View source for CsgG

{{Short description|Outer-membrane channel of Escherichia coli whose engineered form is the pore of Oxford Nanopore sequencers}}
{{Infobox protein
| name = CsgG
| organism = ''Escherichia coli''
| symbol = ''csgG'' (UniProt P0AEA2)
| structure_pdb = 4UV3 (membrane form), 4UV2
| pore_diameter = 0.9 nm at the constriction
| subunits = 9 (homononamer; 262 residues each after processing)
| function = Exports the curli subunit CsgA across the outer membrane
}}
'''CsgG''' is a protein of the outer membrane of ''Escherichia coli'' and related bacteria. Nine copies assemble into a channel through which the cell pushes out CsgA, the subunit of curli, the amyloid fibers that hold bacterial biofilms together. In 2016 [[Oxford Nanopore Technologies]] introduced a pore developed from CsgG as the pore of its R9 chemistry, and its descendants still read DNA in the company's sequencers.<ref name="ontr9">{{cite web |url=https://nanoporetech.com/news/news-update-new-r9-nanopore-faster-more-accurate-sequencing-and-new-ten-minute-preparation |title=Update: New "R9" nanopore for faster, more accurate sequencing, and new ten minute preparation kit |publisher=Oxford Nanopore Technologies |date=May 26, 2016 |access-date=October 25, 2026}}</ref>

== Curli and the secretion channel ==
Curli were described in 1989 as surface structures of ''E. coli'' that bind the host protein fibronectin.<ref name="olsen">{{cite journal |last1=Olsén |first1=A. |last2=Jonsson |first2=A. |last3=Normark |first3=S. |title=Fibronectin binding mediated by a novel class of surface organelles on ''Escherichia coli'' |journal=Nature |volume=338 |issue=6217 |pages=652-655 |year=1989 |doi=10.1038/338652a0}}</ref> In 2002 biochemical and imaging experiments showed that they are amyloid, the class of fiber associated with Alzheimer's and prion diseases. Unlike amyloid in those diseases, the authors wrote, curli formation is "a productive pathway requiring a specific assembly machinery".<ref name="chapman">{{cite journal |last1=Chapman |first1=M. R. |last2=Robinson |first2=L. S. |last3=Pinkner |first3=J. S. |last4=Roth |first4=R. |last5=Heuser |first5=J. |last6=Hammar |first6=M. |last7=Normark |first7=S. |last8=Hultgren |first8=S. J. |title=Role of ''Escherichia coli'' curli operons in directing amyloid fiber formation |journal=Science |volume=295 |issue=5556 |pages=851-855 |year=2002 |doi=10.1126/science.1067484}}</ref> The genes lie in two operons, one holding the fiber proteins CsgB and CsgA and the other ''csgDEFG''. CsgA is the major subunit, and CsgE, CsgF and CsgG are non-structural proteins that help build it.<ref name="hammar">{{cite journal |last1=Hammar |first1=M. |last2=Arnqvist |first2=A. |last3=Bian |first3=Z. |last4=Olsén |first4=A. |last5=Normark |first5=S. |title=Expression of two ''csg'' operons is required for production of fibronectin- and Congo red-binding curli polymers in ''Escherichia coli'' K-12 |journal=Molecular Microbiology |volume=18 |issue=4 |pages=661-670 |year=1995 |doi=10.1111/j.1365-2958.1995.mmi_18040661.x}}</ref><ref name="robinson">{{cite journal |last1=Robinson |first1=L. S. |last2=Ashman |first2=E. M. |last3=Hultgren |first3=S. J. |last4=Chapman |first4=M. R. |title=Secretion of curli fibre subunits is mediated by the outer membrane-localized CsgG protein |journal=Molecular Microbiology |volume=59 |issue=3 |pages=870-881 |year=2006 |doi=10.1111/j.1365-2958.2005.04997.x}}</ref>

CsgG is a lipoprotein. It is made as a chain of 277 residues, loses a signal peptide of 15, and its new N-terminal cysteine is lipidated, which anchors it in the membrane.<ref name="uniprot">{{cite web |url=https://www.uniprot.org/uniprotkb/P0AEA2 |title=P0AEA2: Curli production assembly/transport component CsgG, ''Escherichia coli'' (strain K12) |publisher=UniProt |access-date=October 25, 2026}}</ref> A stretch of 22 amino acids at the start of CsgA is enough to send other proteins to the channel. Overproducing CsgG makes the bacterium susceptible to the antibiotic erythromycin.<ref name="robinson" />

== Structure ==
The structure was published in 2014. Nine units that resemble anticodon-binding domains form a 36-stranded β-barrel that crosses the membrane and opens onto a cage-shaped vestibule in the periplasm. Between the two lies a constriction 0.9 nm wide, made of three stacked rings of phenylalanine, asparagine and tyrosine, and the authors suggested that it guides the unfolded polypeptide through.<ref name="goyal">{{cite journal |last1=Goyal |first1=P. |last2=et al. |title=Structural and mechanistic insights into the bacterial amyloid secretion channel CsgG |journal=Nature |volume=516 |issue=7530 |pages=250-253 |year=2014 |doi=10.1038/nature13768 |pmid=25219853}}</ref> The two structures, released together on September 24, 2014, are a non-lipidated, soluble pre-pore form (PDB 4UV2, resolution 2.8 Å) and the membrane-bound form (4UV3, 3.59 Å).<ref name="pdb">{{cite web |url=https://www.rcsb.org/structure/4UV3 |title=4UV3: Structure of the curli transport lipoprotein CsgG in its membrane-bound conformation |publisher=RCSB Protein Data Bank |access-date=October 25, 2026}}</ref> The channel has no gate of its own. The authors called it "ungated" and "non-selective", and the cell's control is a second protein: CsgE assembles into a nonamer that binds the periplasmic face and closes it.<ref name="goyal" /><ref name="goyal" />

[[File:Biological_nanopore_profiles.svg|thumb|right|upright=1.4|alt=Three side-on outlines of protein pores in a membrane band, with the narrowest part of each channel marked|Outlines of three protein pores at a common scale, with the narrowest part of each marked: alpha-hemolysin (7 subunits, 1.4 nm), MspA (8 subunits, 1.2 nm wide and no more than 0.6 nm long) and CsgG (9 subunits, 0.9 nm). The outlines are schematic.]]
For comparison, [[alpha-hemolysin]] has seven subunits and a narrowest point of 1.4 nm, and [[MspA]] has eight, with a constriction no more than 0.6 nm long. CsgG has the narrowest opening of the three, and its barrel has the most strands.

== As a sequencing pore ==
Oxford Nanopore announced in May 2016 that the pore of its R9 chemistry was CsgG, licensed from a laboratory at VIB in Belgium and from University College London, and that sequencing speed would rise from about 70 to about 250 bases a second per pore.<ref name="ontr9" /> The structure had been solved by the laboratory of Han Remaut at VIB, and the two organizations announced a collaboration in March 2016.<ref name="vib">{{cite news |url=https://www.bionity.com/en/news/157213/new-dna-sequencing-nanopore.html |title=New DNA sequencing nanopore: VIB collaborates with Oxford Nanopore Technologies |work=Bionity |date=2016-03-10}}</ref>

The natural constriction proved useful twice. A 2020 paper combined CsgG with the 35-residue N-terminal region of CsgF, its partner protein outside the cell. The first 33 residues of CsgF bind inside the barrel and form a second constriction about 25 Å from the first, and a prototype pore with two constrictions improved single-read accuracy by 25 to 70% in runs of identical bases up to nine long.<ref name="dual">{{cite journal |last1=Van der Verren |first1=S. E. |last2=et al. |title=A dual-constriction biological nanopore resolves homonucleotide sequences with high fidelity |journal=Nature Biotechnology |volume=38 |pages=1415-1420 |year=2020 |doi=10.1038/s41587-020-0570-8}}</ref> In 2024 an unfoldase, ClpX, moved protein strands hundreds of residues long through CsgG on a commercial sensor array in steps of two residues (see [[Nanopore protein sequencing]]).<ref name="motone">{{cite journal |last1=Motone |first1=K. |last2=et al. |title=Multi-pass, single-molecule nanopore reading of long protein strands |journal=Nature |volume=633 |issue=8030 |pages=662-669 |year=2024 |doi=10.1038/s41586-024-07935-7 |pmid=39261738}}</ref>

== At Molecular Reality ==
[[Molecular Reality Corporation]] builds [[solid-state nanopore]]s, but its atlas of the field, [[The Hole World]], draws two of its 2,112 entries with the CsgG archetype, a barrel of nine subunits, and one of them is labeled "CsgG + motor" and belongs to Oxford Nanopore. Alpha-hemolysin is drawn 33 times.<ref name="hw">{{cite web |url=https://molecularreality.com/the_hole_world |title=The Hole World: Molecular Sensing Atlas |publisher=Molecular Reality Corporation |date=2026-09-26}}</ref> The company's quiz game [[JeoPOREdy!]] has a clue about Oxford Nanopore that says its "commercial pores are now built from a mutant of the curli transporter CsgG".<ref name="jeop">{{cite web |title=JeoPOREdy! |website=EQBio-OS |publisher=Molecular Reality Corporation |date=2026-04}}</ref>

== See also ==
* [[Biological nanopore]]
* [[Protein Data Bank]]
* [[History of nanopore sequencing]]

== References ==
{{Reflist}}

{{Nanopores}}
{{Portal bar|Nanopores|Molecular Reality}}

[[Category:Nanopores]]
[[Category:Bacterial proteins]]
[[Category:Membrane proteins]]
[[Category:Escherichia coli]]