Structural analysis of A-protein of cucumber green mottle mosaic virus and tobacco mosaic virus by synchrotron small-angle X-ray scattering.

The size and shape of A-protein of tobacco mosaic virus coat protein (TMVP) and cucumber green mottle mosaic virus coat protein (CGMMVP) were evaluated by means of small-angle X-ray scattering (SAXS) using a synchrotron radiation source, complemented by electron microscopic observations. The results imply that TMV and CGMMV A-proteins are composed of three and two subunits, respectively, stacked in the shape of an isosceles triangular prism at lower ionic strength. Considering the difference of the A-protein structure at higher and lower ionic strength, the globular core structure was proposed as a subunit which might be modeled as a thin isosceles triangular prism composed of four globular cores joined by rather flexible segments. These cores correspond probably to four helical regions in a subunit, and rearrange their relative positions according to the external conditions. A slight rearrangement of core positions in a subunit may result in the formation of A-proteins of various shapes.