Provided herein are methods and compositions for controlling assembly of modified viral core proteins, for example, into a viral capsid or a nanocage. In some embodiments, the disclosed modified viral core proteins comprise at least one mutation or modification that can substantially prevent assembly of the viral core proteins until assembly is desired. In some embodiments, assembly of the viral core proteins may be triggered, for example, by contacting the viral core proteins with a reducing agent and/or by reducing the concentration of a denaturant. The viral core proteins may self-assemble to form a viral capsid or nanocage.