Calcium ions modulate the mechanics of tomato bushy stunt virus.

Viral particles are endowed with physicochemical properties whose modulation confers certain metastability to their structures to fulfill each task of the viral cycle. Here, we investigate the effects of swelling and ion depletion on the mechanical stability of individual tomato bushy stunt virus nanoparticles (TBSV-NPs). Our experiments show that calcium ions modulate the mechanics of the capsid: the sequestration of calcium ions from the intracapsid binding sites reduces rigidity and resilience in ∼24% and 40%, respectively. Interestingly, mechanical deformations performed on native TBSV-NPs induce an analogous result. In addition, TBSV-NPs do not show capsomeric vacancies after surpassing the elastic limit. We hypothesize that even though there are breakages among neighboring capsomers, RNA-capsid protein interaction prevents the release of capsid subunits. This work shows the mechanical role of calcium ions in viral shell stability and identifies TBSV-NPs as malleable platforms based on protein cages for cargo transportation at the nanoscale.