chitosome

GO:0045009 · Membrane organelle · Membrane bounded organelle · Proposed

An intracellular membrane-bounded particle found in fungi and containing chitin synthase; it synthesizes chitin microfibrils. Chitin synthase activity exists in chitosomes and they are proposed to act as a reservoir for regulated transport of chitin synthase enzymes to the division septum. [GO:0045009]

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
chitosome membrane	Lipid	part of	Reviewed label only (reviewed name; no accession asserted)			ESSENTIAL	Lipid-bilayer boundary of the intracellular chitosome microvesicle that carries latent fungal chitin synthase. `GO:0045009` GO:0045009 defines the chitosome as an intracellular membrane-bounded fungal particle. `DOI:10.1073/pnas.85.22.8516` Leal-Morales et al. 1988 separated low-density Saccharomyces chitosome microvesicles from plasma membrane vesicles in cell-free homogenates.
fungal chitin synthases	Protein	part of	Reviewed label only (reviewed name; no accession asserted)			UNKNOWN	Latent membrane-enzyme cargo that is stored in chitosomes and delivered to the fungal cell surface for localized chitin microfibril synthesis. `GO:0045009` GO:0045009 defines chitosomes as fungal membrane-bounded particles containing chitin synthase and synthesizing chitin microfibrils. `DOI:10.1091/mbc.9.6.1565` Ziman et al. 1998 analyzed regulated Chs3p anterograde transport from the chitosome to the plasma membrane in Saccharomyces cerevisiae.

Taxonomic distribution

NCBITaxon:4751 Fungi — VARIABLE: Chitosome evidence spans multiple fungal lineages, but this first record treats them as documented fungal chitin-synthase vesicles rather than asserting a universal organelle across all fungi. [DOI:10.1111/j.1567-1364.2006.00158.x]

Canonical examples

NCBITaxon:4932 Saccharomyces cerevisiae — Budding-yeast model in which Chs6p-dependent Chs3p traffic from the chitosome to the plasma membrane was genetically dissected. [DOI:10.1091/mbc.9.6.1565]

Functions

chitin synthase storage and delivery — Chitosomes act as intracellular membrane-bounded carriers that store latent fungal chitin synthase and deliver it to cell-surface sites for localized chitin microfibril synthesis.
- GO:0045009 GO:0045009 describes chitosomes as proposed reservoirs for regulated chitin-synthase transport to the division septum.
- DOI:10.1091/mbc.9.6.1565 Ziman et al. 1998 tested Chs6p-dependent anterograde transport of Saccharomyces Chs3p from the chitosome to the plasma membrane.
fungal wall chitin microfibril synthesis — The chitosome-associated synthase pool supports synthesis of chitin microfibrils used in the fungal cell-wall matrix after enzyme delivery to the cell surface.
- DOI:10.1111/j.1567-1364.2006.00158.x Bartnicki-Garcia 2006 reviews chitosomes as microvesicular carriers of chitin-synthetase activity for fungal wall chitin microfibril construction.
- DOI:10.1083/jcb.136.1.95 Santos and Snyder 1997 linked Chs3p localization to polarized chitin synthesis sites in Saccharomyces vegetative and mating cells.

Related pathways

chitin biosynthesis (gomodel:YeastPathways_PWY3O-15) — SUPPORTS_ENZYME_DELIVERY; target taxon NCBITaxon:559292.
The S. cerevisiae exemplar stores and delivers Chs3p to the plasma membrane, where it contributes to chitin synthesis. The target is the narrower S288C model; this does not assert that chitin synthesis occurs inside the chitosome.
Basis: The primary Chs3p trafficking experiment supplies the structure-pathway connection; CHS3 is an explicit target-pathway participant.. PathwayMech revision: d8ec2b69f7a4c615aecddff80498fef1e1fab0ce.
- DOI:10.1091/mbc.9.6.1565 Ziman et al. 1998 show Chs6p-dependent transport of Chs3p from the chitosome to the plasma membrane in S. cerevisiae. Abstract read through Europe PMC on 2026-10-05.

Mechanism graphs

Chitosomes store and deliver chitin synthase (FUNCTION)

A chitosome is a fungal cytoplasmic vesicle that carries latent chitin synthase. In Saccharomyces, Chs3p traffics from the chitosome to the plasma membrane, where localized synthase activity contributes to chitin microfibril synthesis for the fungal cell wall.

Subject	Predicate	Object	Evidence
chitosome membrane	bounds	chitosome	`GO:0045009` GO:0045009 defines chitosomes as intracellular membrane-bounded particles. `DOI:10.1073/pnas.85.22.8516` Leal-Morales et al. 1988 resolved Saccharomyces chitosomes as low-density microvesicles distinct from plasma membrane vesicles.
fungal chitin synthases	are stored in	chitosome	`GO:0045009` GO:0045009 defines chitosomes by their chitin synthase cargo and chitin-microfibril synthesis activity. `DOI:10.1111/j.1567-1364.2006.00158.x` Bartnicki-Garcia 2006 reviews chitosomes as fungal reservoirs for latent chitin synthetase.
chitosome	delivers	fungal chitin synthases	`DOI:10.1091/mbc.9.6.1565` Ziman et al. 1998 tested Chs6p-dependent movement of Chs3p from the Saccharomyces chitosome to the plasma membrane. `DOI:10.1083/jcb.136.1.95` Santos and Snyder 1997 showed that polarized Chs3p targeting requires Chs5p and Myo2p.
chitosome	targets cargo to	cell septum	`GO:0045009` GO:0045009 names regulated transport of chitin synthase enzymes to the division septum in the chitosome definition.
fungal chitin synthases	synthesize	chitin	`GO:0045009` GO:0045009 states that chitosomes synthesize chitin microfibrils. `DOI:10.1111/j.1567-1364.2006.00158.x` Bartnicki-Garcia 2006 reviews chitosomes as vehicles for cell-surface construction of fungal chitin microfibrils.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral identifiers for the fungal chitin synthase cargo class and for any chitosome-specific membrane term before grounding these reviewed labels. GO provides an exact whole-chitosome term but no exact chitosome-membrane child term, and the chitosome literature spans several chitin synthase isoenzymes rather than one curated universal cargo family.
INTERPRETATION (RESOLVED): Keep chitin microfibrils as the synthesis product, not as resident chitosome components. Chitosomes are vesicular carriers for chitin synthase activity; chitin microfibrils are synthesized after that activity is deployed for fungal wall construction.

Evidence

GO:0045009 Gene Ontology cellular-component term for the chitosome.
GO:0031410 Gene Ontology cellular-component parent term for cytoplasmic vesicles.
DOI:10.1111/j.1567-1364.2006.00158.x Bartnicki-Garcia S 2006, Chitosomes: past, present and future, FEMS Yeast Research.
DOI:10.1073/pnas.85.22.8516 Leal-Morales CA, Bracker CE and Bartnicki-Garcia S 1988, Localization of chitin synthetase in cell-free homogenates of Saccharomyces cerevisiae: chitosomes and plasma membrane, Proceedings of the National Academy of Sciences.
DOI:10.1083/jcb.136.1.95 Santos B and Snyder M 1997, Targeting of Chitin Synthase 3 to Polarized Growth Sites in Yeast Requires Chs5p and Myo2p, Journal of Cell Biology.
DOI:10.1091/mbc.9.6.1565 Ziman M, Chuang JS, Tsung M, Hamamoto S and Schekman R 1998, Chs6p-dependent anterograde transport of Chs3p from the chitosome to the plasma membrane in Saccharomyces cerevisiae, Molecular Biology of the Cell.

Curation history

2026-09-23T03:32:33Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-23T03:34:19Z codex · EXPANDED_RECORD — Added cytoplasmic-vesicle parentage, chitosome membrane and reviewed-label-only chitin synthase components, chitosome synonymy, fungal scope, a Saccharomyces example, chitin-synthase delivery functions, DOI/GO evidence, a nonmechanistic delivery graph, and component-grounding and chitin product-boundary discussions.
2026-09-23T03:39:02Z codex · NORMALIZED_GRAPH_LABELS — Aligned grounded chitosome graph node labels with GO:0030428 and CHEBI:17029 canonical labels for the id-label correspondence gate.
2026-09-23T03:48:15Z codex · ADDRESS_REVIEW — Resolved PR #1349 review issues #1350 and #1351 by removing unsupported paraphrase synonyms and narrowing the Saccharomyces canonical example to the Ziman et al. Chs3p transport claim.
2026-10-05T17:45:01Z codex · ADD_PATHWAY_LINK — Added an evidence-backed, organism-scoped PathwayMech link with a full source commit and explicit relation limits.

Source: data/structures/membrane_organelle/chitosome.yaml