The present invention features, inter alia, compositions and methods for the treatment of cancer and infectious disease. The compositions include engineered proteins that specifically bind a metal chelate and may be bispecific. For example, the engineered proteins may bind (a) a target (e.g., a cellular protein) on a cancerous cell or a pathogen and (b) a metal chelate comprising DOTA, or an active variant thereof, and a metal ion such as a radionuclide. By virtue of the multiple binding sites, the engineered protein effectively delivers a metal chelate to a cell one wishes to destroy.