Production of integral membrane proteins (IMPs) in a folded state is a key prerequisite for their functional and structural studies. In cell-free (CF) expression systems membrane mimicking components could be added to the reaction mixture that promotes IMP production in a soluble form. Here lipid-protein nanodiscs (LPNs) of different lipid compositions (DMPC, DMPG, POPC, POPC/DOPG) have been compared with classical membrane mimicking media such as detergent micelles, lipid/detergent bicelles and liposomes by their ability to support CF synthesis of IMPs in a folded and soluble state. Three model membrane proteins of different topology were used: homodimeric transmembrane (TM) domain of human receptor tyrosine kinase ErbB3 (TM-ErbB3, 1TM); voltage-sensing domain of K(+) channel KvAP (VSD, 4TM); and bacteriorhodopsin from Exiguobacterium sibiricum (ESR, 7TM). Structural and/or functional properties of the synthesized proteins were analyzed. LPNs significantly enhanced synthesis of the IMPs in a soluble form regardless of the lipid composition. A partial disintegration of LPNs composed of unsaturated lipids was observed upon co-translational IMP incorporation. Contrary to detergents the nanodiscs resulted in the synthesis of ~80% active ESR and promoted correct folding of the TM-ErbB3. None of the tested membrane mimetics supported CF synthesis of correctly folded VSD, and the protocol of the domain refolding was developed. The use of LPNs appears to be the most promising approach to CF production of IMPs in a folded state. NMR analysis of (15)N-Ile-TM-ErbB3 co-translationally incorporated into LPNs shows the great prospects of this membrane mimetics for structural studies of IMPs produced by CF systems.

The membrane-active, cationic, β-hairpin peptide, arenicin, isolated from marine polychaeta Arenicola marina exhibits a broad spectrum of antimicrobial activity. The peptide in aqueous solution adopts the significantly twisted β-hairpin conformation without pronounced amphipathicity. To assess the mechanism of arenicin action, the spatial structure and backbone dynamics of the peptide in membrane-mimicking media and its pore-forming activity in planar lipid bilayers were studied. The spatial structure of the asymmetric arenicin dimer stabilized by parallel association of N-terminal strands of two β-hairpins was determined using triple-resonance nuclear magnetic resonance (NMR) spectroscopy in dodecylphosphocholine (DPC) micelles. Interaction of arenicin with micelles and its oligomerization significantly decreased the right-handed twist of the β-hairpin, increased its amphipathicity, and led to stabilization of the peptide backbone on a picosecond to nanosecond time scale. Relaxation enhancement induced by water-soluble (Mn(2+)) and lipid-soluble (16-doxylstearate) paramagnetic probes pointed to the dimer transmembrane arrangement. Qualitative NMR and circular dichroism study of arenicin-2 in mixed DPC/1,2-dioleoyl-sn-glycero-3-phosphoglycerol bicelles, sodium dodecyl sulfate micelles, and lipid vesicles confirmed that a similar dimeric assembly of the peptide was retained in membrane-mimicking systems containing negatively charged lipids and detergents. Arenicin-induced conductance was dependent on the lipid composition of the membrane. Arenicin low-conductivity pores were detected in the phosphatidylethanolamine-containing lipid mixture, whereas the high-conductivity pores were observed in an exclusively anionic lipid system. The measured conductivity levels agreed with the model in which arenicin antimicrobial activity was mediated by the formation of toroidal pores assembled of two, three, or four β-structural peptide dimers and lipid molecules. The structural transitions involved in arenicin membrane-disruptive action are discussed.

The structure and dynamics of the isolated voltage-sensing domain (VSD) of the archaeal potassium channel KvAP was studied by high-resolution NMR. The almost complete backbone resonance assignment and partial side-chain assignment of the (2)H,(13)C,(15)N-labeled VSD were obtained for the protein domain solubilized in DPC/LDAO (2:1) mixed micelles. Secondary and tertiary structures of the VSD were characterized using secondary chemical shifts and NOE contacts. These data indicate that the spatial structure of the VSD solubilized in micelles corresponds to the structure of the domain in an open state of the channel. NOE contacts and secondary chemical shifts of amide protons indicate the presence of tightly bound water molecule as well as hydrogen bond formation involving an interhelical salt bridge (Asp62-R133) that stabilizes the overall structure of the domain. The backbone dynamics of the VSD was studied using (15)N relaxation measurements. The loop regions S1-S2 and S2-S3 were found mobile, while the S3-S4 loop (voltage-sensor paddle) was found stable at the ps-ns time scale. The moieties of S1, S2, S3, and S4 helices sharing interhelical contacts (at the level of the Asp62-R133 salt bridge) were observed in conformational exchange on the micros-ms time scale. Similar exchange-induced broadening of characteristic resonances was observed for the VSD solubilized in the membrane of lipid-protein nanodiscs composed of DMPC, DMPG, and POPC/DOPG lipids. Apparently, the observed interhelical motions represent an inherent property of the VSD of the KvAP channel and can play an important role in the voltage gating.