acidocalcisome

GO:0020022 · Membrane organelle · Membrane bounded organelle · Proposed

An acidic electron-dense membrane-bounded organelle found in diverse microbes that stores polyphosphate, pyrophosphate, calcium and other cations and uses proton pumps and cation transporters to maintain its ion-rich matrix. [GO:0020022]

Synonyms: metachromatic granule; volutin granule; polyphosphate vacuole

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

Components

Component	Type	Part of / acts on	Grounding	Essential	Role
polyphosphate	Small molecule	part of	`CHEBI:16838`	ESSENTIAL	Polyanionic phosphate polymer that chelates calcium and other cations in the electron-dense acidocalcisome core. `DOI:10.1038/nrmicro1097` Docampo et al. 2005 review polyphosphate and pyrophosphate as major anionic stores in acidocalcisomes. `DOI:10.1074/jbc.M304548200` Seufferheld et al. 2003 detected phosphorus-rich volutin granules with acidocalcisome-like properties in Agrobacterium tumefaciens.
bound calcium and magnesium cations	Small molecule	part of		ESSENTIAL	Divalent cations complexed with polyphosphate in the acidocalcisome matrix. `DOI:10.1038/nrmicro1097` Docampo et al. 2005 review calcium and other cations as conserved matrix constituents of acidocalcisomes. `DOI:10.1074/jbc.M304548200` Seufferheld et al. 2003 analysed acidocalcisome-like bacterial granules as calcium- and polyphosphate-rich compartments.
acidocalcisome membrane	Lipid	part of		ESSENTIAL	Membrane boundary that encloses the acidic polyphosphate and cation store. `DOI:10.1074/jbc.M304548200` Seufferheld et al. 2003 reported membrane-bounded acidocalcisome-like volutin granules in bacteria. `DOI:10.1128/MMBR.00042-23` Docampo and Huang 2024 review acidocalcisomes as membrane-bound polyphosphate organelles across bacteria and eukaryotes.
acidocalcisome proton pumps	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	ESSENTIAL	Membrane pumps that acidify the organelle lumen and support ion uptake. `DOI:10.1038/nrmicro1097` Docampo et al. 2005 review proton pumps as acidocalcisome membrane components. `DOI:10.1074/jbc.M304548200` Seufferheld et al. 2003 localized vacuolar proton pyrophosphatase activity to bacterial acidocalcisome-like organelles.
acidocalcisome cation transporters	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	UNKNOWN	Calcium and cation transporters that move ions across the acidocalcisome membrane. `DOI:10.1128/MMBR.00042-23` Docampo and Huang 2024 review cation transport systems of acidocalcisome membranes.

Taxonomic distribution

NCBITaxon:2 Bacteria — VARIABLE: Acidocalcisome-like volutin granules have been characterized in bacteria including Agrobacterium tumefaciens and Rhodospirillum rubrum. [DOI:10.1038/nrmicro1097]
NCBITaxon:2759 Eukaryota — VARIABLE: Acidocalcisomes are widespread in diverse microbial eukaryotes and metazoan cell types but are not universal across all eukaryotes. [DOI:10.1128/MMBR.00042-23]

Canonical examples

NCBITaxon:358 Agrobacterium tumefaciens — Bacterial model in which volutin granules were shown to have acidocalcisome-like membrane, cation and polyphosphate properties. [DOI:10.1074/jbc.M304548200]

Functions

polyphosphate storage — Acidocalcisomes store a dense polyphosphate and pyrophosphate matrix that buffers phosphate and bound cations.
- DOI:10.1038/nrmicro1097 Docampo et al. 2005 review acidocalcisomes as polyphosphate-rich organelles.
calcium storage — The organelle stores calcium and other cations complexed with polyphosphate in an acidic lumen.
- DOI:10.1128/MMBR.00042-23 Docampo and Huang 2024 review the ion storage roles of acidocalcisomes.

Mechanism graphs

Proton pumps and polyphosphate build an acidic cation store (FUNCTION)

The acidocalcisome membrane bounds a polyphosphate-rich matrix. Proton pumps acidify the lumen, and the anionic polyphosphate matrix chelates calcium and other cations to create an electron-dense storage compartment.

Subject	Predicate	Object	Evidence
acidocalcisome membrane	bounds	acidocalcisome	`DOI:10.1074/jbc.M304548200` Seufferheld et al. 2003 reported membrane-bounded acidocalcisome-like organelles in bacteria.
acidocalcisome proton pumps	acidify	acidic acidocalcisome lumen	`DOI:10.1038/nrmicro1097` Docampo et al. 2005 review V-H+-PPase and V-H+-ATPase activities in acidocalcisome membranes.
polyphosphate	accumulates in	acidocalcisome	`DOI:10.1074/jbc.M304548200` Seufferheld et al. 2003 characterized bacterial volutin granules as phosphorus-rich acidocalcisome-like organelles.
polyphosphate	chelates	bound calcium and magnesium cations	`DOI:10.1128/MMBR.00042-23` Docampo and Huang 2024 review cations bound to acidocalcisome polyphosphate stores.
acidic acidocalcisome lumen	stabilizes	polyphosphate	`DOI:10.1038/nrmicro1097` Docampo et al. 2005 describe acidity and polyphosphate-rich ion storage as defining acidocalcisome properties.

Discussions

CURATION_TODO (OPEN): Resolve exact bacterial and eukaryotic acidocalcisome proton-pump and cation-transporter families rather than grouping all acidification and ion-transport proteins together. The first record captures the transport roles required to maintain an acidic cation store, but V-H+-PPase, V-H+-ATPase and calcium transporter complements vary by lineage.

Evidence

DOI:10.1038/nrmicro1097 Docampo R, de Souza W, Miranda K, Rohloff P and Moreno SNJ 2005, Acidocalcisomes - conserved from bacteria to man, Nature Reviews Microbiology.
DOI:10.1074/jbc.M304548200 Seufferheld et al. 2003, Identification of organelles in bacteria similar to acidocalcisomes of unicellular eukaryotes, Journal of Biological Chemistry.
DOI:10.1128/MMBR.00042-23 Docampo and Huang 2024 review acidocalcisome cell biology, biochemistry and physiological roles.

Curation history

2026-09-04T19:15:51Z claude · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-04T19:16:22Z claude · EXPANDED_RECORD — Added acidocalcisome synonyms, components, taxonomy, examples, functions, evidence, a polyphosphate-cation-storage graph, and a transporter-grounding todo.
2026-09-04T19:18:54Z claude · FIXED_GROUNDING_LABEL — Aligned CHEBI:16838 and GO:0006797 display labels with ontology canonicals.
2026-09-04T19:27:49Z claude · FIXED_FUNCTION_GROUNDING — Removed the overbroad GO:0006797 function grounding from acidocalcisome polyphosphate storage.

Source: data/structures/membrane_organelle/acidocalcisome.yaml