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 (FlhA_C). FlhA_C and a 38 kDa fragment of FlhA_C (FlhA_C38K) were purified and crystallized. The crystals were obtained by the sitting-drop vapour-diffusion technique with PEG 8000 as a precipitant. FlhA_C crystals grew in the tetragonal space group I4₁/I4₃, with unit-cell parameters a = b = 216.6, c = 65.0 Å. FlhA_C38K was crystallized in an orthorhombic form, with unit-cell parameters a = 53.0, b = 93.1, c = 186.5 Å. X-­ray diffraction data from crystals of FlhA_C and the SeMet derivative of FlhA_C were collected to 2.9 and 3.2 Å, respectively.