a b s t r a c tSome pathogenic mutations associated with Alzheimer's disease are thought to affect structuraldynamic properties and the lateral dimerization of amyloid precursor protein (APP) in neuron membrane. Dimeric structure of APP transmembrane fragment Gln 686 -Lys 726 was determined in membrane-mimicking dodecylphosphocholine micelles using high-resolution NMR spectroscopy.The APP membrane-spanning a-helix Lys 699 -Lys 724 self-associates in a left-handed parallel dimer through extended heptad repeat motif I 702 X 3 M 706 X 2 G 709 X 3 A 713 X 2 I 716 X 3 I 720 X 2 I 723 , whereas the juxtamembrane region Gln 686 -Val 695 constitutes the nascent helix, also sensing the dimerization. The dimerization mechanism of APP transmembrane domain has been described at atomic resolution for the first time and is important for understanding molecular events of APP sequential proteolytical cleavage resulting in amyloid-b peptide. Structured summary of protein interactions:APPjmtm and APPjmtm bind by comigration in gel electrophoresis (View interaction) APPjmtm and APPjmtm bind by nuclear magnetic resonance (View interaction).

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Antimicrobial peptides (AMPs) are ubiquitous agents that play a crucial role in the host defense systems of bacteria, fungi, plants and animals [1][2][3]. In higher vertebrates, these peptides work in synergy with the adaptive immune system and form the basis of 'so-called' innate immunity [2]. At the same time, AMPs of plants provide one of the major barriers for invading pathogens and significantly enhance the Cyclotides are a family of bioactive plant peptides that are characterized by a circular protein backbone and three conserved tightly packed disulfide bonds. The antimicrobial and hemolytic properties of cyclotides, along with the relative hydrophobicity of the peptides, point to the biological membrane as a target for cyclotides. To assess the membrane-induced conformation and orientation of cyclotides, the interaction of the Mo¨bius cyclotide, kalata B1, from the African perennial plant Oldenlandia affinis, with dodecylphosphocholine micelles was studied using NMR spectroscopy. Under conditions where the cyclotide formed a well-defined complex with micelles, the spatial structure of kalata B1 was calculated from NOE and J couplings data, and the model for the peptide-micelle complex was built using 5-and 16-doxylstearate relaxation probes. The binding of divalent cations to the peptide-micelle complex was quantified by Mn 2+ titration. The results show that the peptide binds to the micelle surface, with relatively high affinity, via two hydrophobic loops (loop 5, Trp19-Val21; and loop6, Leu27-Val29). The charged residues (Glu3 and Arg24), along with the cation-binding site (near Glu3) are segregated on the other side of the molecule and in contact with polar head groups of detergent. The spatial structure of kalata B1 is only slightly changed during incorporation into micelles and represents a distorted triple-stranded b-sheet cross-linked by a cystine knot. Detailed structural analysis and comparison with other knottins revealed structural conservation of the two-disulfide motif in cyclic and acyclic peptides. The results thus obtained provide the first model for interaction of cyclotides with membranes and permit consideration of the cyclotides as membrane-active cationic antimicrobial peptides.

Structural studies of membrane proteins are of great importance and interest, with solution and solid state NMR spectroscopy being very promising tools for that task. However, such investigations are hindered by a number of obstacles, and in the first place by the fact that membrane proteins need an adequate environment that models the cell membrane. One of the most widely used and prospective membrane mimetics is isotropic bicelles. While large anisotropic bicelles are well-studied, the field of small bicelles contains a lot of "white spots". The present work reports the radii of particles and concentration of the detergents in the monomeric state in solutions of isotropic bicelles, formed by 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO), and sodium cholate, as a function of lipid/detergent ratio and temperature. These parameters were measured using (1)H NMR diffusion spectroscopy for the bicelles composed of lipids with saturated fatty chains of different length and lipids, containing unsaturated fatty acid residue. The influence of a model transmembrane protein (membrane domain of rat TrkA) on the properties of bicelles and the effect of the bicelle size and composition on the properties of the transmembrane protein were investigated with heteronuclear NMR and nuclear Overhauser effect spectroscopy. We show that isotropic bicelles that are applicable for solution NMR spectroscopy behave as predicted by the theoretical models and are likely to be bicelles rather than mixed micelles. Using the obtained data, we propose a simple approach to control the size of bicelles at low concentrations. On the basis of our results, we compared different rim-forming agents and selected CHAPS as a detergent of choice for structural studies in bicelles, if the deuteration of the detergent is not required.