Solid-state NMR investigations on the structure and topological equilibria of polypeptides associated with biological membranes

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Abstract
Peptides and proteins have been labelled with 15N specifically, selectively or uniformly by chemical or biochemical methods and reconstituted into oriented lipid bilayers. Thereafter, the orientation of polypeptide α-helices with respect to the bilayer surface has been determined by proton-decoupled 15N solid-state NMR spectroscopy. Hydrophobic peptides such as the channel-forming domains of Vpu of HIV-1 or M2 of Influenza A adopt stable transmembrane alignments. This orientation is in agreement with models suggesting the transient channel formation by transmembrane helical bundles. The size distribution of such oligomers is dependent on a multitude of experimental parameters. In contrast, a wide variety of peptide antibiotics and other amphipathic α-helices adopt stable orientations along the bilayer surface. For other peptides, equilibria are observed between in-plane and transmembrane topologies. These include designed sequences that change alignment in a pH dependent manner as well as peptides whose lengths do not match the bilayer thickness. A thermodynamic model is presented that describes in-plane-to-transmembrane topological transitions. Topological equilibria are also observed for larger membrane proteins such as some of the pore-forming domains of colicins.

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