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Significance Despite considerable advances in de novo protein design in recent years, it still remains challenging to engineer proteins with large internal cavities that can be functionalized to become biotechnological tools, such as specific binders, sensors, or catalysts. In this work, we outline a computational strategy to combine multiple de novo designed domains into symmetric protein assemblies that enclose large internal chambers. The high stability of de novo scaffolds enables ready functionalization of these chambers; for instance, with specific metal-binding sites, as demonstrated here by generating a lanthanide-binding protein with ultra-high affinity., De novo protein design has succeeded in generating a large variety of globular proteins, but the construction of protein scaffolds with cavities that could accommodate large signaling molecules, cofactors, and substrates remains an outstanding challenge. The long, often flexible loops that form such cavities in many natural proteins are difficult to precisely program and thus challenging for computational protein design. Here we describe an alternative approach to this problem. We fused two stable proteins with C2 symmetry—a de novo designed dimeric ferredoxin fold and a de novo designed TIM barrel—such that their symmetry axes are aligned to create scaffolds with large cavities that can serve as binding pockets or enzymatic reaction chambers. The crystal structures of two such designs confirm the presence of a 420 cubic Ångström chamber defined by the top of the designed TIM barrel and the bottom of the ferredoxin dimer. We functionalized the scaffold by installing a metal-binding site consisting of four glutamate residues close to the symmetry axis. The protein binds lanthanide ions with very high affinity as demonstrated by tryptophan-enhanced terbium luminescence. This approach can be extended to other metals and cofactors, making this scaffold a modular platform for the design of binding proteins and biocatalysts.