A variant of CfaE, donor strand complemented CfaE (dscCfaE), containing a C-terminal hairpin linker followed by the first 19 amino acid residues derived from the N-terminus of the major fimbrial subunit CfaB was purified to homogeneity. The dscCfaE protein was readily crystallized and the structure was determined. The dscCfaE molecule consists of two domains of roughly equal size. The N-terminal domain of CfaE is referred to as the adhesin domain (CfaEad) and is represented in the structure from residues A23 to D200. It consists of one anti-parallel beta-sheet (Sheet 1) and one mixed beta-sheet (Sheet 2). The C-terminal domain immediately follows the short three-residue linker (K201-G202-N203) and mediates attachment of the adhesive subunit to the main body of the fimbria. It is therefore termed the pilin domain (CfaEpd). The pilin domain folds into a beta-sandwich with a topology reminiscent of the adhesin domain. Both CfaEad and CfaEpd beta-structures display a topology that resembles the v-type Ig fold with nine beta-strands. In order to understand how the major and minor subunits are assembled into a CFA/I fimbria, we further engineered the donor strand complemented CfaEB complex (dscCfaEB) construct, which was expressed in E. coli. The recombinant dscCfaEB protein was purified and crystallized. The crystal structure of the CfaEB complex was determined, providing structural information on not only the major subunit CfaB, but also the geometry of the connection between the major and minor subunit. In addition to the CfaEB complex, we also determined crystal structures for the major-major subunit complexes CfaBB and CfaBBB, providing a basis for constructing a model of CFA/I pilus consistent with EM reconstructions of purified CFA/I pilus. EM studies of ETEC strains observed two-distinct forms of CFA/I pili, a helical filament and an extended, unwound conformation. Modeling and corroborative mutational data indicate that proline isomerization is involved in the conversion between the helical and extended forms of CFA/I fimbriae. This also suggested the existence of a possible prolinyl isomerase activity for CfaC. Our findings affirm the strong structural similarities seen between Class 5 fimbriae and Class 1 pili in the absence of significant primary sequence similarity. They also suggest that morphological and biochemical differences between fimbrial types, regardless of class, provide structural specialization that facilitates survival of each bacterial pathotype in its preferred host microenvironment. Our model supports the notion that bacteria use antigenic variation to evade host immune response in that residues occupying the predicted surface-exposed face of CfaB and related Class 5 pilins show much higher genetic sequence variability than the remainder of the pilin protein. Located at the upper surface of CfaEad distal to the CfaEpd, R181, which was previously known to be important for binding, is found in a positively charged depression and surrounded by a cluster of residues that are highly conserved in the Class 5 fimbrial adhesins, including residues from three different loops (i.e., B'-C, D'-E, and F-G loops). This pocket thus appears to be a suitable location to which a negatively charged sialylated receptor might bind. To confirm the role of this domain, R67, which is adjacent to R181, was mutated to alanine (dscCfaE/R67A) and purified. Bead-adsorbed dscCfaE/R67A failed to agglutinate human erythrocytes, similar to our previous findings for the dscCfaE/R181A mutant. These results implicate the pocket anchored by these two residues as the putative receptor-binding domain. To determine the role in hemagglutination of individual residues in the neighborhood of R181, we introduced site-specific mutations into CfaE in the plasmid pMAM2, which encodes all components of the CFA/I and directs surface expression of mutant fimbriae with single site mutations of CfaE. Twelve such mutations involving residues that are either invariant (fully conserved) or are subclass-specific for Class 5 ETEC fimbrial adhesins were introduced. All positively charged residues (R181, R182, R67) are absolutely required for receptor binding and cluster together to form a positively charged center. The positively charged center of the binding pocket is surrounded by a band of subclass-specific residues. Mutations of those residues display altered interactions with red cells and several show discriminatory behavior to either human type-A or bovine red cell species. Recently, we have also determined the crystal structure of CfaA, the chaperone component that is essential for assembly of CFA/I fimbriae.

National Institute of Health (NIH)
National Cancer Institute (NCI)
Investigator-Initiated Intramural Research Projects (ZIA)
Project #
Application #
Study Section
Project Start
Project End
Budget Start
Budget End
Support Year
Fiscal Year
Total Cost
Indirect Cost
National Cancer Institute Division of Basic Sciences
Zip Code
Bao, Rui; Liu, Yang; Savarino, Stephen J et al. (2016) Off-pathway assembly of fimbria subunits is prevented by chaperone CfaA of CFA/I fimbriae from enterotoxigenic E. coli. Mol Microbiol 102:975-991
Bao, Rui; Esser, Lothar; Poole, Steven et al. (2014) Preliminary X-ray diffraction analysis of CfaA, a molecular chaperone essential for the assembly of CFA/I fimbriae of human enterotoxigenic Escherichia coli. Acta Crystallogr F Struct Biol Commun 70:196-9
Bao, Rui; Fordyce, April; Chen, Yu-Xing et al. (2014) Structure of CfaA suggests a new family of chaperones essential for assembly of class 5 fimbriae. PLoS Pathog 10:e1004316
Bao, Rui; Nair, Manoj K M; Tang, Wai-kwan et al. (2013) Structural basis for the specific recognition of dual receptors by the homopolymeric pH 6 antigen (Psa) fimbriae of Yersinia pestis. Proc Natl Acad Sci U S A 110:1065-70
Liu, Yang; Esser, Lothar; Interlandi, Gianluca et al. (2013) Tight conformational coupling between the domains of the enterotoxigenic Escherichia coli fimbrial adhesin CfaE regulates binding state transition. J Biol Chem 288:9993-10001
Bao, Rui; Esser, Lothar; Sadhukhan, Annapurna et al. (2012) Crystallization and preliminary X-ray diffraction analysis of PsaA, the adhesive pilin subunit that forms the pH 6 antigen on the surface of Yersinia pestis. Acta Crystallogr Sect F Struct Biol Cryst Commun 68:1243-6
Li, Yong Fu; Poole, Steven; Rasulova, Fatima et al. (2009) Crystallization and preliminary X-ray diffraction analyses of several forms of the CfaB major subunit of enterotoxigenic Escherichia coli CFA/I fimbriae. Acta Crystallogr Sect F Struct Biol Cryst Commun 65:242-7
Li, Yong-Fu; Poole, Steven; Nishio, Kazuya et al. (2009) Structure of CFA/I fimbriae from enterotoxigenic Escherichia coli. Proc Natl Acad Sci U S A 106:10793-8