This fold comprises 4, 6, 7 or 8 antiparallel beta sheets with a radial arrangement in space, giving the overall structure the appearance of a propellor. The beta sheets pack face to face, and form a closed structure, in which each beta sheet packs against two neighbours. The sheets have a mean twist and orientation that depends on the number of beta sheets that form the propellor. The beta sheets have the same topology, i.e. the propellor blades are structurally similar units (four antiparallel strands with +1, +1, +1 connectivity). An example of a six-bladed beta propellor is neuraminidase (CATH domain ID 1nscA00).
Structural principles for the propeller assembly of beta-sheets: the preference for seven-fold symmetry.