type I pilus

GO:0140621 · Appendage · Appendage · Proposed

A mannose-sensitive adhesive bacterial pilus assembled by the chaperone-usher pathway. In the Escherichia coli model, a helical FimA rod carries an ordered FimH-FimG-FimF tip, is polymerized through the FimD usher, and ends in a basal FimI terminator. Other type I systems, including Salmonella, have different subunit inventories. [GO:0140621]

Synonyms: type 1 pilus; type 1 fimbria; mannose-sensitive pilus

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Stoichiometry	Role
FimA major pilin	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	fimA	up to 3000 copies per Escherichia coli type I pilus rod	In the Escherichia coli model, polymerizes into the right-handed helical rod that forms most of the extracellular type I pilus. `DOI:10.1128/ecosalplus.ESP-0007-2017` Werneburg and Thanassi 2018 review FimA as the major type 1 pilus subunit assembled by the chaperone-usher pathway. `DOI:10.1038/s41467-025-60325-z` Bachmann et al. 2025 structurally analyze the Escherichia coli type 1 pilus rod as a helical assembly of FimA.
FimG tip adaptor subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	fimG		In the Escherichia coli model, links the distal FimH adhesin to FimF in the short flexible tip fibrillum. `DOI:10.1073/pnas.92.6.2081` Jones et al. 1995 isolated type 1 pilus tip fibrillae containing FimH and FimG. `DOI:10.1038/s41467-025-60325-z` Bachmann et al. 2025 place FimG between FimH and FimF in the type 1 pilus flexible tip fibrillum.
FimF tip adaptor subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	fimF		In the Escherichia coli model, joins the FimH/FimG tip to the helical FimA rod. `DOI:10.1038/s41467-025-60325-z` Bachmann et al. 2025 model FimF between FimG and the FimA rod in the assembled type 1 pilus.
FimH tip adhesin	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	fimH	1 distal adhesin per adhesive tip	In the Escherichia coli model, caps the distal tip and binds mannose-containing receptors on host surfaces. `DOI:10.1073/pnas.92.6.2081` Jones et al. 1995 identified FimH in the type 1 pilus tip fibrillum and localized the mannose-binding adhesin to that distal tip. `DOI:10.1128/ecosalplus.ESP-0007-2017` Werneburg and Thanassi 2018 review FimH as the mannose-binding adhesin of type 1 pili.
FimI terminator subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	fimI	1 basal terminator per assembled rod	In the Escherichia coli model, terminates the FimA rod and anchors the assembled pilus to the FimD outer-membrane assembly platform. `DOI:10.1038/s41467-025-60325-z` Bachmann et al. 2025 describe a single FimI subunit terminating the FimA rod and anchoring it to FimD.

Taxonomic distribution

NCBITaxon:2 Bacteria — VARIABLE: Type 1 pili are one chaperone-usher pilus subtype in Gram-negative bacteria, characterized in Escherichia coli and Salmonella rather than universal across bacteria. [DOI:10.1128/ecosalplus.ESP-0007-2017]

Canonical examples

NCBITaxon:562 Escherichia coli — The Escherichia coli Fim system is the structural and biochemical model for type I pilus rod and tip assembly. [DOI:10.1128/ecosalplus.ESP-0007-2017]

Functions

type I pilus assembly GO:0140623 — In Escherichia coli, the FimC chaperone and FimD usher assemble Fim subunits in order to build the tip fibrillum, helical FimA rod and FimI-terminated basal end.
- DOI:10.1128/ecosalplus.ESP-0007-2017 Werneburg and Thanassi 2018 review assembly of E. coli and Salmonella type 1 pili through the chaperone-usher pathway.
- DOI:10.1038/s41467-025-60325-z Bachmann et al. 2025 resolve the E. coli tip-to-rod transition and FimI-terminated assembly platform. The 2018 review described FimI termination as unconfirmed; the later structure supports the termination claim here.
mannose-sensitive adhesion — A distal FimH adhesin at the tip of the pilus binds mannosylated host-surface receptors.
- DOI:10.1073/pnas.92.6.2081 Jones et al. 1995 mapped FimH to the type 1 pilus tip and separated the mannose-binding adhesin from the FimA rod.
- DOI:10.1128/ecosalplus.ESP-0007-2017 Werneburg and Thanassi 2018 review mannose-sensitive FimH-mediated adhesion by type 1 pili.

Mechanism graphs

E. coli FimC and FimD polymerize the type I pilus tip and rod (ASSEMBLY)

In Escherichia coli, FimC carries folded pilus subunits to the outer-membrane FimD usher. FimD accepts the FimH adhesin, FimG adaptor, and FimF adaptor before adding FimA repeatedly into the helical rod, then the FimI terminator caps the basal end and anchors the rod at the usher.

Subject	Predicate	Object	Evidence
FimC chaperone	delivers to	FimD usher	`DOI:10.1128/ecosalplus.ESP-0007-2017` Werneburg and Thanassi 2018 review the FimC periplasmic chaperone and FimD outer-membrane usher steps of type 1 pilus biogenesis.
FimD usher	assembles tip from	FimH tip adhesin	`DOI:10.1128/ecosalplus.ESP-0007-2017` Werneburg and Thanassi 2018 describe ordered chaperone-usher incorporation of the type 1 pilus tip subunits.
FimH tip adhesin	connects to	FimG tip adaptor subunit	`DOI:10.1038/s41467-025-60325-z` Bachmann et al. 2025 structurally model FimG immediately proximal to FimH in the type 1 pilus tip.
FimG tip adaptor subunit	connects to	FimF tip adaptor subunit	`DOI:10.1038/s41467-025-60325-z` Bachmann et al. 2025 place FimF proximal to FimG in the ordered FimH-FimG-FimF tip fibrillum.
FimF tip adaptor subunit	connects to	FimA major pilin	`DOI:10.1038/s41467-025-60325-z` Bachmann et al. 2025 model FimF as the adaptor between FimG and the helical FimA rod.
FimA major pilin	polymerizes into	type I pilus	`DOI:10.1038/s41467-025-60325-z` Bachmann et al. 2025 describe the rod as a helical FimA assembly.
FimI terminator subunit	terminates	type I pilus	`DOI:10.1038/s41467-025-60325-z` Bachmann et al. 2025 assign the basal rod-ending density to a single FimI subunit bound to FimD.
FimD usher	carries out	type I pilus assembly	`DOI:10.1038/nrmicro.2017.40` Hospenthal, Costa and Waksman 2017 review the outer-membrane usher as the assembly platform for chaperone-usher pili.

Discussions

CURATION_TODO (RESOLVED): Keep FimC and FimD out of the component table for this first type I pilus record. FimC and FimD are required for ordered pilus biogenesis and are therefore shown in the assembly graph, but this record denotes the extracellular appendage built from FimA, FimF, FimG, FimH and FimI rather than the periplasmic chaperone or outer-membrane usher that polymerizes and secretes those subunits.
CURATION_TODO (OPEN): Resolve exact source-neutral family groundings for the FimA, FimG, FimF, FimH and FimI constituent subunits. The record intentionally uses reviewed labels for the E. coli type I pilus subunits until InterPro, Pfam or NCBIfam accessions can be verified without overclaiming a broader chaperone-usher domain as an exact subunit family.
INTERPRETATION (RESOLVED): Keep the E. coli subunit model distinct from other type I pilus inventories. Mannose-sensitive type I pili occur in both E. coli and Salmonella, but shared subtype identity does not imply identical Fim subunits. The component roles and graph are explicitly the E. coli model; the broader taxonomic distribution is retained.

Evidence

DOI:10.1128/ecosalplus.ESP-0007-2017 Werneburg GT and Thanassi DG 2018, 'Pili Assembled by the Chaperone/Usher Pathway in Escherichia coli and Salmonella', EcoSal Plus.
DOI:10.1038/nrmicro.2017.40 Hospenthal MK, Costa TRD and Waksman G 2017, 'A comprehensive guide to pilus biogenesis in Gram-negative bacteria', Nature Reviews Microbiology.
DOI:10.1038/s41467-025-60325-z Bachmann et al. 2025, 'Structures of the Escherichia coli type 1 pilus during pilus rod assembly and after assembly termination', Nature Communications.
DOI:10.1073/pnas.92.6.2081 Jones et al. 1995, 'FimH adhesin of type 1 pili is assembled into a fibrillar tip structure in the Enterobacteriaceae', Proceedings of the National Academy of Sciences.

Curation history

2026-09-11T01:45:47Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-11T01:46:57Z codex · EXPANDED_RECORD — Added exact synonymy, GO pilus parentage, FimA/FimF/FimG/FimH/FimI constituents, E. coli taxonomy, GO:0140623 assembly and mannose-sensitive adhesion functions, literature evidence, a FimC/FimD assembly graph, and grounding/boundary discussions.
2026-09-11T01:53:52Z codex · ADDRESS_REVIEW — Resolved PR #385 issues #386 and #387 by splitting the grouped FimF/FimG tip-adaptor component into ordered FimG and FimF constituents, updating the assembly graph to preserve the FimH-FimG-FimF-FimA order, and attaching the family-grounding TODO to every reviewed-label-only subunit.
2026-10-11T06:33:22Z codex · ADDRESS_SAVED_REVIEW — Addressed #2182, #2183: Separated broad type I identity from the E. coli Fim inventory and added the confirmed 2025 FimI termination source to the assembly function.

Source: data/structures/appendage/type_i_pilus.yaml