Comparison of properties of particles of Cucumber mosaic virus and Tomato aspermy virus based on the analysis of molecular surfaces of capsids.

The plant RNA viruses Cucumber mosaic virus (CMV) and Tomato aspermy virus (TAV) (genus Cucumovirus) have similar icosahedral particles, the crystal structures of which have been reported recently. Similarity in particle structure agrees with reports of stable capsids assembled from their capsid proteins and of viable recombinant viruses with chimeric capsid proteins derived from CMV and TAV. However, differences between the cucumoviruses have been reported for physicochemical properties. Here, structural and electrostatic features of the molecular surfaces are studied to investigate their relationship with these observations. Two coat-protein recombinants with structures modelled by taking CMV and TAV as templates were also included in the analysis. Results show that there exists an external region of negative electrostatic potential that has arisen from strictly conserved charged residues situated near the external HI loop of the subunits in the capsomers. This negative domain surrounds the fivefold and quasi-sixfold axes and locates above regions of positive potential that extend to cover, nearly homogeneously, the inner surface of capsids, where interaction with encapsidated RNA occurs. Differences between the outer electrostatic distributions in CMV and TAV explain the distinct response of both viruses to variations in physicochemical conditions required for particle stability and are essential to rationalize the biological activity of the coat-protein recombinants, in spite of their seemingly distinct electrostatic characteristics.