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Structural Studies of Fimbriae of Enterotoxigenic E. Coli (ETEC)


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. 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. <P>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. More recently, for the first time, we elucidate atomic structures of an ETEC major pilin subunit, CfaB from colonization factor antigen I (CFA/I) fimbriae. These data are used to construct models for two morphological forms of CFA/I fimbriae that are both observed in vivo, the helical filament into which it is typically assembled, 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.<P> Our findings affirm the strong structural similarities seen between Class 5 fimbriae (from bacteria primarily causing gastrointestinal disease) and Class 1 pili (from bacteria that cause urinary, respiratory and other infections) 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. Lastly, we present structural evidence for bacterial use of antigenic variation to evade host immune responses, 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. <P>We have been trying to obtain the structure of CS3 fimbril for some times but so far a solution to its structure determination remain elusive. Instead, we have made significant progress toward a solution for the structure of Psa, which is a CS3 homolog from the pathogenic bacterium Yersinia pestis. The pH 6 antigen or Psa fimbriae of Yersinia pestis bind to two receptors, beta1-linked galactosyl residues in glycosphingolipids and phosphocholine group in phospholipids. Despite the ubiquitous presence of either moiety on the surface of many mammalian cells, Y. pestis appears to prefer interacting with certain types of human cells such as macrophages and alveolar epithelial cells of the lung.<P>The molecular mechanism of this apparent selectivity is not clear. Site-directed mutagenesis of the consensus choline-binding motif in the sequence of PsaA, the subunit of the Psa fimbrial homopolymer, identified residues that abolish either galactosylceramide or phosphatidylcholine binding or both. The crystal structure of the ternary complex of an in cis donor-strand complemented PsaA, galactose and phosphocholine reveals separate galactose and phosphocholine binding sites that share a common structural motif, thus suggesting potential interaction between the two sites. Mutagenesis of this shared structural motif identified Tyr126, which is part of the choline-binding consensus sequence but is found in direct contact with the galactose in the structure of PsaA, important for both receptor binding. This is the first structural resolution of a fimbrial subunit that forms a polymeric polyadhesin describing a unique arrangement of dual receptor binding sites. These findings move the field forward by providing insights into new types of multiple receptor-ligand interactions and should steer research into the synthesis of dual receptor inhibitor molecules to slow down the rapid progression of plague. Even more recently, we have also determined the crystal structure of CfaA, the chaperone component that is essential for assembly of CFA/I fimbriae.

Xia, Di
DHHS/NIH - National Cancer Institute
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