In this work, we provide evidence of a mechanism to reinforce the strength of an icosahedral virus by using its genomic DNA as a structural element. The mechanical properties of individual empty capsids and DNA-containing virions of the minute virus of mice are investigated by using atomic force microscopy. The stiffness of the empty capsid is found to be isotropic. Remarkably, the presence of the DNA inside the virion leads to an anisotropic reinforcement of the virus stiffness by ≈3%, 40%, and 140% along the fivefold, threefold, and twofold symmetry axes, respectively. A finite element model of the virus indicates that this anisotropic mechanical reinforcement is due to DNA stretches bound to 60 concavities of the capsid. These results, together with evidence of biologically relevant conformational rearrangements of the capsid around pores located at the fivefold symmetry axes, suggest that the bound DNA may reinforce the overall stiffness of the viral particle without canceling the conformational changes needed for its infectivity.

Neutralizing monoclonal antibodies (nMAbs) elicited against foot-and-mouth disease virus (FMDV) of serotype C were assayed with field isolates and variant FMDVs using several immunoassays. Of a total of 36 nMAbs tested, 23 recognized capsid protein VP1 and distinguished at least 13 virion conformation-independent epitopes involved in neutralization of FMDV C. Eleven epitopes of FMDV C-S8c1 have been located in segments 138–156 or 192–209 of VP1 by quantifying the reactivity of nMAbs with synthetic peptides and with nMAb-resistant mutants of FMDV C-S8c1 carrying defined amino acid substitutions. The main antigenic site of FMDV C-S8c1 (VP1 residues 138 to 150) consists of multiple (at least 10), distinguishable, overlapping epitopes. Some amino acid replacements abolished one of the epitopes, whereas other replacements affected several epitopes in this region. The conservative substitution His(146) → Arg, found in many nMAb-resistant mutants analysed, abolished the reactivity of the virus with all nMAbs that recognized epitopes in the main antigenic site of FMDV C-S8c1. This indicates that a minimum genetic change can result in a highly amplified phenotypic effect, as regards the antigenicity of FMDV.