Combining sites of bacterial fimbriae.

The few known crystal structures of receptor-binding domains of fimbrial tip adhesins, FimH, PapGII, and F17G, tell us that each of these structures is unique and surprising. Despite little to no sequence identity, common to them all is their variable immunoglobulin (Ig)-fold. Nevertheless, their glycan-binding sites have evolved in different locations onto this similar scaffold, and with distinct, highly specific binding properties. Difficult to capture is the often dominant role played by the fimbrial shaft in host cell recognition and biofilm formation. The major pilin FaeG, building up the shaft of F4 fimbriae, also harbors the carbohydrate receptor-binding property and has thereto an enlarged Ig-domain, with the insertion of two beta-strands and two alpha-helices. Bordetella and CFA/I fimbriae combine a tip adhesin with major subunit adhesins. Still other fimbriae incorporate a specialized invasin at the very tip of polyadhesive fibers for uptake of bacteria in cells of the immune system and host epithelia. Finally, glycan recognition by fimbrial adhesins has often been found to coincide with the binding of cell-surface integrins and components of the extracellular matrix, such as collagen IV and laminin.