Crystallization and preliminary X-ray analysis of the C-terminal cytoplasmic domain of FlhA, a membrane-protein subunit of the bacterial flagellar type III protein-export apparatus.

The axial components of the bacterial flagellum and the scaffolding proteins for its assembly are exported through the flagellar-specific type III protein-export apparatus, which is believed to be located on the cytoplasmic surface of the basal body. FlhA is an essential component of the type III export apparatus of Salmonella and consists of two major portions: an N-terminal transmembrane domain and a C-terminal cytoplasmic domain (FlhAC). FlhAC and a 38 kDa fragment of FlhAC (FlhAC38K) were purified and crystallized. The crystals were obtained by the sitting-drop vapour-diffusion technique with PEG 8000 as a precipitant. FlhAC crystals grew in the tetragonal space group I4(1)/I4(3), with unit-cell parameters a = b = 216.6, c = 65.0 A. FlhAC38K was crystallized in an orthorhombic form, with unit-cell parameters a = 53.0, b = 93.1, c = 186.5 A. X-ray diffraction data from crystals of FlhAC and the SeMet derivative of FlhAC were collected to 2.9 and 3.2 A, respectively.