In Gram-negative bacteria the biogenesis of fimbriae (or pili) requires a two- component assembly and transport system which is composed of a periplasmic chaperone and an outer membrane protein which has been termed a molecular 'usher' [1-3]. The usher protein is rather large (from 86 to 100 Kd) and seems to be mainly composed of membrane-spanning beta-sheets, a structure reminiscent of porins. Although the degree of sequence similarity of these proteins is not very high they share a number of characteristics. One of these is the presence of two pairs of cysteines, the first one located in the N-terminal part and the second at the C-terminal extremity that are probably involved in disulphide bonds. The best conserved region is located in the central part of these proteins [4-5].

1: Chaperone-assisted self-assembly of pili independent of cellular energy. Jacob-Dubuisson F, Striker R, Hultgren SJ; J Biol Chem. 1994;269:12447-12455. PMID:7909802

2: Permissive linker insertion sites in the outer membrane protein of 987P fimbriae of Escherichia coli. Schifferli DM, Alrutz MA; J Bacteriol. 1994;176:1099-1110. PMID:7906265

3: Structural and evolutionary relationships between two families of bacterial extracytoplasmic chaperone proteins which function cooperatively in fimbrial assembly. Van Rosmalen M, Saier MH Jr; Res Microbiol. 1993;144:507-527. PMID:7906046

4: Insights into pilus assembly and secretion from the structure and functional characterization of usher PapC. Huang Y, Smith BS, Chen LX, Baxter RH, Deisenhofer J; Proc Natl Acad Sci U S A. 2009;106:7403-7407. PMID:19380723

5: Fiber formation across the bacterial outer membrane by the chaperone/usher pathway. Remaut H, Tang C, Henderson NS, Pinkner JS, Wang T, Hultgren SJ, Thanassi DG, Waksman G, Li H; Cell. 2008;133:640-652. PMID:18485872