Fimbrial usher protein

{{update|structure (need to add beta barrel; need new pic)|date=December 2020}}

{{Pfam_box

| Symbol = Usher

| Name = Fimbrial Usher protein

| image = PDB_1zdv_EBI.jpg

| width =

| caption = Structure of the type 1 pilus assembly platform FimD(25-139).{{cite journal |vauthors=Nishiyama M, Horst R, Eidam O |title=Structural basis of chaperone–subunit complex recognition by the type 1 pilus assembly platform FimD |journal=EMBO J. |volume=24 |issue=12 |pages=2075–86 |date=June 2005 |pmid=15920478 |pmc=1150887 |doi=10.1038/sj.emboj.7600693 |display-authors=etal}}

| Pfam= PF00577

| InterPro= IPR000015

| SMART=

| PROSITE = PDOC00886

| SCOP =

| TCDB = 1.B.11

| OPM family=187

| OPM protein=4j3o

| PDB=

}}

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.{{cite journal |vauthors=Hultgren SJ, Jacob-Dubuisson F, Striker R |title=Chaperone-assisted self-assembly of pili independent of cellular energy |journal=J. Biol. Chem. |volume=269 |issue=17 |pages=12447–12455 |year=1994 |doi=10.1016/S0021-9258(18)99895-9 |pmid=7909802 |doi-access=free }}{{cite journal |vauthors=Schifferli DM, Alrutz MA |title=Permissive linker insertion sites in the outer membrane protein of 987P fimbriae of Escherichia coli |journal=J. Bacteriol. |volume=176 |issue=4 |pages=1099–1110 |year=1994 |doi=10.1128/JB.176.4.1099-1110.1994 |pmid=7906265 |pmc=205162}}{{cite journal |vauthors=Saier Jr MH, Van Rosmalen M |title=Structural and evolutionary relationships between two families of bacterial extracytoplasmic chaperone proteins which function cooperatively in fimbrial assembly |journal=Res. Microbiol. |volume=144 |issue=7 |pages=507–527 |year=1993 |pmid=7906046 |doi=10.1016/0923-2508(93)90001-I|doi-access=free }}

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{{cite journal | vauthors = Capitani G, Eidam O, Grütter MG | year = 2006 | title = Evidence for a novel domain of bacterial outer membrane ushers | url = https://pubmed.ncbi.nlm.nih.gov/17066380 | journal = Proteins | volume = 65 | issue = 4| pages = 816–23 | doi = 10.1002/prot.21147 | pmid = 17066380 | s2cid = 28766740 }} 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).{{cite journal | vauthors = Phan G, Remaut H, Wang T, Allen WJ, Pirker KF, Lebedev A et al | year = 2011 | title = Crystal structure of the FimD usher bound to its cognate FimC-FimH substrate | journal = Nature | volume = 474 | issue = 7349| pages = 49–53 | doi = 10.1038/nature10109 | pmid = 21637253 | pmc=3162478}} 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.