Protein cages that readily encapsulate active enzymes of interest present useful nanotools for delivery and catalysis, wherein those with programmable disassembly characteristics serve as particularly attractive platforms. Here we establish a general guest packaging system based on an artificial protein cage, TRAP-cage, the disassembly of which can be induced by the addition of reducing agents. In this system, TRAP-cage with SpyCatcher moieties in the lumen was prepared using genetic modification of the protein building block and assembled into a cage structure with either monovalent gold ions or molecular crosslinkers. The resulting protein cage can efficiently capture guest proteins equipped with a SpyTag by simply mixing them in aqueous solution. This post-assembly loading system which circumvents the exposure of guests to thiol-reactive crosslinkers, enables packaging of enzymes possessing a catalytic cysteine or a metal cofactor while retaining their catalytic activity.