Stentor Km fiber

cellstructuremech:stentor_km_fiber · Cytoskeleton · Other · Proposed

A longitudinal cortical bundle of overlapping microtubule ribbons in Stentor, characterized ultrastructurally in S. coeruleus. Ribbons originate beside successive basal-body pairs and overlap alongside a ciliary row; the bundle is distinct from the deeper contractile myonemes. [DOI:10.1083/jcb.57.3.704]

Image (not hosted — CC_BY_NC_SA_4_0): PMC2108994.1 Figure 8

Figure 8: transverse section of a Km fiber in an extended Stentor coeruleus cell, with a microtubule-numbering diagram. Scale bar: 0.1 micrometer. TEM · Stentor coeruleus (NCBITaxon:5963) · B. Huang and D. R. Pitelka, Figure 8, The contractile process in the ciliate, Stentor coeruleus. I. The role of microtubules and filaments, Journal of Cell Biology 57 (1973), doi:10.1083/jcb.57.3.704, via PubMed Central; CC BY-NC-SA 4.0. Link only; no repository copy or adaptation. · CC_BY_NC_SA_4_0 · source: PMC PMC2108994.1 Figure 8 · DOI:10.1083/jcb.57.3.704

Synonyms: km fibers

Part of: GO:0030863 — cortical cytoskeleton (no local record)

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
Alpha tubulin	Protein	part of	`InterPro:IPR002452`			UNKNOWN	Tubulin subunit of the microtubules forming the ribbons. `DOI:10.1371/journal.pbio.1001861` Tubulin RNAi Results and Figure 2D-E identify cortical microtubule bundles by anti-alpha-tubulin staining. `InterPro:IPR002452` Authority entry verifies the Alpha tubulin family and alpha/beta microtubule composition.
Beta tubulin	Protein	part of	`InterPro:IPR002453`			UNKNOWN	Partner tubulin subunit; depletion disrupts the normal cortical microtubule rows, without defining a minimal Km-fiber assembly requirement. `DOI:10.1371/journal.pbio.1001861` RNAi Results and Figure 2 show beta-tubulin transcript depletion and disorganized tubulin-positive cortical rows; this is not a Km-specific knockout. `InterPro:IPR002453` Authority entry verifies the Beta tubulin family and its role in microtubules.

Canonical examples

NCBITaxon:5963 Stentor coeruleus — Experimental species for the fixed-state ultrastructural comparison. ENA verifies the species identifier; no genus-wide prevalence is asserted. [DOI:10.1083/jcb.57.3.704]

Functions

Cell re-extension — Implicated in cell re-extension through relative displacement of overlapping ribbons; a bridge-driven motor cycle remains a model.
- DOI:10.1083/jcb.57.3.704 Results and Discussion compare contracted and relaxed cells. Reduced ribbon overlap accompanies extension; selective disruption of the two fiber systems was unsuccessful.

Mechanism graphs

Nonmechanistic summary of beta-tubulin RNAi effects on cortical rows (ASSEMBLY)

Subject	Predicate	Object	Evidence
Beta-tubulin RNAi	produces	Disorganized cortical microtubule rows	`DOI:10.1371/journal.pbio.1001861` Figure 2D-E and RNAi Results. Nonmechanistic intervention summary: tubulin depletion is not specific to Km fibers and does not isolate their contribution to whole-cell shape or establish the native extension motor.

Discussions

CURATION_TODO (RESOLVED): Distinguish the Km bundle from a single microtubule, myoneme or striated rootlet. Basal-body association and similar fiber names do not make different cortical assemblies equivalent.
CURATION_TODO (OPEN): Identify the molecular bridges and test how Km fibers generate extension force. Morphological bridges do not identify a motor protein or prove an ATPase cycle.
CURATION_TODO (RESOLVED): Keep neighboring organelles and broadly localized regulators outside the constituent list. Cortical localization alone does not establish membership of the Km bundle.

Evidence

DOI:10.1083/jcb.57.3.704 Huang and Pitelka (1973), PMID 4633444. Primary PDF Methods, Km-fiber Results, Discussion and Figure 8 inspected. Fixed-state comparisons support sliding; molecular force generation remains unresolved.
DOI:10.1371/journal.pbio.1001861 Slabodnick et al. (2014). Publisher tubulin-RNAi Results, Figure 2 legend, and RNAi, antibody and immunofluorescence Methods read. No Mob1 phenotype is attributed to tubulin.
GO:0097427 Verified non-obsolete microtubule bundle: broader is-a parent, not exact identity.
GO:0030863 — cortical cytoskeleton Verified non-obsolete cortical cytoskeleton: encompassing framework, used as part_of.

Curation history

2026-10-04T01:16:22Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-10-04T01:17:38Z codex · EXPANDED_RECORD — Added verified tubulin families, species example, bounded extension function, nonmechanistic RNAi graph, identity and motor limits, and inspected link-only Figure 8.

Source: data/structures/cytoskeleton/stentor_km_fiber.yaml