Protein ORIGAMI: A program for the creation of 3D paper models of folded peptides.

Protein ORIGAMI (http://ibg.kit.edu/protein_origami) is a browser-based web application that allows the user to create straightforward 3D paper models of folded peptides for research, teaching and presentations. An amino acid sequence can be turned into α-helices, ß-strands and random coils that can be printed out and folded into properly scaled models, with a color code denoting the biophysical characteristics of each amino acid residue (hydrophobicity, charge, etc.). These models provide an intuitive visual and tactile understanding of peptide interactions with other partners, such as helix-helix assembly, oligomerization, membrane binding, or pore formation. Helices can also be displayed as a helical wheel or helical mesh in 2D graphics, to be used in publications or presentations. The highly versatile programme Protein ORIGAMI is also suited to create less conventional helices with arbitrary pitch (e.g., 310 -helix, π-helix, or left-handed helices). Noncanonical amino acids, labels and different terminal modifications can be defined and displayed at will, and different protonation states can be shown. In addition to the web application, the program source code can be downloaded and installed locally on a PC. The printed paper models can be readily used for daily research and discussions, just as for educational purposes and teaching. © 2018 by The International Union of Biochemistry and Molecular Biology, 46:403-409, 2018.

Folding and self-assembly of the TatA translocation pore based on a charge zipper mechanism.

We propose a concept for the folding and self-assembly of the pore-forming TatA complex from the Twin-arginine translocase and of other membrane proteins based on electrostatic "charge zippers." Each subunit of TatA consists of a transmembrane segment, an amphiphilic helix (APH), and a C-terminal densely charged region (DCR). The sequence of charges in the DCR is complementary to the charge pattern on the APH, suggesting that the protein can be "zipped up" by a ladder of seven salt bridges. The length of the resulting hairpin matches the lipid bilayer thickness, hence a transmembrane pore could self-assemble via intra- and intermolecular salt bridges. The steric feasibility was rationalized by molecular dynamics simulations, and experimental evidence was obtained by monitoring the monomer-oligomer equilibrium of specific charge mutants. Similar "charge zippers" are proposed for other membrane-associated proteins, e.g., the biofilm-inducing peptide TisB, the human antimicrobial peptide dermcidin, and the pestiviral E(RNS) protein.

Peptide-lipid interactions of the stress-response peptide TisB that induces bacterial persistence.

The bacterial stress-response peptide TisB in Escherichia coli has been suggested to dissipate the transmembrane potential, such that the depletion of ATP levels induces the formation of dormant persister cells which can eventually form biofilms. We studied the structure and membrane interactions of TisB to find out whether it forms pores or other proton-selective channels. Circular dichroism revealed an amphiphilic α-helical structure when reconstituted in lipid vesicles, and oriented circular dichroism showed that the helix assumes a transmembrane alignment. The addition of TisB to dye-loaded vesicles caused leakage only at very high peptide concentration, notably with a Hill coefficient of 2, which suggests that dimers must be involved. Coarse-grained molecular dynamics simulations showed that membrane binding of monomeric TisB is rapid and spontaneous, and transmembrane insertion is energetically feasible. When TisB oligomers are assembled as transmembrane pores, these channels collapse during the simulations, but transmembrane dimers are found to be stable. Given the pattern of charges on the amphiphilic TisB helix, we postulate that antiparallel dimers could be assembled via a ladder of salt bridges. This electrostatic charge-zipper could enable protons to pass along a wire of trapped water molecules across the hydrophobic membrane.