Tad pilus

cellstructuremech:tad_pilus · Appendage · Appendage · Proposed

A tight-adherence (Tad/Flp) subtype of type IV pilus involved in bacterial adhesion and colonization. In the Caulobacter vibrioides model, polar PilA filaments extend and retract during surface exploration and promote holdfast-dependent permanent attachment; this dynamic behavior is not asserted for every Tad system. [DOI:10.1128/mBio.01237-19]

Synonyms: tight-adherence pilus; type IVc pilus; Flp 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	Essential	Role
PilA major pilin	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	pilA			Polymerizes into the polar Tad pilus filament that extends and retracts during Caulobacter surface exploration. `DOI:10.1128/mBio.01237-19` Sangermani et al. 2019 image dynamic Caulobacter Tad pili and identify PilA as the pilin building the polar filament. `DOI:10.1126/science.aan5706` Ellison et al. 2017 fluorescently labeled Caulobacter PilA to directly observe Tad pilus extension and retraction.

Taxonomic distribution

NCBITaxon:155892 Caulobacter vibrioides — UNIVERSAL: Caulobacter vibrioides encodes polar Tad pili used by swarmer cells for dynamic surface colonization before holdfast-mediated permanent attachment. [DOI:10.1128/mBio.01237-19]

Canonical examples

NCBITaxon:565050 Caulobacter vibrioides NA1000 — Caulobacter vibrioides NA1000 is the model strain in direct-imaging and optical-trapping studies of dynamic Tad pili. [DOI:10.1128/mBio.01237-19]

Functions

Tad pilus extension and retraction — The CpaF motor extends and retracts polar PilA filaments so Caulobacter cells can repeatedly probe nearby surfaces.
- DOI:10.1126/science.aan5706 Ellison et al. 2017 directly observed extension and retraction of labeled Caulobacter Tad pili.
- DOI:10.1038/s41467-024-50280-6 Hohl et al. 2024 show bidirectional processing by the Tad pilus CpaF motor.
- DOI:10.1126/sciadv.aay2591 Ellison et al. 2019 correlate CpaF ATPase mutations with reduced extension/retraction rates, ATP hydrolysis and retraction force in Caulobacter; the tested Asticcacaulis system differs in the fraction of dynamic pili.
surface colonization — Cycles of Tad pilus extension, surface contact and retraction help Caulobacter cells remain near a surface long enough to deposit a holdfast.
- DOI:10.1128/mBio.01237-19 Sangermani et al. 2019 propose that the flagellum and Tad pili cooperate to progressively drive Caulobacter surface colonization.

Mechanism graphs

Caulobacter CpaF extends and retracts PilA Tad pili (FUNCTION)

In Caulobacter, PilA subunits polymerize into a polar Tad pilus filament, the CpaF motor drives filament extension and retraction, and surface-contact dynamics support early colonization.

Subject	Predicate	Object	Evidence
PilA major pilin	polymerizes into	Tad pilus	`DOI:10.1128/mBio.01237-19` Sangermani et al. 2019 track dynamic Caulobacter Tad pili built from PilA pilin.
CpaF Tad pilus motor	extends and retracts	Tad pilus	`DOI:10.1038/s41467-024-50280-6` Hohl et al. 2024 structurally and biochemically analyze bidirectional pilus processing by CpaF. `DOI:10.1126/science.aan5706` Ellison et al. 2017 show that Caulobacter Tad pili retract and that retraction obstruction stimulates surface sensing. `DOI:10.1126/sciadv.aay2591` Ellison et al. 2019 correlate CpaF ATPase mutations with reduced extension/retraction rates, ATP hydrolysis and retraction force in Caulobacter; the tested Asticcacaulis system differs in the fraction of dynamic pili.
Tad pilus	contacts	abiotic surface	`DOI:10.1128/mBio.01237-19` Sangermani et al. 2019 image Tad pilus extension toward the surface during Caulobacter colonization assays.
Tad pilus	supports	surface colonization	`DOI:10.1128/mBio.01237-19` Sangermani et al. 2019 describe Tad pili as dynamic structures that promote surface colonization.

Discussions

CURATION_TODO (RESOLVED): Keep CpaF and the rest of the Cpa assembly machinery out of the component table for this first Tad pilus record. This record denotes the extracellular PilA filament rather than the complete Tad envelope machine. CpaF is therefore shown as the extension-retraction motor in the function graph, while exact structural boundaries for the rest of the Cpa machinery remain out of scope.
CURATION_TODO (OPEN): Resolve exact source-neutral family groundings for the PilA major pilin. The record intentionally uses a reviewed label until an exact InterPro, Pfam or NCBIfam identifier for Caulobacter PilA-family Tad pilins can be verified.
INTERPRETATION (OPEN): Determine how broadly the Caulobacter polar retraction mechanism applies across Tad systems. The record retains the broad Tad subtype identity while curating the demonstrated Caulobacter PilA/CpaF model. Differences between species and unresolved conservation prevent treating dynamic polar retraction as a defining property of every Tad pilus.

Evidence

DOI:10.1128/mBio.01237-19 Sangermani M et al. 2019, 'Tad Pili Play a Dynamic Role in Caulobacter crescentus Surface Colonization', mBio.
DOI:10.1126/science.aan5706 Ellison CK et al. 2017, 'Obstruction of pilus retraction stimulates bacterial surface sensing', Science.
DOI:10.1038/s41467-024-50280-6 Hohl M et al. 2024, 'Bidirectional pilus processing in the Tad pilus system motor CpaF', Nature Communications.

Curation history

2026-09-11T02:41:52Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-11T02:42:31Z codex · EXPANDED_RECORD — Added Tad/Flp synonymy, type IV pilus parentage, PilA as the major pilin constituent, Caulobacter taxonomy, extension-retraction and surface-colonization functions, a CpaF/PilA dynamics graph, DOI evidence, and grounding/boundary discussions.
2026-09-11T02:51:48Z codex · ADDRESS_REVIEW — Resolved PR #395 issues #396 and #397 by collapsing duplicate CpaF retraction edges into one motor edge and typing the terminal surface-colonization outcome as a biological process.
2026-10-11T06:33:19Z codex · ADDRESS_SAVED_REVIEW — Addressed #2181: Bounded the generic Tad identity separately from Caulobacter polar PilA/CpaF dynamics and attached the direct 2019 motor evidence.

Source: data/structures/appendage/tad_pilus.yaml