distal connecting fiber

cellstructuremech:distal_connecting_fiber · Cytoskeleton · Other · Proposed

A centrin-containing fibrous bridge connecting the distal regions of paired flagellar basal bodies in green algae. It is distinct from proximal connecting fibers and nucleus-directed rootlets; cross-striation differs between the documented examples. [DOI:10.1083/jcb.105.2.903]

Image (not hosted — CC_BY_NC_SA_4_0): PMC2114749.1 Figure 9

Figure 9: distal-fiber immunogold localization and basal-body geometry in Spermatozopsis similis. Panels B-E show labeled distal fibers; F shows the separate nucleus connector and G the preimmune control. Source scale bars: 300 nm. TEM · Spermatozopsis similis (NCBITaxon:3192) · G. I. McFadden, D. Schulze, B. Surek, J. L. Salisbury and M. Melkonian, Figure 9, Journal of Cell Biology 105 (1987), doi:10.1083/jcb.105.2.903, via PubMed Central; CC BY-NC-SA 4.0. Link only; no repository copy or adaptation. · CC_BY_NC_SA_4_0 · source: PMC PMC2114749.1 Figure 9 · DOI:10.1083/jcb.105.2.903

Synonyms: distal basal body connecting fiber; distal striated fiber; DSF; distal fiber

Part of: GO:0005856 — 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
Distal connecting fiber centrins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)			UNKNOWN	Calcium-binding constituents of the distal fiber. Centrin sequence substitutions affect normal distal-fiber assembly in the tested Chlamydomonas strains; this is not a universal essentiality assertion. `DOI:10.1083/jcb.105.2.903` Anti-20-kDa-protein immunogold labels the distal connecting fiber of Spermatozopsis similis; apparatus immunoblots detect the antigen. No exact Spermatozopsis sequence is assigned. `DOI:10.1083/jcb.119.6.1613` E101K centrin and position-101 revertants alter normal distal striated fiber formation. Class-2 revertants retain abnormal half fibers despite normal nucleus-basal body connectors and transition-region structures.

Organism-specific protein examples

Named proteins in which each component's role was established, with the organism the evidence comes from. The component above is the taxon-agnostic family; these are the specific entries behind it.

Distal connecting fiber centrins

Accession	Protein	Gene	Organism	Entry
`UniProtKB:P05434`	Caltractin		Chlamydomonas reinhardtii (`NCBITaxon:3055`)	REVIEWED
Role and evidence (2 references) Species-level centrin example, not a distal-fiber-specific paralogue or an entire fiber inventory. `UniProtKB:P05434` Verified reviewed entry, recommended name, organism and sequence-source references; no primary gene symbol is supplied. `DOI:10.1083/jcb.119.6.1613` The centrin/caltractin sequence and vfl2 linkage identify the studied protein; Methods use pCaBP4 and the sequence publications also cited by UniProt. Mutants and intragenic revertants distinguish distal-fiber assembly phenotypes.

Canonical examples

NCBITaxon:3192 Spermatozopsis similis — SAG 1.85 is explicitly used in Methods and was verified against the culture collection. The distal fiber is unstriated in this example; extracted-apparatus reorientation and immunogold localization were studied. [DOI:10.1083/jcb.105.2.903]
NCBITaxon:3055 Chlamydomonas reinhardtii — Wild-type, vfl2 and intragenic-revertant comparisons resolve the cross-striated distal fiber from other centrin assemblies. No collection accession is inferred from the mutant names. [DOI:10.1083/jcb.119.6.1613]

Functions

interbasal-body linkage — The distal fiber forms a physical bridge between paired flagellar basal bodies, separately from the proximal fibers and nucleus-directed connectors.
- DOI:10.1083/jcb.105.2.903 Ultrastructure and Figure 9 distinguish the interbasal bridge from adjacent root structures.
- DOI:10.1083/jcb.119.6.1613 Distal half-fiber assembly and continuity between the basal bodies are assessed separately from other centrin structures.
calcium-dependent basal-body reorientation — Distal-fiber contraction reorients paired basal bodies in Spermatozopsis similis. Chlamydomonas preparations show smaller, reversible calcium-dependent angle changes; the two experimental responses are not treated as identical.
- DOI:10.1083/jcb.105.2.903 Reorientation persists when beating is blocked or axonemes are mechanically removed, supporting a separate contractile apparatus.
- DOI:10.1002/(SICI)1097-0169(1998)41:1<49::AID-CM4>3.0.CO;2-A Primary abstract reports calcium-dependent interaxonemal-angle changes in pf18 and pf18fa1 preparations and lack of that response in pf18vfl2. Full paper unread; vfl2 is not a selective distal-fiber perturbation.

Mechanism graphs

Calcium-driven distal-fiber contraction in extracted Spermatozopsis apparatuses (FUNCTION)

A physical contractile mechanism, with constituent and geometry context. Elevated calcium condenses the fiber and reorients basal bodies; attached axonemes change their relative orientation. This does not require ongoing beating and does not establish the molecular force-generating cycle.

Subject	Predicate	Object	Evidence
Distal connecting fiber centrins	are constituents of	Spermatozopsis distal connecting fiber	`DOI:10.1083/jcb.105.2.903` Figure 9 and immunoblot results; no complete molecular inventory or stoichiometry is implied.
Spermatozopsis distal connecting fiber	undergoes calcium-induced	Distal-fiber contraction	`DOI:10.1083/jcb.105.2.903` Figure 9 contrasts expanded and condensed fibers in the extracted apparatus.
Elevated calcium in the extracted-apparatus assay	triggers	Distal-fiber contraction	`DOI:10.1083/jcb.105.2.903` Results, Figures 3-5 and 9; lack of added ATP is not proof of an ATP-independent complete contraction-extension cycle.
Distal-fiber contraction	reorients	Basal bodies reoriented toward parallel	`DOI:10.1083/jcb.105.2.903` Orthovanadate, DTT/ATP-omission and mechanical-amputation experiments support separation from axonemal motility.
Basal bodies reoriented toward parallel	repositions attached axonemes into	Parallel projection of attached axonemes	`DOI:10.1083/jcb.105.2.903` Figures 3-5 and Results distinguish axonemal geometry from beating. No requirement for the fiber to initiate flagellar beating is asserted.

Discussions

CURATION_TODO (RESOLVED): Keep the interbasal distal fiber separate from proximal fibers and nucleus-directed rootlets. Shared centrin and adjacent attachment sites do not establish equivalent structures.
CURATION_TODO (OPEN): Resolve exact centrin-family grounding and additional distal-fiber constituents. Immunoreactivity and one verified species sequence do not establish a complete cross-taxon inventory.
CURATION_TODO (OPEN): Resolve native force generation and species-specific reextension mechanisms. Preparations differ in reversibility and do not identify every biochemical intermediate.
CURATION_TODO (OPEN): Reconcile experimental conditions and isolate the distal fiber's contribution to flagellar coordination. Multistructure mutants cannot by themselves establish a distal-fiber-specific necessity or absence of function.
CURATION_TODO (RESOLVED): Do not transfer the entire vfl2 segregation phenotype to the distal fiber. The centrin mutation affects several distinct assemblies.
CURATION_TODO (OPEN): Extend directly demonstrated taxonomic scope and resolve an exact reorientation function term. Two studied species do not establish clade-wide prevalence or identical geometry.

Evidence

DOI:10.1083/jcb.105.2.903 Full ten-page primary PDF text read; Figure 9 visually inspected. Identity, fiber-localized antigen and calcium-triggered mechanical response.
DOI:10.1083/jcb.119.6.1613 Primary Methods, mutant/revertant Results and Table II inspected; normal distal-fiber assembly is separable from other centrin assemblies.
DOI:10.1083/jcb.98.3.818 Full seven-page primary PDF text read. Pleiotropic vfl3 comparisons preserve the study-specific negative results on waveform, synchrony and shock response.
DOI:10.1002/(SICI)1097-0169(1998)41:1<49::AID-CM4>3.0.CO;2-A Complete primary abstract read through Europe PMC; full paper and figures unread. Chlamydomonas angle reversibility is not transferred to Spermatozopsis.
DOI:10.1073/pnas.1518527113 Primary abstract and significance statement read through NCBI EFetch. Full text unavailable through attempted PMC web, Europe PMC and current PMC OA routes; later coupling evidence remains abstract-bounded.
GO:0043232 Verified intracellular membraneless organelle as a broad is-a parent, not exact identity.
GO:0005856 — cytoskeleton Verified cytoskeleton term for parthood, not equivalence to the whole framework.
https://sagdb.uni-goettingen.de/detailedList.php?str_number=1.85 Authority confirms SAG 1.85 as Spermatozopsis similis; strain label agrees with the 1987 Methods.

Curation history

2026-10-04T04:19:00Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-10-04T04:20:58Z codex · EXPANDED_RECORD — Added distinct distal-fiber identity, centrin composition and verified protein example, two canonical species, bounded linkage/reorientation functions, an extracted-apparatus graph, link-only micrograph and explicit mutant, synchronization and contractile-cycle limitations.

Source: data/structures/cytoskeleton/distal_connecting_fiber.yaml