We demonstrate a method for generating discretely structured protein nanotubes from the simple ring-shaped building block, homohexameric Hcp1 from Pseudomonas aeruginosa. Our design exploited the observation that the crystal lattice of Hcp1 contains rings stacked in a repeating head-to-tail pattern. High-resolution detail of the ring-ring interface allowed the selection of sites for specific cysteine mutations capable of engaging in disulfide bond formation across rings, thereby generating stable Hcp1 nanotubes. Protein nanotubes containing up to 25 subunits ( approximately 100 nm in length) were self-assembled under simple conditions. Furthermore, we demonstrate that the tube ends and interior can be independently and specifically functionalized to generate nanocapsules.