Fimbrial usher protein explained

The fimbrial usher protein is involved in biogenesis of the pilus in Gram-negative bacteria. The biogenesis of some fimbriae (or pili) requires a two-component assembly and transport system which is composed of a periplasmic chaperone and a pore-forming outer membrane protein which has been termed a molecular 'usher'; this is the chaperone-usher pathway.^{[1] [2] [3]}

The usher protein has a molecular weight ranging from 86 to 100 kDa and is composed of a membrane-spanning 24-stranded beta barrel domain, reminiscent of porins, and of four periplasmic soluble domains: an N-terminal one of about 120 residues (NTD), a 'middle' domain of about 80 residues^[4] located as a soluble insertion within the beta barrel region of the sequence (plug domain) and two IG-like domains (each about 80 residues long) at the C-terminus (CTD1 and CTD2).^[5] 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 disulfide bond-forming cysteines, the first one located in the NTD and the second in CTD2. The best conserved region of the sequence corresponds to the plug domain.

Notes and References

1. Hultgren SJ, Jacob-Dubuisson F, Striker R . Chaperone-assisted self-assembly of pili independent of cellular energy . J. Biol. Chem. . 269 . 17 . 12447–12455 . 1994 . 10.1016/S0021-9258(18)99895-9 . 7909802 . free .
2. Schifferli DM, Alrutz MA . Permissive linker insertion sites in the outer membrane protein of 987P fimbriae of Escherichia coli . J. Bacteriol. . 176 . 4 . 1099–1110 . 1994 . 10.1128/JB.176.4.1099-1110.1994 . 7906265 . 205162.
3. Saier Jr MH, Van Rosmalen M . Structural and evolutionary relationships between two families of bacterial extracytoplasmic chaperone proteins which function cooperatively in fimbrial assembly . Res. Microbiol. . 144 . 7 . 507–527 . 1993 . 7906046 . 10.1016/0923-2508(93)90001-I. free .
4. Capitani G, Eidam O, Grütter MG . 2006 . Evidence for a novel domain of bacterial outer membrane ushers . Proteins . 65 . 4. 816–23 . 10.1002/prot.21147 . 17066380 . 28766740 .
5. Phan G, Remaut H, Wang T, Allen WJ, Pirker KF, Lebedev A et al . 2011 . Crystal structure of the FimD usher bound to its cognate FimC-FimH substrate . Nature . 474 . 7349. 49–53 . 10.1038/nature10109 . 21637253 . 3162478.