polar tube

GO:0044099 · Spore · Filament · Proposed

A microsporidian invasion tube that is coiled inside the spore as the polar filament and rapidly everts from the anterior pole during germination, forming a conduit for the sporoplasm toward a host cell. [GO:0044099]

Synonyms: polar filament

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
polar tube proteins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)			UNKNOWN	Lineage-specific structural proteins that build the compact polar filament in the dormant spore and the everted polar tube after germination. `DOI:10.1016/j.ijpara.2005.04.003` Xu and Weiss 2005 reviewed PTP1, PTP2 and PTP3 as identified protein constituents of the microsporidian polar tube. `DOI:10.1016/j.jprot.2022.104617` Lv et al. 2022 compared Nosema bombycis polar filaments and everted polar tubes by proteomic profiling. `DOI:10.1371/journal.pbio.3002533` Sharma et al. 2024 combined cryo-electron tomography with affinity purification and mass spectrometry of germinated microsporidian polar tubes.

Taxonomic distribution

NCBITaxon:6029 Microsporidia — COMMON: The polar tube is a defining invasion apparatus of microsporidian spores; this scope is tied to the spore stage, not every vegetative cell. [DOI:10.1186/s13071-023-05908-9]

Canonical examples

NCBITaxon:27978 Nosema bombycis — Silkworm-infecting microsporidian in which proteomic profiles of compact polar filaments and everted polar tubes were compared. [DOI:10.1016/j.jprot.2022.104617]

Functions

host cell invasion — The polar tube rapidly everts from the spore, penetrates or apposes the host-cell surface and establishes the route used to initiate infection.
- DOI:10.1016/j.ijpara.2005.04.003 Xu and Weiss 2005 reviewed polar-tube discharge and host-cell invasion by microsporidian spores.
- DOI:10.1186/s13071-023-05908-9 Chen et al. 2023 reviewed polar-tube origin, structure, composition and function in microsporidia.
sporoplasm delivery — After firing, the everted tube acts as a narrow conduit for microsporidian sporoplasm and associated cargo released from the spore.
- DOI:10.1128/mbio.02749-23 Lv et al. 2024 visualized polaroplast-derived membranes moving through fired Nosema bombycis polar tubes during sporoplasm membrane formation.
- DOI:10.1371/journal.pbio.3002533 Sharma et al. 2024 imaged cargo-containing states of Vairimorpha necatrix polar tubes during germination by cryo-electron tomography.

Mechanism graphs

Polar tube eversion opens a conduit from the spore (FUNCTION)

The dormant spore stores the tube in a compact polar-filament state. Germination ejects and everts the tube, creating a conduit that carries sporoplasm cargo out of the spore.

Subject	Predicate	Object	Evidence
polar filament	everts into	polar tube	`DOI:10.1186/s13071-023-05908-9` Chen et al. 2023 reviewed the transition from polar filament to discharged polar tube. `DOI:10.1371/journal.pbio.3002533` Sharma et al. 2024 captured Vairimorpha necatrix polar-tube states during germination.
polar tube	conducts	sporoplasm	`DOI:10.1016/j.ijpara.2005.04.003` Xu and Weiss 2005 reviewed the tube as the route for sporoplasm passage into a new host cell. `DOI:10.1371/journal.pbio.3002533` Sharma et al. 2024 imaged cargo-containing polar tubes during Vairimorpha necatrix germination.

Discussions

CURATION_TODO (OPEN): Resolve exact family groundings for polar tube proteins. This record models polar tube proteins as an aggregate constituent because known PTPs are lineage-specific and divergent. A follow-up pass should decide whether any exact InterPro, Pfam or NCBIfam families can be asserted for individual PTP classes without collapsing distinct proteins.

Evidence

GO:0044099 Gene Ontology cellular-component term for polar tube.
DOI:10.1016/j.ijpara.2005.04.003 Xu and Weiss 2005, The microsporidian polar tube: a highly specialised invasion organelle, International Journal for Parasitology.
DOI:10.1186/s13071-023-05908-9 Chen et al. 2023, The microsporidian polar tube: origin, structure, composition, function, and application, Parasites & Vectors.
DOI:10.1016/j.jprot.2022.104617 Lv et al. 2022, Proteomic profile of polar filament and polar tube from fungal pathogen microsporidium Nosema bombycis provides new insights into its unique invasion organelle, Journal of Proteomics.
DOI:10.1128/mbio.02749-23 Lv et al. 2024, Microsporidia dressing up: the spore polaroplast transport through the polar tube and transformation into the sporoplasm membrane, mBio.
DOI:10.1371/journal.pbio.3002533 Sharma et al. 2024, Ultrastructural insights into the microsporidian infection apparatus reveal the kinetics and morphological transitions of polar tube and cargo during host cell invasion, PLOS Biology.

Curation history

2026-09-10T13:40:23Z claude · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-10T13:41:30Z claude · EXPANDED_RECORD — Added polar filament synonyms, an aggregate polar tube protein component, Microsporidia and Nosema bombycis scope, host-invasion and sporoplasm-delivery functions, record evidence, a nonmechanistic eversion graph, and a PTP grounding TODO.
2026-09-10T13:42:02Z claude · TIGHTEN_EVIDENCE — Narrowed the Nosema bombycis canonical example to what the cited mBio paper directly shows and shifted the sporoplasm-conduction graph edge from polaroplast-membrane evidence to cryo-ET cargo evidence.
2026-09-10T13:50:21Z claude · ADDRESS_REVIEW — Addressed PR #374 review issues #375 and #376 by removing the source-local PT abbreviation and changing the Nosema bombycis canonical example to cite the polar filament/tube proteomics paper directly.

Source: data/structures/spore/polar_tube.yaml