Force Spectroscopy reveals mechanical structure of capsids

Abstract

Force spectroscopy measurements are effectively used in the research of single-molecules. The tools and methods of this technique can also be used for the investigation of complex protein assemblies such as viral capsids. Viral capsids need to encapsidate their genome to protect it from external influences. Therefore they need to with-stand considerable forces from the outside, but also from the inside, as the genome is often packaged at near crystalline densities. To get a better understanding of the mechanics of these nanocontainers, force spectroscopy is an ideal tool to obtain insight into their mechanical structure. Force spectroscopy on viruses can be preformed by indenting viral capsids with an atomic force microscope (AFM). Unexpectedly, most experimental studies on the deformation of empty viral capsids demonstrate a linear relation between indentation depth and force. For thin shells this might be expected for deformation in the order of the thickness of the shell but experimentally a similar behavior has been shown to exist for thick shells. Numerical simulations guided by the Foppl- von Karman (FvK) number (a dimensionless number relating the “in-plane” elasticity of the shell to its “out-of-plane” bending rigidity) have been able to explain these results for shells with either small (>150) or large (<∼500) FvK numbers. For shells with an FvK number between those values a non-linear response is expected. Here we report nanoindentation experiments on different viral capsids to investigate this elastic deformation. Moreover, we explore when elastic deformation fails resulting in capsids deformation and collapse.