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 FliHC fragment consisting of residues 99-235 was co-purified with FliI and the FliHC2-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 P2(1)2(1)2(1), with unit-cell parameters a=133.7, b=147.3, c=164.2 Å, and diffracted to 3.0 Å resolution.