Structure of the stacked disk aggregate of tobacco mosaic virus protein

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.