Crystallization and preliminary X-ray analysis of the FliH-FliI complex responsible for bacterial flagellar type III protein export

The bacterial flagellar proteins are translocated into the central channel of the flagellum by a specific protein-export apparatus for self-assembly at the distal growing end. FliH and FliI are soluble components of the export apparatus and form an FliH₂-FliI heterotrimer in the cytoplasm. FliI is an ATPase and the FliH₂-FliI complex delivers export substrates from the cytoplasm to an export gate made up of six integral membrane proteins of the export apparatus. In this study, an FliH_C fragment consisting of residues 99-­235 was co-purified with FliI and the FliH_C2-FliI complex was crystallized. Crystals were obtained using the hanging-drop vapour-diffusion technique with PEG 400 as a precipitant. The crystals belonged to the orthorhombic space group P2₁2₁2₁, with unit-cell parameters a = 133.7, b = 147.3, c = 164.2 Å, and diffracted to 3.0 Å resolution.