Nature has provided us with a range of reactive nanoscale platforms, in the form of protein cage architectures such as viral capsids and the cages of ferritin-like proteins. Protein cage architectures have clearly demarcated exterior, interior, and interface surfaces consisting of precisely located chemical functionalities. In the present work, we demonstrate that the small heat shock protein (MjHsp) cage from Methanococcus jannaschii is a new and versatile nanoscale platform whose exterior and interior surfaces are amenable to both genetic and chemical modification. Wild type and genetic mutants of the Hsp cage are shown to react with activated fluorescein molecules in a site specific manner. In addition, the 12 nm Hsp cage serves as a size constrained reaction vessel for the oxidative mineralization of iron, resulting in the formation of monodispersed 9 nm iron oxide nanoparticles. These results demonstrate the utility of the Hsp cage to serve as a nanoscale platform for the synthesis of both soft (orga...