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High-resolution two-dimensional (2D) 1H-15N heteronuclear correlation (HETCOR) spectroscopy has been used to characterize the structure and dynamics of 15N-backbone labeled antimicrobial piscidin 1 (p1) oriented in “native-like” hydrated lipid bilayers. Piscidin belongs to a family of amphipatic cationic antimicrobial peptides, which are membrane-active and have broad spectrum antimicrobial activity on bacteria. When the 1H chemical shifts are encoded by the 1H-15N dipolar couplings, 2D dipolar-encoded HETCOR (i.e., de-HETCOR) solid-state NMR spectra yield high resolution 1H and 15N chemical shifts as well as 1H-15N heteronuclear dipolar couplings. Several advantages of HETCOR and de-HETCOR techniques that emerge from our investigations could facilitate the atomic-level investigations of structure-function relationships in membrane-active peptides and membrane-bound species. First, the de-HETCOR NMR spectrum of a ten-site 15N-labeled sample of p1 aligned in hydrated lipid bilayers can resolve resonances that are overlapped in the standard HETCOR spectrum. Second, the resolution in de-HETCOR spectra of p1 improves significantly at higher magnetic field due to an enhanced alignment that improves spectrum definition uniformly. Third, the HETCOR spectrum of 15N-K14 p1 oriented in hydrated lipid bilayers displays not only the expected crosscorrelation between the chemical shifts of bonded amide 1H and 15N spins but also a cross peak between the 1H chemical shift from bulk water and the 15N chemical shift from the labeled amide nitrogen. This information provides new insights into the intermolecular interactions of an amphipathic antimicrobial peptide optimized to partition at the water-bilayer interface and may have implications at the biological level.
Since the earliest demonstration1 that high-resolution structural constraints could be obtained by solid-state NMR (ssNMR) of proteins in anisotropic environments, high-resolution structural NMR studies of membrane-bound proteins and peptides oriented in “native-like” hydrated lamellar-phase lipid environments have intensified.2 Information from these studies feed a larger body of knowledge aimed at improving our understanding of structure-function relationships in biomolecules that perform vital functions at cell membranes. With improved resolution in recently developed Separated-Local-Field (SLF) experiments3, the correlation between the orientation-dependent, anisotropic 1H-15N heteronuclear Dipolar Couplings (DCs) and 15N Chemical Shifts (CSs) has become an effective way4,5 to characterize the helical tilt, τ (i.e. the angle between the peptide helical axis and the bilayer normal) of uniformly labeled aligned proteins and peptides. However, until recently6–8, the CSs of amide protons have received little attention in the structural and topological studies of aligned proteins or peptides, although they may yield complementary information, such as hydrogen bonding partnership and geometry in peptide planes8,9. In this communication, we demonstrate the resolution improvement of 1H-15N Heteronuclear Correlation (HETCOR)7 NMR spectroscopy at high static magnetic field, B0, and its advantages in characterizing both the topology and solvent interactions of 15Nα-labeled piscidin aligned in hydrated lipid bilayers.
Piscidin, an Amphipathic Cationic Antimicrobial Peptide (ACAP) from fish, is believed to play a fundamental and direct role in the fight against many aquatic bacterial infections10,11. The recent surge in bacterial resistance to the most potent conventional antibiotics has motivated active research on ACAPs since they evidence little, if any, resistance effects12. The actual mode of action of most ACAPs remains undetermined13 even though some theoretical models, which involve interactions with cell membranes14–17, exist18–22. A better understanding of structure-function relationships in ACAPs, as needed to design improved therapeutics20,23,24, may be obtained by analyzing the high-resolution structures of their membrane-bound states. Our prior high-resolution ssNMR studies25,26 have indicated that piscidin isoforms 1 and 3 (p1 and p3), which differ significantly in their antimicrobial and hemolytic effects, adopt an α-helical structure and lie almost in the plane of the bilayer, where they experience fast motion about the bilayer normal. Since piscidins are well adapted to partition at the water-bilayer interface25–27, and piscidin-bilayer interactions may have biological implications, they represent a suitable ground to test 1H-15N HETCOR to not only obtain high-resolution structures of peptides and proteins but also gain insight into their interactions with bilayers and the aqueous environment.
Fig. 1A shows the 2D HETCOR spectrum of amidated 15N-I5F6G8I9V10V12G13I16L19V20 p1 oriented in lipid bilayers. As described previously26, this peptide sequence FFHHIFRGIVHVGK-TIHRLVTG was chemically synthesized and a hydrated sample aligned on glass plates was prepared by mixing dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol in a 3:1 molar ratio and adding p1 at a 1:20 peptide/lipid molar ratio. At 21.1 T, nine resonances are identified in Fig. 1A with the 15N CSs spreading from 37 to 63 ppm, in contrast to values of ~ 180 ppm for transmembrane peptides2. As shown in the Supporting Information, the 1H-15N HETCOR resonances of an ideal α-helix form characteristic patterns, from which τ values can be uniquely determined without resonance assignments. The simulated “red” curve in Fig. 1A indicates that τ for p1 is about 86°±4° with respect to B0 and the bilayer normal, which is consistent with prior general observations on singly-labeled p125,26. While the 15N resonance at 63 ppm does not fit the simulation in Fig. 1A, presumably due to variations in the local chemical environment of p1, its 1H-15N DC follows the trend of the other residues, implying that the whole helix adopts a relatively constant orientation.
When the 1H CSs are encoded by the 1H-15N DCs, the dipolar-encoded HETCOR (de-HETCOR) spectrum of the same sample (c.f. Fig. 1B) displays two distinct groups of resonances split by the DCs. Unlike the SLF spectra4,5 where the DCs result in two identical wheels mirrored along the zero frequency in the 1H-15N DC dimension, the de-HECTOR spectrum shows two asymmetric resonance patterns. For the upper group (i.e., in the low 1H CS range) in Fig. 1B, ten resonances are clearly identified, implying that the resonances at the 15N CS of ~52 ppm, which were overlapped in the HETCOR spectrum, are now resolved in the de-HETCOR spectrum. The curves simulated for τ = 86°±4° fit the experimental data very well, confirming again that the peptide helical axis is almost perpendicular to the bilayer normal.
Fig. 2 shows the 15N and 1H slices taken from the de-HETCOR spectra of the ten-site labeled sample at different B0 strengths for the 15N resonance at 37.8 ppm. The 15N linewidth (LW) at half-height was 2.2 ppm at 14.1 T and 1.5 ppm at 21.1 T. Similarly, the 1H LW at half-height of the two split resonances decreased from 2.0 (left) and 1.8 ppm (right) to 1.5 (left) and 1.2 ppm (right) when B0 increased from 14.1 to 21.1 T. That is, the improvement in spectral resolution at 21.1 T in both the 1H and 15N CS dimensions, as measured by the ratio of the LWs, reached ~150%, commensurate with the increase in B0, implying that the absolute LWs in unit of Hertz remained almost the same even though B0 increased. This result indicates that the uniform enhancement of resolution in both the 1H and 15N dimensions at 21.1 T is due to a combination of shear alignment between glass plates and anisotropy of the diamagnetic susceptibility at higher B028. With simultaneously improved resolution of 1H and 15N CSs as well as 1H-15N DCs at higher B0, de-HETCOR emerges as an advantageous technique for structural studies of multiply-labeled aligned membrane-bound species that intrinsically yield crowded spectra.
An important aspect of the HETCOR spectra is to provide information about the chemical environment of amide protons. Fig. 3 shows the HETCOR spectrum of 15N-K14 p1 oriented in lipid bilayers. The 15N-oriented peak at 53.8 ppm correlates with two 1H resonances at 10.7 and 4.1 ppm. As indicated by the red curve in Fig. 1A, the resonance at 10.7 ppm originates from the backbone amide proton of K14, while the resonance at 4.1 ppm appears to be the isotropic signal from bulk water protons. Such a resonance at ~ 4 ppm is also present in the HETCOR spectrum of 15N-H4 p1 oriented in hydrated lipids (data not shown), but not observed in the spectrum of the ten-site labeled p1 that features 15N labels on the hydrophobic side of the amphipathic helix. This suggests that the unique crosscorrelations in the 2D HETCOR spectra of 15N-H4 p1 and 15N-K14 p1 may be due to the location of H4 and K14 on the hydrophilic side of piscidin’s amphipathic helix (c.f. Fig. 3). The slice shown in Fig. 3 indicates that the 1H LWs for the resonances at 10.7 and 4.1 ppm appear to be broader (~2.8 ppm) than the corresponding LWs of the hydrophobic sites (c.f. Fig. 2), implying that protons from water molecules in the aqueous environment exchange with the amide proton at the K14 site at a slow rate29. To the best of our knowledge, this is the first direct evidence that piscidin bound to lipid bilayers interacts with the aqueous environment. This chemical information, which relates to the location of the peptide in the lipids, is useful to better understand the biological activity of piscidin, as the depth of insertion of membrane-active peptides is likely related to their ability to disrupt bilayers and induce cell death15–18. Therefore, with improved resolution at ultra-high field and the ability to access 1H CSs, the HETCOR technique featured here provides a new opportunity to characterize structure-function relationships of membrane-bound species, especially at the water-bilayer interface.
NMR experiments performed at the NHMFL were supported in part by NIH R01 AI23007 (RF), NSF Cooperative Agreement DMR-0654118, and the State of Florida. MC, EDG, and DJH are grateful for support from NSF (CHE-0832571), Research Corporation, Dreyfus Foundation, and PLU. The authors thank Dr. Cross (NHMFL) for helpful discussions and Dr. Ball (U. TX) for the synthesis of the ten-site labeled p1.