Crystallization and preliminary X-ray diffraction analyses of several forms of the CfaB major subunit of enterotoxigenic Escherichia coli CFA/I fimbriae

Enterotoxigenic Escherichia coli (ETEC), a major global cause of diarrhea, initiates the pathogenic process via fimbriae-mediated attachment to the small intestinal epithelium. A common prototypic ETEC fimbria, colo­nization factor antigen I (CFA/I), consists of a tip-localized minor adhesive subunit CfaE and the stalk-forming major subunit CfaB, both of which are necessary for fimbrial assembly. To elucidate the structure of CFA/I at atomic resolution, three recombinant proteins were generated consisting of fusions of the minor and major subunits (CfaEB) and of two (CfaBB) and three (CfaBBB) repeats of the major subunit. Crystals of CfaEB diffracted X-rays to 2.1 A resolution and displayed the symmetry of space group P21. CfaBB exhibited a crystal diffraction limit of 2.3 A resolution and had the symmetry of space group P21212. CfaBBB crystallized in the monoclinic space group C2 and diffracted X-­rays to 2.3 A resolution. These structures were determined using the molecular-replacement method.