Determination of the structure of a membrane-incorporated ion channel. Solid-state nuclear magnetic resonance studies of gramicidin A

Melittin-induced changes in lipid multilayers. A solid-state NMR study

Solid-state 1H, 13C, 14N, and 31P NMR spectroscopy was used to study the effects of the bee venom peptide, melittin, on aligned multilayers of dimyristoyl-, dilauryl- and ditetradecyl-phosphatidylcholines above the gel to liquid-crystalline transition temperature, Tc. Both 31P spectra from the lipid headgroups and 1H resonances from the lipid acyl chain methylene groups indicate that the peptide does not affect the mosaic spread of the lipid molecules at lipid:peptide molar ratios of 10:1, or higher. None of the samples prepared above Tc showed any evidence of the formation of hexagonal or isotropic phases. Melittin-induced changes in the chemical shift anisotropy of the headgroup phosphate and the lipid carbonyl groups, and in the choline 14N quadrupole splittings, show that the peptide has effects on the headgroup order and on the molecular organization in the sections of the acyl chains nearest to the bilayer surface. The spin-lattice relaxation time for the lipid acyl chain methylene protons was found to increase and the rotating-frame longitudinal relaxation time to markedly decrease with the addition of melittin, suggesting that motions on the nanosecond time scale are restricted, whereas the slower, collective motions are enhanced in the presence of the peptide.

The effect of changes in gramicidin conformation on bilayer lipid properties

The effects of two different gramicidin conformations on lipid phase behaviour and dynamics are compared. Samples of chain-perdeuterated dimyristoylphosphatidylcholine containing gramicidin were first prepared with gramicidin in a state having a circular dichroism spectrum generally identified as corresponding to the non-channel conformation. The effects, on bilayer lipid properties, of gramicidin in this conformation were then determined using deuterium nuclear magnetic resonance measurements of acyl chain orientational order and transverse relaxation times as a function of temperature. These samples were then incubated at 65 degrees C to convert the gramicidin to a state with a circular dichroism spectrum of the type generally identified with the channel conformation. The nuclear magnetic resonance measurements were then repeated. In the gel phase, it was found that transverse relaxation time and chain orientational order of the lipid were insensitive to gramicidin conformation. In the liquid crystalline phase, gramicidin in the channel conformation was found to have a slightly larger effect on transverse relaxation and orientational order than gramicidin in the non-channel conformation. The perturbation of the phase behavior by gramicidin was found to be relatively insensitive to gramicidin conformation.

Molecular dynamics computations and solid state nuclear magnetic resonance of the gramicidin cation channel

This paper reports on a coupled approach to determining the structure of the gramicidin A ion channel, utilizing solid state nuclear magnetic resonance (NMR) of isotopically labeled gramicidin channels aligned parallel to the magnetic field direction, and molecular dynamics (MD). MD computations using an idealized right-handed beta-helix as a starting point produce a refined molecular structure that is in excellent agreement with atomic resolution solid state NMR data. The data provided by NMR and MD are complementary to each other. When applied in a coordinated manner they provide a powerful approach to structure determination in molecular systems not readily amenable to x-ray diffraction.

Ion transport in a model gramicidin channel. Structure and thermodynamics

The potential of mean force for Na+ and K+ ions as a function of position in the interior of a periodic poly(L,D)-alanine model for the gramicidin beta-helix is calculated with a detailed atomic model and realistic interactions. The calculated free energy barriers are 4.5 kcal/mol for Na+ and 1.0 kcal/mol for K+. A decomposition of the free energy demonstrates that the water molecules make a significant contribution to the free energy of activation. There is an increase in entropy at the transition state associated with greater fluctuations. Analysis reveals that the free energy profile of ions in the periodic channel is controlled not by the large interaction energy involving the ion but rather by the weaker water-water, water-peptide and peptide-peptide hydrogen bond interactions. The interior of the channel retains much of the solvation properties of a liquid in its interactions with the cations. Of particular importance is the flexibility of the helix, which permits it to respond to the presence of an ion in a fluidlike manner. The distortion of the helix is local (limited to a few carbonyls) because the structure is too flexible to transmit a perturbation to large distances. The plasticity of the structure (i.e., the property to deform without generating a large energy stress) appears to be an essential factor in the transport of ions, suggesting that a rigid helix model would be inappropriate.

Photo-modulated ion channels based on covalently linked gramicidins

The covalent coupling of two gramicidin A monomers proved to be a useful tool for the rational design of ion channels with predictable electrophysiological properties (Stankovic, C.J., Heinemann, S.H., Delfino, J.M., Sigworth, F.J. and Schreiber, S.L. (1989) Science 244, 813-817; Stankovic, C.J., Heinemann, S.H. and Schreiber, S.L. (1990) J. Am. Chem. Soc. 112, 3702-3704). Herein we report on our first efforts to equip such channels with an artificial gating mechanism. Gramicidin monomers were covalently linked with 3,3'-azobis(benzeneacetic acid). Based on computer modeling of the beta-helix channel motif, this linker in its dark-adapted (trans) form does not allow for the formation of unimolecular ion channels, while the photo-activated (cis) form was expected to provide this possibility. The electrophysiological assays showed that (A) the trans-isomer does form characteristic ion channels, and (B) irradiation transforms these channels into a new distinct, flickering channel type in a reversible manner. The results are discussed in the framework of intermolecular gramicidin aggregates.

The structure of gramicidin A in dimethylsulfoxide/acetone

It has been demonstrated by two-dimensional NMR cross-relaxation spectroscopy that gramicidin A exists in dimethylsulfoxide/acetone solution in random coil form. This contradicts earlier conclusions by Hawkes et al. [Hawkes, G. E., Lian, L. Y., Randall, E. W., Sales, K. D. & Curzon, E. H. (1987) Eur. J. Biochem. 166, 437-445] that were based on the interpretation of vicinal proton coupling constants.

Solid-state 13C-NMR studies of the effects of sodium ions on the gramicidin A ion channel

End-to-end helical dimers of gramicidin A form transmembrane pores in lipid bilayers, through which monovalent ions may pass. The groups within the peptide that interact with these ions have been studied by application of solid-state spectroscopic methods to a series of gramicidin A analogues synthesized with 13C in selected peptide carbonyl groups. The resonances of D-Leu10, D-Leu12 and D-Leu14 analogues were perturbed in the presence of 0.16 M sodium ions, whereas the resonances of the carbonyls of Gly2, Ala3, D-Leu4 and Val7, which are closer to the formylated N-terminal end of the peptide, were unaffected. The observed changes in chemical shift anisotropy are indicative of a change in orientation of the abovementioned leucine carbonyls.