While in vitro protein crystallization has been studied for decades to facilitate the production of crystals for structural analysis, ubiquitous insect viruses have evolved proteins that readily crystallize in vivo despite the complexity of the cellular environment. Such viral crystals are among the most striking examples of protein self-assembly, leading to the formation of ultra-stable microcrystals filling most of the infected cells.
These crystals were initially characterised as infectious micro-capsules that package up to hundreds of virus particles. The crystalline matrix forms an armour that allows the virus to persist for years in the environment like bacterial spores. In addition, insect poxviruses also produce a second type of in vivo crystals called spindles that do not contain virus particles but strongly enhance the virulence of these pathogens. Interestingly, administration of spindles can also potentiate the effect of unrelated viruses and toxins.
Over the last few years, we have developed X-ray microcrystallography methods allowing structure determination of these different in vivo crystals directly purified from infected insects. This presentation will compare the molecular organisation and specific characteristics of the four classes of viral, in vivo crystals known to date and present our attempts to engineer in vivo crystals as antigen-delivery vehicles for vaccination.