A nanowire comprising a purified protein filament, such as a pilus, isolated from a bacterium, such as Geobacter sulfurreducens, is provided. Such a purified pilus can contain peptide subunits capable of assembling into the protein filament and establishing an electrical connection with an insoluble electron acceptor. The novel nanowires can be produced via a novel single step. Such nanowires are useful in applications requiring rectifying behavior.