microneme

GO:0020009 · Membrane organelle · Membrane bounded organelle · Proposed

A small, elongated secretory organelle that forms part of the apical complex, located along the main axis of an apicomplexan parasite cell within the extreme apical region and at the periphery under the inner membrane complex. Of the specialized secretory compartments identified in apicomplexans, micronemes discharge their contents first, during initial contact of the parasite's apical pole with the host cell surface. Micronemal proteins function during parasite attachment and penetration into the target cell. [GO:0020009]

Synonyms: sarconeme

Part of: apical complex (GO:0020007)

Has part: microneme membrane (GO:0033163); microneme lumen (GO:0034494)

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

Components

Component	Type	Part of / acts on	Grounding	Essential	Role
microneme membrane	Lipid	part of	`GO:0033163`	ESSENTIAL	Lipid bilayer that bounds the microneme and encloses its secretory lumen. `DOI:10.1111/cmi.13018` Dubois and Soldati-Favre 2019 review Toxoplasma microneme biogenesis and exocytosis.
microneme adhesins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	UNKNOWN	Secreted adhesive proteins that are released from micronemes during host-cell attachment and invasion. `DOI:10.1007/978-0-387-78267-6_2` Carruthers and Tomley 2008 review apicomplexan microneme proteins used for motility and invasion. `PMID:9208224` Carruthers and Sibley 1997 observed microneme secretion during Toxoplasma invasion of fibroblasts.
MIC2-M2AP adhesive protein complex	Protein complex	part of	Reviewed label only (reviewed name; no accession asserted)	UNKNOWN	Toxoplasma microneme adhesin complex required for efficient attachment and rapid host-cell invasion. `DOI:10.1093/emboj/cdg217` Huynh et al. 2003 connected secretion of the MIC2-M2AP adhesive complex to rapid host-cell invasion. `DOI:10.1371/journal.ppat.0020084` Huynh and Carruthers 2006 established Toxoplasma MIC2 as a major determinant of invasion and virulence.

Taxonomic distribution

NCBITaxon:2759 Eukaryota — RARE: Micronemes are apical-complex secretory organelles of apicomplexan microbial eukaryotes. [DOI:10.1007/978-0-387-78267-6_2]
NCBITaxon:5794 Apicomplexa — COMMON: Apicomplexans use micronemes as early-discharging apical secretory organelles for attachment and invasion. [DOI:10.1007/978-0-387-78267-6_2]

Canonical examples

NCBITaxon:5811 Toxoplasma gondii — Apicomplexan model in which sequential microneme secretion, the MIC2-M2AP complex, and PKG-regulated exocytosis have been experimentally studied. [PMID:9208224]

Functions

regulated microneme exocytosis — Micronemes discharge adhesins in a regulated secretion step that precedes host-cell invasion.
- PMID:9208224 Carruthers and Sibley 1997 showed microneme secretion occurs early during Toxoplasma host-cell invasion.
- DOI:10.1074/jbc.m115.700518 Brown et al. 2016 connected serum albumin to PKG-dependent microneme secretion in Toxoplasma gondii.
microneme-mediated host cell invasion — Secreted microneme adhesins support parasite attachment, motility, and invasion of host cells.
- DOI:10.1093/emboj/cdg217 Huynh et al. 2003 showed that the MIC2-M2AP adhesive protein complex is required for rapid host-cell invasion.
- DOI:10.1371/journal.ppat.0020084 Huynh and Carruthers 2006 showed that MIC2 contributes to Toxoplasma invasion and virulence.

Mechanism graphs

Regulated microneme exocytosis releases adhesins for invasion (FUNCTION)

A microneme membrane encloses the secretory lumen; signaling-triggered exocytosis releases microneme adhesins, including the MIC2-M2AP complex, that support host-cell attachment and invasion.

Subject	Predicate	Object	Evidence
microneme membrane	bounds	microneme	`DOI:10.1111/cmi.13018` Dubois and Soldati-Favre 2019 review microneme biogenesis and secretory organelle architecture.
microneme lumen	is part of	microneme	`DOI:10.1111/cmi.13018` Dubois and Soldati-Favre 2019 review microneme biogenesis and exocytosis.
microneme adhesins	reside in	microneme lumen	`DOI:10.1007/978-0-387-78267-6_2` Carruthers and Tomley 2008 review apicomplexan microneme adhesins.
MIC2-M2AP adhesive protein complex	resides in	microneme lumen	`DOI:10.1093/emboj/cdg217` Huynh et al. 2003 studied MIC2-M2AP as a microneme adhesive protein complex.
PKG-dependent signaling	triggers	microneme exocytosis	`DOI:10.1074/jbc.m115.700518` Brown et al. 2016 found that serum albumin stimulates PKG-dependent microneme secretion.
microneme exocytosis	releases	microneme adhesins	`PMID:9208224` Carruthers and Sibley 1997 showed microneme secretion accompanies the first phase of Toxoplasma invasion.
MIC2-M2AP adhesive protein complex	mediates	host cell invasion	`DOI:10.1093/emboj/cdg217` Huynh et al. 2003 showed that secretion of the MIC2-M2AP complex is required for rapid host-cell invasion.
microneme adhesins	support	host cell invasion	`DOI:10.1007/978-0-387-78267-6_2` Carruthers and Tomley 2008 review microneme adhesins as apicomplexan proteins involved in motility and invasion.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral groundings for microneme adhesins and the MIC2-M2AP adhesive protein complex. The first record uses microneme-localized adhesive protein cohorts because the cited proteins span multiple unrelated adhesin families and no exact cross-source complex CURIE was curated for MIC2-M2AP.

Evidence

PMID:9208224 Carruthers and Sibley 1997, Sequential protein secretion from three distinct organelles of Toxoplasma gondii accompanies invasion of human fibroblasts, European Journal of Cell Biology.
DOI:10.1007/978-0-387-78267-6_2 Carruthers and Tomley 2008, Microneme proteins in apicomplexans, Subcellular Biochemistry.
DOI:10.1111/cmi.13018 Dubois and Soldati-Favre 2019, Biogenesis and secretion of micronemes in Toxoplasma gondii, Cellular Microbiology.
DOI:10.1093/emboj/cdg217 Huynh et al. 2003, Rapid invasion of host cells by Toxoplasma requires secretion of the MIC2-M2AP adhesive protein complex, EMBO Journal.
DOI:10.1371/journal.ppat.0020084 Huynh and Carruthers 2006, Toxoplasma MIC2 is a major determinant of invasion and virulence, PLOS Pathogens.
DOI:10.1074/jbc.m115.700518 Brown et al. 2016, Serum albumin stimulates protein kinase G-dependent microneme secretion in Toxoplasma gondii, Journal of Biological Chemistry.

Curation history

2026-09-10T04:51:36Z claude · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-10T04:52:37Z claude · EXPANDED_RECORD — Added apical-complex parthood, microneme membrane/lumen parts, membrane, adhesin and MIC2-M2AP component cohorts, rare eukaryotic and common Apicomplexa taxonomic scope, Toxoplasma gondii as a canonical example, regulated-exocytosis and host-invasion functions, a microneme exocytosis graph, and a grounding TODO for microneme adhesin cohorts.
2026-09-10T04:53:15Z claude · FIXED_CAUSAL_GRAPH — Connected the MIC2-M2AP adhesive protein complex to the microneme lumen in the microneme exocytosis graph.
2026-09-10T05:04:01Z claude · FIXED_EVIDENCE_SCOPE — Narrowed the microneme secretion signal node to PKG-dependent signaling and cited broad review evidence on the generic microneme-adhesin invasion edge.
2026-09-20T21:58:46Z codex · ADDRESSED_REVIEW — Retyped the microneme lumen graph node as CELLULAR_LOCALIZATION.

Source: data/structures/membrane_organelle/microneme.yaml