Crystallographic analysis of Bacillus subtilis CsaA

Bacillus subtilis CsaA (BsCsaA) has been proposed to act as a protein-secretion chaperone in the Sec-dependent translocation pathway, possibly compensating for the lack of SecB in the Gram-positive eubacterium Bacillus subtilis. This paper presents the cloning, purification, crystallization and structures of BsCsaA in two space groups (P42(1)2 and P3(2)21) solved and refined to resolutions of 1.9 and 2.0 angstrom, respectively. These structures complement the previously available crystal structure of CsaA from the Gram-negative eubacterium Thermus thermophilus (TtCsaA) and provide a direct structural basis for the interpretation of previously available biochemical data on BsCsaA. The sequence and structure of the proposed substrate-binding pocket are analyzed and discussed. A comparison with the TtCsaA structure reveals a different pattern of electrostatic potential in the vicinity of the binding site, which overlaps with a region of high sequence variability. In addition, the dimerization interface of this homodimeric protein is analyzed and discussed.