IMSS Seminar DATE: 2005-07-04 10:00 - 11:30 PLACE: 4-go-kan, 2F Rinko-shitsu TITLE: NEUTRON AND X-RAY SCATTERING STUDIES OF TOXIN ASSAULT ON MODEL LIPID MEMBRANE SPEAKER: Dr. J. Majewski  (Manuel Lujan Neutron Scattering Center, Los Alamos National Laboratory, USA) LANGUAGE: English URL: http://pfwww.kek.jp/pf-seminar/ ABSTRACT: Many bacterial toxins bind to and gain entrance to target cells through specific interactions with membrane components. One such example is cholera toxin (CTAB5), a pathologically active agent secreted by the bacterium Vibrio cholera. The toxin has an AB5 arrangement of subunits. Five identical B subunits (CTB5), each composed of 103 amino acids, form a pentameric ring with a vertical height of 3.2 nm and a radius of 3.1 nm CTB5 is responsible for binding the toxin to its cell-surface receptor, ganglioside GM1. The single A-unit is a disulfide-linked dimmer composed of an A1 and A2-subunit that is aligned through the central pore "doughnut hole" of the B5 subunit. It has been proposed that the A1 peptide crosses the cell membrane and reaches the cytoplasmic face. There, it interacts with integral membrane proteins, disrupting their normal function, resulting in a large efflux of water and ions from the cell (severe diarrhea). Although much is known about the structure and catalytic activity of cholera toxin, the mechanism by which cholera toxin crosses the plasma membrane remains unknown. Using neutron/x-ray reflectivity and x-ray grazing incidence diffraction (GID), we have characterized the structure of mixed DPPE:GM1 lipid monolayers before and during the binding of CTAB5 or its B subunit CTB5. Structural parameters such as the density and thickness of the lipid layer, extension of the GM1 oligosaccharide headgroup, and orientation and position of the protein upon binding are reported. Both CTAB5 and CTB5 were measured to have ~50% coverage when bound to the lipid monolayer. X-ray GID experiments show that both the lipid monolayer and the cholera toxin layer are crystalline. Back