The co-self-assembly of proteins and nucleic acids (NAs) produces complex biomolecular machines (e.g., ribosomes and telomerases) that represent some of the most daunting targets for biomolecular design. Despite significant advances in protein and DNA or RNA nanotechnology, the construction of artificial nucleoprotein complexes has largely been limited to cases that rely on the NA-mediated spatial organization of protein units, rather than a cooperative interplay between protein- and NA-mediated interactions that typify natural nucleoprotein assemblies. We report here a structurally well-defined synthetic nucleoprotein assembly that forms through the synergy of three types of intermolecular interactions: Watson-Crick base pairing, NA-protein interactions, and protein-metal coordination. The fine thermodynamic balance between these interactions enables the formation of a crystalline architecture under highly specific conditions.