Self-assembled cage structures of nanometre dimensions can be used as constrained environments for the preparation of nanostructured materials(1,2) and the encapsulation of guest molecules(3), with potential applications in drug delivery(4) and catalysis(5). In synthetic systems the number of subunits contributing to cage structures is typically rather small(3,6). But the protein coats of viruses (virions) commonly comprise hundreds of subunits that self-assemble into a cage for transporting viral nucleic acids. Many virions, moreover, can undergo reversible structural changes that open or close gated pores to allow switchable access to their interior(7). Here we show that such a virion - that of the cowpea chlorotic mottle virus - can be used as a host for the synthesis of materials. We report the mineralization of two polyoxometalate species (paratungstate and decavanadate) and the encapsulation of an anionic polymer inside this virion, controlled by pH-dependent gating of the virion＇s pores. The diversity in size and shape of such virus particles make this a versatile strategy for materials synthesis and molecular entrapment.