The transfer of electrons through protein complexes is central to cellular respiration. Exploiting proteins for charge transfer in a controllable fashion has the potential to revolutionize the integration of biological systems and electronic devices. Here we characterize the structure of an ultrastable protein filament and engineer the filament subunits to create electronically conductive nanowires under aqueous conditions. Cryoelectron microscopy was used to resolve the helical structure of gamma-prefoldin, a filamentous protein from a hyperthermophilic archaeon. Conjugation of tetra-heme c3-type cytochromes along the longitudinal axis of the filament created nanowires capable of long-range electron transfer. Electrochemical transport measurements indicated networks of the nanowires capable of conducting current between electrodes at the redox potential of the cytochromes. Functionalization of these highly engineerable nanowires with other molecules, such as redox enzymes, may be useful for bioelectronic applications.