Crystallization and preliminary X-ray analysis of the periplasmic domain of FliP, an integral membrane component of the bacterial flagellar type III protein-export apparatus

The bacterial flagellar proteins are transported via a specific export apparatus to the distal end of the growing structure for their self-assembly. FliP is an essential membrane component of the export apparatus. FliP has an N-terminal signal peptide and is predicted to have four transmembrane (TM) helices and a periplasmic domain (FliP_P) between TM-2 and TM-3. In this study, FliP_P from Thermotoga maritima (TmFliP_P) and its selenomethionine derivative (SeMet-TmFliP_P) were purified and crystallized. TmFliP_P formed a homotetramer in solution. Crystals of TmFliP_P and SeMet-TmFliP_P were obtained by the hanging-drop vapour-diffusion technique with 2-methyl-2,4-pentanediol as a precipitant. These two crystals grew in the hexagonal space group P6₂22 or P6₄22, with unit-cell parameters a = b = 114.9, c = 193.8 Å. X-ray diffraction data were collected from crystals of TmFliP_P and SeMet-TmFliP_P to 2.4 and 2.8 Å resolution, respectively.