Dynamic assembly of pentamer-based protein nanotubes

The molecular mechanisms and the geometrical theory underlying the polymorphic behavior of protein cages provide a basis for designing ones with the desired morphology and assembly properties. We show here that a circularly permuted cage-forming enzyme can controllably assemble into a variety of hollow spherical and cylindrical structures composed entirely of pentamers. A dramatic cage-to-tube transformation is facilitated by an untethered alpha-helix domain that prevents the 3-fold symmetry interaction and imparts a torsion between the building blocks. The unique double- and triple-stranded helical arrangements of subunits are mathematically optimal tiling patterns for this type of pentamer. These structural insights afford guidelines for the design of customized protein nanotubes for smart delivery and nanoreactor systems.