Ushers are a family of integral outer membrane proteins participating in the chaperone (usher) secretion pathway, a terminal branch of the General Secretion Pathway (GSP) dedicated to the biogenesis of adhesive surface structures associated with pathogenesis. As the name of the pathway implies, these proteins work in conjunction with a periplasmic chaperone in order to assemble and secrete more than 30 different surface molecules in a broad range of Gram-negative bacteria. The most studied example of the chaperone (usher) pathway is the assembly and biogenesis of the type 1 and P pili, expressed by the uropathogenic Escherichia coli. Members of the Usher family (like PapC, FasD, FaeD and FimD), display properties of ß-barrel proteins and form channels consisting of 24 beta-strands, as shown in the crystal structure of PapC. It has been proposed that Ushers form twin dimeric pores in the outer membrane and the C-terminal parts of the sequences are responsible for the dimerization. In addition, the N-terminal 100-120 amino acids are believed to be involved in the recognition of the periplasmic chaperone.

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