Fimbrial usher protein


The fimbrial usher protein is involved in biogenesis of the pilus in Gram-negative bacteria. The biogenesis of fimbriae 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'.
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, a 'middle' domain of about 80 residues located as a soluble insertion within the beta barrel region of the sequence and two IG-like domains at the C-terminus. 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.