The membrane topology of the peptide 18A, a derivative of apolipoprotein A-I, is investigated in structural detail. Apolipoprotein A-I is the dominant protein component of high density lipoproteins with important functions in cholesterol metabolism. 18A (Ac-DWLKA FYDKV AEKLK EAF- NH2) was designed to mimic the structure of tandem domains of class A amphipathic helices and has served as a lead peptide for biomedical applications. At low peptide-to-lipid ratios 18A partitions into phosphatidylcholine membranes with helix topologies parallel to the membrane surface, an alignment that is maintained when disc-like bicelles form at higher peptide-to-lipid ratios. Notably, the bicelles interact cooperatively with the magnetic field of the NMR spectrometer, thus the bilayer normal is oriented perpendicular to the magnetic field direction. A set of peptides that totals four 15N or 2H labelled positions of 18A allowed the accurate analysis of tilt and azimuthal angles relative to the membrane surface under different conditions. The topology agrees with a double belt arrangement forming a rim that covers the hydrophobic fatty acyl chains of the bicelles. In another set of experiments, it was shown that POPC nanodiscs prepared in the presence of diisobutylene/maleic acid (DIBMA) polymers can also be made to align in the magnetic field. Finally, the transmembrane domains of the DQ alpha-1 and DQ beta-1 subunits of the major histocomptability complex (MHC) class II have been prepared and reconstituted into magnetically oriented bicelles for NMR structural analysis.
Keywords: Apo A-I mimetic; Bicelle; DIBMA polymer; DQA1 of MHC II; DQB1; Oriented bilayer; Rim structure; Solid-state NMR; helix topology.
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