Fimbriae, hydrophobic

The subunits of fimbriae are constructed essentially as Ig-like P-sandwiches, but with a circular permutation that positions the sequence corresponding to the seventh, C-terminal, Ig P-strand (strand G of a canonical Ig domain) at the N-terminus of the polypeptide sequence [32-35] (Figs. 3a and 4a). In a typical Ig fold, the top edge of the sandwich, defined by the A and F strands, is capped by the C-terminal G strand, which is hydrogen bonded to the F strand and provides hydrophobic residues to the core of the fold. Free pilin subunits are only marginally stable, and no structure for a monomeric pilin in the absence of chaperone has been obtained. Sometimes, in the absence of the chaperone, soluble, domain-swapped pilin dimers [36] or trimers [37, 38] are formed and have been reported in their crystal structures. However, the oligomerization of pilins in this manner is a dead-end process, and the pilins in these oligomers are not able to assemble into fibrillar structures. 

The attachment of cells to a surface (Figure 7.1a) is complicated, and can be due to either electrostatic forces or covalent binding between the cell membrane and the surface [12]. This process is affected by the diverse characteristics of the substratum, the adhesive molecules secreted by the cells, the growth medium, and, especially, the cell surface. Properties of cell surface such as hydrophobicity, extracellular polymeric substance production, and the presence of fimbria and flagella play important roles in determining the rate and extent of attachment. 

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