mucocyst

GO:0036000 · Membrane organelle · Membrane bounded organelle · Proposed

A small subcellular vesicle, surrounded by a membrane, in the pellicle of ciliate protozoans that discharges a mucus-like secretion. [GO:0036000]

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

Components

Component	Type	Part of / acts on	Grounding	Genes	Essential	Role
mucocyst membrane	Lipid	part of	Reviewed label only (reviewed name; no accession asserted)		ESSENTIAL	Membrane that bounds the Tetrahymena mucocyst and encloses its regulated secretory cargo. `DOI:10.1534/g3.116.028878` Kontur et al. 2016 describe Tetrahymena mucocysts as secretory granules that undergo a maturation process. `DOI:10.1371/journal.pgen.1010194` Kuppannan et al. 2022 analyze Tetrahymena mucocysts as lysosome-related organelles that dock at the cell periphery and undergo regulated exocytosis.
granule lattice proteins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	GRL	UNKNOWN	Major dense-core cargo proteins that coassemble into the crystalline lattice of the mucocyst lumen. `DOI:10.1128/EC.00151-09` Rahaman et al. 2009 distinguish Grl proteins as Tetrahymena dense-core granule proteins that coassemble into the lumenal crystalline lattice. `DOI:10.1534/g3.116.028878` Kontur et al. 2016 connect mucocyst maturation defects to altered processing of granule cargo proteins.
granule tip proteins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	GRT	UNKNOWN	Dense-core granule cargo proteins that occupy a mucocyst tip domain and are transported independently from Grl lattice cargo. `DOI:10.1128/EC.00151-09` Rahaman et al. 2009 analyze Tetrahymena Grt1p and Grt2p as dense-core granule proteins distinct from the Grl lattice-protein cargo class.
MDD mucocyst docking and discharge complex	Protein complex	acts on	Reviewed label only (reviewed name; no accession asserted)	MDL1	UNKNOWN	Mucocyst-associated membrane complex that localizes to the organelle tip and is required for efficient docking and regulated exocytosis. `DOI:10.1371/journal.pgen.1010194` Kuppannan et al. 2022 identify Mdl1p and associated MDD proteins as a Tetrahymena mucocyst membrane complex needed for organelle docking and regulated exocytosis.

Taxonomic distribution

NCBITaxon:5911 Tetrahymena thermophila — COMMON: Mucocysts are dense-core regulated secretory organelles characterized in Tetrahymena thermophila. [DOI:10.1534/g3.116.028878]

Canonical examples

NCBITaxon:5911 Tetrahymena thermophila — Ciliate model in which mucocyst dense-core cargo sorting, maturation and docking have been studied genetically and cell biologically. [DOI:10.1534/g3.116.028878]

Functions

regulated mucocyst exocytosis — Docked mucocysts release dense-core cargo from the Tetrahymena cell surface in response to extracellular stimulation.
- DOI:10.1371/journal.pgen.1010194 Kuppannan et al. 2022 link the MDD complex to docking and regulated exocytosis of Tetrahymena mucocysts.
- DOI:10.1128/EC.00151-09 Rahaman et al. 2009 describe Grl lattice expansion as a propulsive mechanism during dense-core granule release.

Mechanism graphs

Mucocyst core proteins mature before docking and discharge (FUNCTION)

A membrane-bounded Tetrahymena mucocyst stores Grl lattice and Grt tip proteins as dense-core cargo; the MDD membrane complex localizes to the mucocyst tip and supports docking and regulated exocytosis.

Subject	Predicate	Object	Evidence
mucocyst membrane	bounds	mucocyst	`GO:0036000` The GO mucocyst definition describes the structure as a membrane-surrounded vesicle.
mucocyst lumen	is part of	mucocyst	`DOI:10.1128/EC.00151-09` Rahaman et al. 2009 describe the crystalline Grl lattice inside Tetrahymena dense-core granules.
granule lattice proteins	coassemble into	mucocyst crystalline lattice	`DOI:10.1128/EC.00151-09` Rahaman et al. 2009 distinguish the Grl class as cargo that coassembles into a crystalline dense-core granule lattice.
mucocyst crystalline lattice	is stored in	mucocyst lumen	`DOI:10.1128/EC.00151-09` Rahaman et al. 2009 describe Grl proteins as forming a crystalline lattice within the Tetrahymena dense-core granule lumen.
granule tip proteins	sort independently to	mucocyst	`DOI:10.1128/EC.00151-09` Rahaman et al. 2009 show that Grt and Grl proteins are likely to be independently transported during Tetrahymena dense-core granule biosynthesis.
MDD mucocyst docking and discharge complex	localizes to tip of	mucocyst	`DOI:10.1371/journal.pgen.1010194` Kuppannan et al. 2022 found that Mdl1p and other putative MDD complex members localize to Tetrahymena mucocysts.
MDD mucocyst docking and discharge complex	supports	regulated exocytosis	`DOI:10.1371/journal.pgen.1010194` Kuppannan et al. 2022 connect Mdl1p and MDD complex members to mucocyst docking and discharge.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral family or complex groundings for the mucocyst membrane, Grl proteins, Grt proteins and MDD complex. The first mucocyst record curates the membrane, major dense-core cargo cohorts and the docking/discharge complex from Tetrahymena studies, but exact cross-taxon CURIEs for those local component classes were not curated.
INTERPRETATION (RESOLVED): Tetrahymena maturation factors such as Mma1p, AP-3 and endosomal syntaxin were not modeled as mature mucocyst components. Mma1p and membrane-trafficking factors can be required for mucocyst biogenesis, but the first record only models cargo or machinery associated with mature docked or discharge-competent mucocysts. Proteins that act upstream in sorting or maturation should be added only if evidence places them in the mature organelle or at the assembled organelle boundary.

Evidence

GO:0036000 Gene Ontology cellular-component term for mucocyst.
DOI:10.1534/g3.116.028878 Kontur et al. 2016, Whole Genome Sequencing Identifies a Novel Factor Required for Secretory Granule Maturation in Tetrahymena thermophila, G3 Genes|Genomes|Genetics.
DOI:10.1128/EC.00151-09 Rahaman et al. 2009, Independent Transport and Sorting of Functionally Distinct Protein Families in Tetrahymena thermophila Dense Core Secretory Granules, Eukaryotic Cell.
DOI:10.1371/journal.pgen.1010194 Kuppannan et al. 2022, A novel membrane complex is required for docking and regulated exocytosis of lysosome-related organelles in Tetrahymena thermophila, PLOS Genetics.
DOI:10.1091/mbc.e17-01-0018 Kaur et al. 2017, An endosomal syntaxin and the AP-3 complex are required for formation and maturation of candidate lysosome-related secretory organelles (mucocysts) in Tetrahymena thermophila, Molecular Biology of the Cell.

Curation history

2026-09-12T04:24:49Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-12T04:25:53Z codex · EXPANDED_RECORD — Added secretory-granule parentage, mucocyst membrane, Grl, Grt and MDD components, Tetrahymena thermophila taxonomic scope and canonical example, regulated-exocytosis function, a mucocyst docking/discharge graph, and reviewed-label-only component grounding discussions.
2026-09-12T04:30:37Z codex · FIXED_DISCUSSION_ANCHOR — Retargeted the mucocyst maturation-factor boundary discussion from a section-level components anchor to the MDD complex component.
2026-09-12T04:34:15Z codex · ADDRESSED_PR_REVIEW — Addressed PR #579 review issues #580, #581 and #582 by retargeting the Grl coassembly edge to a crystalline-lattice node, removing the MDD abbreviation from gene_symbols, and adding direct Kaur 2017 evidence for AP-3/endosomal syntaxin boundary exclusions.

Source: data/structures/membrane_organelle/mucocyst.yaml