Transmembrane helices of integral membrane proteins often are flanked by interfacial aromatic residues that may serve as anchors to aid the stabilization of a tilted transmembrane orientation. Yet physical factors that govern the orientations or the dynamic averaging of individual transmembrane helices are not well understood and have not been adequately explained. When using solid-state 2H NMR spectroscopy to examine lipid bilayer-incorporated model peptides of the GWALP23 (acetyl-GGALW(LA)6LWLAGA-amide) family, we observe substantial unwinding at the terminals of several tilted helices spanning the membranes of DLPC, DMPC or DOPC lipid bilayers. The fraying of helix ends may be vital for defining the dynamics and orientations of transmembrane helices in lipid-bilayer membranes.

Negatively charged side chains are important for the function of particular ion channels and certain other membrane proteins. To investigate the influence of single glutamic acid side chains on helices that span lipid-bilayer membranes, we have employed GWALP23 (acetyl-GGALW5LALALALALALALW19LAGA-amide) as a favorable host peptide framework. We substituted individual Leu residues with Glu residues (L12E or L14E or L16E) and incorporated specific 2H-labeled alanine residues within the core helical region or near the ends of the sequence. Solid-state 2H-NMR spectra reveal little change for the core labels in GWALP23-E12, –E14 and –E16 over a pH range of 4 to 12.5, with the spectra being broader for samples in DOPC compared to DLPC bilayers. The spectra for samples with deuterium labels near the helix ends on alanines 3 and 21 show modest pH-dependent changes in the extent of unwinding of the helix terminals in DLPC and DOPC bilayers. The combined results indicate minor overall responses of these transmembrane helices to changes in pH, with the most buried residue E12 showing no pH dependence. While the Glu residues E14 and E16 may have high pKa values in the lipid bilayer environment, it is also possible that a paucity of helix response is masking the pKa values. Interestingly, when E16 is present, spectral changes at high pH report significant local unwinding of the core helix. Our results are consistent with the expectation that buried carboxyl groups aggressively hold their protons and/or waters of hydration.

We review the importance of helix motions for the function of several important categories of membrane proteins and for the properties of several model molecular systems. For voltage-gated potassium or sodium channels, sliding, tilting and/or rotational movements of the S4 helix accompanied by a swapping of cognate side-chain ion-pair interactions regulate the channel gating. In the seven-helix G protein-coupled receptors, exemplified by the rhodopsins, collective helix motions serve to activate the functional signaling. Peptides which initially associate with lipid-bilayer membrane surfaces may undergo dynamic transitions from surface-bound to tilted-transmembrane orientations, sometimes accompanied by changes in the molecularity, formation of a pore or, more generally, the activation of biological function. For single-span membrane proteins, such as the tyrosine kinases, an interplay between juxtamembrane and transmembrane domains is likely to be crucial for the regulation of dimer assembly that in turn is associated with the functional responses to external signals. Additionally, we note that experiments with designed single-span transmembrane helices offer fundamental insights into the molecular features that govern protein-lipid interactions. This article is part of a Special Issue entitled: Lipid-protein interactions.

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membrane and the atomic structure has been resolved by X-ray crystallography. The hydrophobic thickness of the protein observed in the structure 29.7A is considerably thinner than that predicted for the membrane 35.5A suggesting a hydrophobic mismatch. This hydrophobic mismatch is expected to aid protein-protein interactions and result in lipid sorting. Furthermore the surface of the protein has several cationic groups that could result in specific interactions with anionic lipid species. In order to investigate the local environment of Aquaporin Z in native E. coli membranes, and so understand the degree of lipid sorting and local protein-protein interactions, we have isolated Styrene-Maleic Acid Lipid Particles. These particles are believed to contain small portions of membrane isolated by the detergent action of the polymer. We have analysed particles isolated from E.coli membranes and containing Aquaporing Z for size, lipid and protein content. We have compared these experimental results to results obtained from coarse grained molecular dynamic simulations. These results are interpreted in terms of the interactions between the aquaporin and its environment.