Secondary structure and orientation of the amphipathic peptide GALA in lipid structures

Abstract

GALA, a synthetic, amphipathic 30-amino-acid peptide, based upon a Glu-Ala-Leu-Ala motive, was designed to mimic the behavior of viral fusion proteins. GALA is a water-soluble peptide with an aperiodic conformation at neutral pH, and becomes an amphipathic a helix as the pH is lowered to 5, where it interacts with phospholipid bilayers. Attenuated total-reflection infrared spectroscopy, using polarized light, provides information on the structure and orientation of the peptide and the lipids, which is not subject to artifacts due to light scattering with large particles. H/2H-exchange rate of the amide N-H group and analysis of the shape of the amide I' by Fourier self-deconvolution and curve fitting indicate that the a-helical content increases from 19% to 69% on lowering the pH. A further increase to 100% c( helix is observed after interaction with palmitoyloleoylglycerophosphocholine (PamOleGroPCho) vesicles. Dichroism data obtained with oriented bilayers of the PamOleGroPCho-GALA complex demonstrate that PamOleGroPCho hydrocarbon chains and the peptide a helical axis are essentially perpendicular (k 15") to the membrane plane. At neutral pH, in the presence of dimyristoylglycerophosphocholine (Myr,GroPCho), GALA is known to form discoidal structures similar to those formed under the same conditions by apolipoproteins A1 and AII. In these discoidal complexes, the a-helical content was estimated to be 65%, with the rest of the structure being essentially unordered. No significant modification of the all-trans conformation of the hydrocarbon chain of Myr2GroPCho was detected upon disc formation. Dichroism measurements show that the a-helical axis is essentially parallel to the hydrocarbon chains. These data support a model in which a discoidal patch of the bilayer is surrounded by amphipathic helices which shield the hydrophobic region of the bilayer from the aqueous environment. The infrared spectrum of GALA in this complex was found to be very similar to those of apolipoproteins A1 and A11 which form discoidal complexes with Myr2GroPCho, but the spectrum is quite different from that of apolipoprotein BlOO in low-density lipoproteins, which does not form discoidal complexes.A comprehensive understanding of the interaction of a peptide with a bilayer requires both dynamic and structural information on the system. Methods for investigation of the dynamics of peptide-lipid interactions have progressed quite far; however, structural investigations have not advanced as rapidly. A delineation of the structural components requires knowledge of the structure of the peptide in the bilayer, the orientation of the peptide in relation to the plane of the membrane, and the structure(s) of the phospholipids in contact with the peptide. Such a detailed structural picture has emerged only for a few peptide-lipid systems [l, 21. Fourier-transformed infrared spectroscopy (FTIR) can be used to assign secondary structure to peptides [3 -51. In addition, the orientation of peptides with a defined secondaryCorrespondence to E. Go...

Secondary structure and dosage of soluble and membrane proteins by attenuated total reflection Fourier‐transform infrared spectroscopy on hydrated films

“…Another example of such an agreement between Fourier self-deconvolution and dichroism measurements for resolving two components, has been reported for the complex poly-(Glu-Ala-Leu-Ala) peptide-Myr2GroPCho complex. The high-frequency component of the a-helix at 1658 cm-' had a well-defined orientation with respect to the membrane plane, while the low-frequency component at 1648 cm-' was not oriented [48]. The observation that at least in some instances membrane-associated structures have vibration frequencies unusual for soluble proteins is potentially crucial for the independent study of the structure and orientation of the polypeptide chain embedded in the lipid bilayer and the structure of the part of the protein protruding into the aqueous environment.…”

Sticky water surfaces: Helix–coil transitions suppressed in a cell-penetrating peptide at the air-water interface

“…This indicates that only partial deuteration of N-H groups in the amide bonds of GALA 0 occurs, as has been observed before for GALA in the helical state for which N-H appear to have extremely slow O-D exchange rates. 13 However, the GALA 7− spectrum does not exhibit any bands related to protonated N-H groups. This indicates that the main chain N-H groups are accessible to the surrounding water and therefore fully deuterated.…”

“…31 The bands at 1568 and 1407 cm −1 -which can serve as valuable probes for the glu- tamic acid protonation state-were not observed in the attenuated total internal reflection FTIR spectra reported before. 13 A detailed view of the amide I region is shown in Fig. 1(b).…”

“…The findings are in line with the previous study. 13 The spectrum of GALA 7− also contains an α-helix contribution at 1653 cm −1 however accounting for only 27%. Similarly, the β-sheet content has significant lower contribution with 15%.…”

“…[13][14][15] Along those lines, multiple investigations of GALA as a vector for drug or gene delivery have been conducted. 9 What remains to be clarified is how hydrophobic surfaces-for example, drug loaded polymeric nanoparticles, biochips, and biotechnical equipment-may influence GALA's pH-driven helix-coil transitions.…”