Two Case Studies where Electron Cryotomography has Revealed Mechanism: ESCRT and the Type VI Secretion System

Abstract

Sometimes all that is really needed to distinguish between competing models is a few really good pictures. Electron cryotomography has increasingly met that need by delivering unprecedented 3-D images of intact cells in a near-native (frozen-hydrated) state to “macromolecular” resolution. I will present two case studies. In the first, cryotomograms of dividing Sulfolobus acidocaldarius cells revealed a thin layer of material at the leading edge of ingressing membranes whose structure and development point uniquely to an “hourglass” model for how the ESCRT machinery drives membrane scission. In the second, cryotomograms of Vibrio cholerae cells revealed extended cytoplasmic tubes in two different conformational states. Identification of the tubes as Type VI Secretion Systems led immediately to a “spring-loaded dagger” model for their ability to deposit toxins into neighboring cells.