Cellvibrio japonicus arabinanase Arb43A hydrolyzes the alpha-1,5-linked L-arabinofuranoside backbone of plant cell wall arabinans. The three-dimensional structure of Arb43A, determined at 1.9 A resolution, reveals a five-bladed beta-propeller fold. Arb43A is the first enzyme known to display this topology. A long V-shaped surface groove, partially enclosed at one end, forms a single extended substrate-binding surface across the face of the propeller. Three carboxylates deep in the active site groove provide the general acid and base components for glycosidic bond hydrolysis with inversion of anomeric configuration.