Biophysical investigations of membrane perturbations by polypeptides using solid-state NMR spectroscopy (review)

Polypeptides have been prepared by solid-phase peptide synthesis and labelled with N-15 at single sites to be used for static or magic angle spinning solid-state NMR spectroscopy. After reconstitution into oriented membranes, the alignment of polypeptide alpha -helices with respect to the bilayer surface is accessible by proton-decoupled N-15 solid-state NMR spectroscopy. In addition, limiting values of rotational diffusion coefficients are obtained. The effects of membrane inserted peptides on the bilayer phospholipids have been investigated by H-2 and P-31 solid-state NMR spectroscopy. Long hydrophobic peptides such as the channel-forming domains of Vpu of HIV-1 or M2 of Influenza A adopt stable alignments approximately parallel to the bilayer normal in agreement with models suggesting transmembrane helical bundle formation. The N-15 chemical shift data agree with tilt angles of similar to 20 and 33 degrees, respectively. In contrast, multi-charged amphipathic alpha -helices adopt stable orientations parallel to the bilayer surface. In the presence of these peptides, decreased order parameters of the fatty acyl chains, membrane thinning, and the loss of long-range order are observed. Peptides that change topology in a pH dependent manner are more potent in antibiotic assays under experimental conditions where they show in-plane alignments. This result suggests that their detergent-like properties, rather than the formation of transmembrane helical bundles, are responsible for their cell-killing activities. Topological equilibria are also observed within proteins or for polypeptides that do not match the hydrophobic thickness of the bilayer.