Geminate structures of African cassava mosaic virus.

Two types of geminate structures were purified from African cassava mosaic geminivirus (ACMV)-infected Nicotiana benthamiana plants and analyzed by electron cryomicroscopy and image reconstruction. After cesium sulfate density gradient centrifugation, they were separated into lighter top (T) and heavier bottom (B) components. T particles comigrated with host proteins, whereas B particles were concentrated in a cesium density typical for complete virions. Both particles were composed of two incomplete icosahedra of 11 capsomers each, but T particles were slightly larger (diameter, 22.5 nm) and less dense in the interior than B particles (diameter, 21.5 nm). T particles were frequently associated with small globules of approximately 14 nm diameter of unknown origin. The overall structure of ACMV, a begomovirus transmitted by whiteflies, was similar to that of Maize streak virus (MSV), a mastrevirus transmitted by leafhoppers, although the vertices of the icosahedra were less pronounced. Models of ACMV coat proteins based on Satellite tobacco necrosis virus support the exposure of parts of the molecule essential for transmission specificity by whiteflies and provide possible structural explanations for the smaller protrusion of the ACMV capsid relative to MSV. The differences of ACMV and MSV virion shapes are discussed with reference to their different animal vectors.