title: Structure of the stacked disk aggregate of tobacco mosaic virus protein.
abstract: The coat protein of tobacco mosaic virus is known to form three different classes of aggregate, depending on environmental conditions, namely helical, disk, and A-protein. Among the disk aggregates, there are four-layer, six-layer, and long stacks, which can be obtained by varying the ionic strength and temperature conditions during the association process. The four-layer aggregate has been crystallized, and its structure solved to atomic resolution. The stacked disk aggregate had been presumed to be built of a polar two-layer disk related to the crystalline A and B rings. A study using monoclonal antibodies specific to the bottom surface of TMV protein demonstrated that the stacked disk aggregate is bipolar, and suggested that the repeating two-layer unit might be similar to the dihedrally symmetrical A-ring pair in the disk crystal. In this paper we present a three-dimensional reconstruction of the stacked disk aggregate obtained by electron microscopy of ice-embedded samples. After modeling of the structure, we found the ring pairs to have the same quaternary structure as the A-ring pair of the four-layer aggregate. The resolution achieved in the image processing of the electron micrographs is on the order of 9 A in the meridional direction and 12 A in the equatorial. The identification of the structure of the stacked disk with the A-ring pair of the disk crystal provides an explanation of the observation that the axial periodicity of the disk pair, which is approximately 53 A when fully hydrated, can shrink to approximately 43 A in the dry state.
authors.affiliation: Institute of Molecular Biophysics, Florida State University, Tallahassee 32306, USA. diaz@sb.fsu.edu
authors.forename: R
authors.initials: R
authors.lastname: Díaz-Avalos
authors.forename: D L
authors.initials: DL
authors.lastname: Caspar
mesh_terms: D000465:Algorithms; D002213:Capsid / Q000737:chemistry / Q000302:isolation & purification / Q000648:ultrastructure*; D007053:Ice; D007091:Image Processing, Computer-Assisted; D008854:Microscopy, Electron / Q000379:methods; D008958:Models, Molecular; D010947:Plants, Toxic; D017433:Protein Structure, Secondary; D014026:Nicotiana / Q000821:virology; D014027:Tobacco Mosaic Virus / Q000737:chemistry / Q000648:ultrastructure*
chemical_list: D007053:Ice