety of Arg1 and the carboxyl group of Arg9, whereas the middle part of the peptide is buried in the micelle. The structure is deposited as Brookhaven PDB file 1 BDK.

Structural studies of a peptide activator of human lecithin-cholesterol acyltransferase

The synthetic lipid-associating peptide, LAP-20 (VSSLLSSLKEYWSSLKESFS), activates lecithin-cholesterol acyltransferase (LCAT) despite its lack of sequence homology to apolipoprotein A-I, the primary in vivo activator of LCAT. Using SDS and dodecylphosphocholine (DPC) to model the lipoprotein environment, the structural features responsible for LAP-20's ability to activate LCAT were studied by optical and two-dimensional 1H NMR spectroscopy. A large blue shift in the intrinsic fluorescence of LAP-20 with the addition of detergent suggested that the peptide formed a complex with the micelles. Analysis of the CD data shows that LAP-20 lacks well defined structure in aqueous solution but adopts helical, ordered conformations upon the addition of SDS or DPC. The helical nature of the peptides in the presence of both lipids was confirmed by upfield H alpha NMR secondary shifts relative to random coil values. Average structures for both peptides in aqueous solutions containing SDS and DPC were generated using distance geometry methods from 329 (SDS) and 309 (DPC) nuclear Overhauser effect-based distance restraints. The backbone (N, Calpha, C=O) RMSD from the average structure of an ensemble of 17 out of 20 calculated structures was 0.41 +/- 0.15 Angstrom for LAP-20 in SDS and 0.41 +/- 0.12 A for an ensemble of 20 out of 20 calculated structures for LAP-20 in DPC. In the presence of SDS, the distance geometry and simulated annealing calculations show that LAP-20 adopts a well defined class A amphipathic helix with distinct hydrophobic and hydrophilic faces. A similar structure was obtained for LAP-20 in the presence of DPC, suggesting that both detergents may be used interchangeably to model the lipoprotein environment. Conformational features of the calculated structures for LAP-20 are discussed relative to models for apolipoprotein A-I activation of LCAT.