Protein cages are hollow protein nanoparticles, such as chaperonins, viral capsids, and ferritins. They represent intriguing self-assembled nanostructures that can be precisely modified on their inner and outer surface by chemical conjugation reactions and by genetic engineering. Moreover, they are monodisperse in size. Therefore, they have tremendous potential for many nanotechnological applications. We have developed the archaeal chaperonin thermosome (THS) into a platform for a multitude of applications, ranging from force-responsive fluorescent sensors for self-reporting materials, to nanoreactors  and drug-delivery vehicles. Compared to other protein cages, the THS offers the advantage of having pores that are large enough to allow macromolecules and nanoparticles to enter and leave the cage, so that the cage can easily load or release these objects. For example, polymers can be synthesized within or conjugated into the protein cage. Conjugates of THS and polymers allow to bind nucleic acids into the THS, thus converting it into an effective  delivery vehicle for small interferring RNA (siRNA), or they can act as anchor for the templated formation of gold nanoparticles.