The transmembrane homotrimer of ADAM 1 in model lipid bilayers

A conserved tetrameric interaction of cry toxin helix α3 suggests a functional role for toxin oligomerization

Crystal (Cry) toxins are widely used for insect control, but their mechanism of toxicity is still uncertain. These toxins can form lytic pores in vitro, and water soluble tetrameric pre-pore intermediates have been reported. Even the precise oligomeric state of the toxin in membranes, trimeric or tetrameric, is still a debated issue. Based on previous reports, we have assumed that interactions between toxin monomers in solution are at least partly mediated by domain I, and we have analyzed in silico the homo-oligomerization tendencies of the domain I α-helices individually. Using many homologous sequences for each α-helix, our strategy allows selection of evolutionarily conserved interactions. These interactions appeared only in helices α3 and α5, but only α3 produced a suitably oriented or α-helical sample in lipid bilayers, forming homotetramers in C14-betaine, and allowing determination of its rotational orientation in lipid bilayers using site-specific infrared dichroism (SSID). The determined orientation in the tetrameric model is in agreement with only one of the evolutionarily conserved models. In addition mutation R99E, which was found to inhibit oligomerization experimentally, greatly destabilized the tetramer in molecular dynamic simulations. In this model, helix 3 is able to form inter-monomer interactions without significant rearrangements of domain I, which is compatible with the available crystal structure of Cry toxins in solution. The model presented here at least partially explains the reported tetrameric oligomerization of Cry toxins in solution and the inhibition of this oligomerization by a synthetic α3 peptide.

Design of transmembrane peptides: coping with sticky situations

Membrane proteins have central roles in cellular processes ranging from nutrient uptake to cell-cell communication, and are key drug targets. However, research on α-helical integral membrane proteins is in its relative infancy vs. water-soluble proteins, largely because of their water insolubility when extracted from their native membrane environment. Peptides with sequences that correspond to the membrane-spanning segments of α-helical integral membrane proteins, termed transmembrane (TM) peptides, provide valuable tools for the characterization of these molecules. Here we describe in detail protocols for the design of TM peptides from the sequences of natural α-helical integral membrane proteins and outline strategies for their synthesis and for improving their solubility properties.

Structure and ion channel activity of the human respiratory syncytial virus (hRSV) small hydrophobic protein transmembrane domain

The small hydrophobic (SH) protein from the human respiratory syncytial virus (hRSV) is a glycoprotein of approximately 64 amino acids with one putative alpha-helical transmembrane domain. Although SH protein is important for viral infectivity, its exact role during viral infection is not clear. Herein, we have studied the secondary structure, orientation, and oligomerization of the transmembrane domain of SH (SH-TM) in the presence of lipid bilayers. Only one oligomer, a pentamer, was observed in PFO-PAGE. Using polarized attenuated total reflection-Fourier transform infrared (PATR-FTIR) spectroscopy, we show that the SH-TM is alpha-helical. The rotational orientation of SH-TM was determined by site-specific infrared dichroism (SSID) at two consecutive isotopically labeled residues. This orientation is consistent with that of an evolutionary conserved pentameric model obtained from a global search protocol using 13 homologous sequences of RSV. Conductance studies of SH-TM indicate ion channel activity, which is cation selective, and inactive below the predicted pK(a) of histidine. Thus, our results provide experimental evidence that the transmembrane domain of SH protein forms pentameric alpha-helical bundles that form cation-selective ion channels in planar lipid bilayers. We provide a model for this pore, which should be useful in mutagenesis studies to elucidate its role during the virus cycle.