Chaperone-usher fimbriae

Pilus biogenesis proceeds via the strand complementation mechanism, the mechanism functions as a direct consequence of the structures of both the chaperones and pilus subunits. Both the G1 extension of the chaperone and the N-terminal extension of the pilus subunits can bind to the hydrophobic P1-P5 cleft. When the G1 is bound it binds in a parallel conformation stabilising the subunit structure but doesn't form a true Ig fold (which is anti-parallel). It also only binds the cleft from sites P1-P4 leaving the P5 pocket exposed. When another subunit binds the hydrophobic cleft it binds in a P5-P2 anti-parallel fashion creating the most energetically favourable conformation of a true Ig-fold.[1]

When a chaperone/subunit complex is brought to the growing usher/pilus complex (note that a chaperone/subunit complex forms the base of this) the NTD and CTDs of the usher dimer bring and hold the chaperone/subunit complex to the pilus base. Here the N-terminal extension of the subunit interacts with the free P5 pocket of the pilus. This binding causes a residue by residue 'unzipping' of the chaperone's G1 strand with concurrent 'zipping' of the N-terminal extension of the chaperone/subunit complex, unknown as the zip in-zip out mechanism.[2]

Once the chaperone has been displaced it diffuses back to the sec machinery to bind another emerging pilus subunit. The usher using is CTDs then translocates the longer pilus across the translocation pore by one residue until the chaperone is in close association with the usher. This translocation is coupled with a slight rotation giving the pilus its helical structure. This growth proceeds until a terminating subunit binds the pilus (if one exists), terminating subunits lack the P5 pocket and thus almost irreversibly bind their chaperone. Since the usher can only translocate pilus subunits and not chaperone/subunit complexes, and the fact that the chaperone can't be energetically out-competed by another subunit (due to lack of P5) this terminating subunit anchors the pilus to the outer membrane and stop any further pilus growth.[1]

Historically classification of fimbriae was made based on their appearance under the microscope creating 4 classes: Afimbrial, type 1 fimbriae, bundle forming (type IV) pilus and curli. However this type of classification doesn't give any measure of relatedness and thus has been superseded by a phylogenetic system.[8]

In chaperone/usher fimbriae there are broadly two types of classification: Based on the type of usher present, or based on the type of chaperone present. Using the usher as the base of classification forms 5 clades: alpha, beta, gamma, pi and sigma, and four sub clades: gamma 1-4. The naming of the beta and gamma clades are simply alphabetical, whereas the others are named by a defining feature: alpha for alternative chaperone/usher family; pi for P-pilus (pyelonephritis associated); and sigma for spore coat pilus from Myxococcus xanthus.[8]

As stated previously there are two types of chaperone; FGL and FGS, and this is the basis of chaperone classification. All FGL chaperones are associated with afimbrial secretion and all FGS chaperones are associated with fimbrial secretion. All FGL chaperones are found in sub-clade gamma-3, all other clades and sub-clades are FGS.[8]

Chaperone/usher fimbriae have many roles, in many stages of pathogenicity in several species. The most well understood are roles in adhesion, innate immune evasion and localisation. Adhesion is facilitated by the lectin domain at the tip of chaperone/usher fimbriae, this domain of FimH is well understood. FimH is the first subunit of the type I pilus (γ-1 clade) in species such as UPEC and binds to D-mannosylated residues in the bladder, allowing pathogen adhesion and colonisation. Immune evasion is also facilitated by the tip adhesin, for example the binding of Decay accelerating factor (DAF) by gamma3 clade fimbriae inhibit effective complement activation. Localisation could be performed by sequential expression of different chaperone/usher systems. Most species of pathogenic bacteria express more than one type of chaperone/usher system, for example in Pseudomonas aeruginosa there are five different systems. UPEC expresses both the type 1 fimbriae and the P-pilus (pap) which it expresses sequentially possibly to facilitate migration from the bladder (type I fimbriae) to the kidney (pap).[9]

This importance in pathogenicity makes the chaperone/usher system an attractive target for novel drug therapies. By targeting pathogenicity instead of the organism (in the case of conventional antibiotics) the strong selective pressure for evolution of resistance is removed. In the case of UPEC there have been some success in stopping pilus formation in clinical trials and the development of an effective vaccine in mice and cynomolgus monkeys based on FimCH complex.[9]