Affinage · Toxoplasma gondii

MIC4

microneme protein MIC4 · UniProt A0A125YP11

Annotated
2026-09-08
34 papers in source corpus 13 papers cited in narrative 13 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

TgMIC4 is a secreted, multi-apple-domain adhesin of Toxoplasma gondii that localizes to the micronemes of all invasive forms and functions both as a host-cell attachment factor during invasion and as a modulator of host innate immunity (PMID:11053441, PMID:22399295). It is delivered to micronemes as a passive cargo of the trimeric MIC4-MIC1-MIC6 complex: MIC4 binds directly to MIC1, while MIC6 provides the cytoplasmic sorting signal and MIC1 serves as a quality-control checkpoint that permits the complex to exit early secretory compartments and reach the organelle (PMID:11157983, PMID:16166092). Host recognition is mediated by a galactose-specific carbohydrate recognition domain in the fifth apple domain, which binds galactose-terminal glycans and is proteolytically cleaved from the complex during invasion (PMID:22399295). Upon secretion onto the parasite surface the complex is trimmed at its N- and C-termini by the GPI-anchored subtilisin TgSUB1, a processing step that supports efficient attachment, gliding, invasion, and virulence (PMID:11053441, PMID:20678172). Beyond invasion, MIC4's carbohydrate recognition domain engages N-glycans on host TLR2 and TLR4 to drive innate immune signaling: TLR2 engagement activates a TAK1/p38/NF-κB axis producing IL-12, TNF-α and IL-6, TLR4 endocytosis drives IL-10 and LPS tolerance, and CRD point mutations abolish this activation (PMID:31226171, PMID:31658592, PMID:33912181). MIC4 also binds T lymphocytes in a carbohydrate-dependent manner and induces ROS- and MAPK-driven caspase-dependent apoptosis (PMID:40333130).

Mechanistic history

Synthesis pass · year-by-year structured walk · 12 steps
  1. 2000 High

    Establishing that MIC4 is a micronemal secreted adhesin defined its role in host attachment and localized the adhesive activity to a C-terminal apple domain.

    Evidence Protein purification, microsequencing, immunolocalization, domain-mapping and host cell binding assays in T. gondii

    PMID:11053441

    Open questions at the time
    • Did not define binding partners within the microneme complex
    • Ligand identity of the adhesive apple domain not resolved
  2. 2000 Medium

    Detection of MIC4 antigen in macrogamete wall-forming bodies raised the possibility of a role beyond invasion in oocyst wall formation.

    Evidence Immuno-EM and immunocytochemistry in T. gondii macrogametes

    PMID:11027789

    Open questions at the time
    • Identity as MIC4 versus a cross-reacting protein not confirmed by western blot
    • Functional contribution to oocyst wall not tested
  3. 2001 High

    Genetic dissection resolved how MIC4 reaches micronemes, showing it is passive cargo escorted by MIC6 within a trimeric MIC4-MIC1-MIC6 complex.

    Evidence Knockouts of MIC1/MIC4/MIC6 with domain mapping and Co-IP trafficking assays in T. gondii

    PMID:11157983

    Open questions at the time
    • Structural basis of MIC4-MIC1 contact not defined
    • MIC4's own contribution to complex stability unclear
  4. 2001 Medium

    Lactose-affinity co-purification confirmed the MIC1-MIC4 physical association but attributed the native lectin activity to MIC1 rather than MIC4.

    Evidence Lactose affinity chromatography, hemagglutination inhibition and blot overlay in T. gondii RH

    PMID:11447133

    Open questions at the time
    • Left MIC4's intrinsic carbohydrate-binding activity unresolved
    • Single-lab biochemical inference
  5. 2005 High

    Structural analysis of MIC1 explained the quality-control basis of complex assembly, showing TSR1 domains engage MIC4 and a Galectin-like fold stabilizes MIC6 to enable secretory exit.

    Evidence X-ray crystallography, NMR, interaction mapping and mutagenesis of TgMIC1 in T. gondii

    PMID:16166092

    Open questions at the time
    • Atomic details of the MIC4-MIC1 interface not solved
    • Dynamics of complex disassembly during secretion unknown
  6. 2009 Medium

    Identifying an aldolase interaction on the MIC6 cytoplasmic tail connected the MIC4-containing complex to the parasite cytoskeleton during invasion.

    Evidence Co-IP and pulldown assays in T. gondii

    PMID:19308454

    Open questions at the time
    • Interaction is with MIC6 tail, not MIC4 directly
    • Single Co-IP/pulldown without reciprocal validation
  7. 2010 High

    Knockout of TgSUB1 established that surface proteolytic trimming of MIC4 is functionally required for efficient attachment, gliding, invasion and virulence.

    Evidence Δsub1 knockout with complementation, surface processing, invasion, gliding and mouse virulence assays in T. gondii

    PMID:20678172

    Open questions at the time
    • Precise cleavage sites on MIC4 not mapped
    • Functional consequence of MIC4-specific trimming not isolated from other substrates
  8. 2012 High

    The MIC4 crystal structure defined six apple domains and localized potent galactose-binding to the fifth domain, which is cleaved during invasion, establishing the molecular basis of its host lectin activity.

    Evidence X-ray crystallography, glycan-array binding assays and structural mutagenesis of TgMIC4 in T. gondii

    PMID:22399295

    Open questions at the time
    • In vivo host glycan targets during invasion not identified
    • Functional consequence of the fifth-domain cleavage not directly tested
  9. 2015 Low

    Biophysical binding kinetics indicated MIC4 prefers galactose-exposed (desialylated) glycoproteins and undergoes a galactose-induced conformational change potentially linked to internalization.

    Evidence QCM-D binding kinetics of recombinant TgMIC4 with fetuin versus asialofetuin

    PMID:26488670

    Open questions at the time
    • Conformational change inferred from kinetics, not directly observed
    • Single-lab biophysical assay with recombinant protein
    • Link to host cell internalization not demonstrated in cells
  10. 2019 High

    Demonstrating that MIC4's CRD binds TLR2/TLR4 N-glycans to drive NF-κB and pro-inflammatory cytokines extended MIC4's function from invasion to direct activation of host innate immunity.

    Evidence Recombinant protein and knockout/CRD-mutant parasite stimulation of macrophages/DCs, TLR glycomutant transfection, Co-IP and cytokine assays in T. gondii

    PMID:31226171 PMID:31658592

    Open questions at the time
    • Contribution relative to MIC1 not fully separable
    • Signaling pathway data partly from transfection/inhibitor systems
  11. 2021 Medium

    Showing that MIC4 drives IL-10 via TLR4 endocytosis and confers LPS tolerance revealed a distinct immunomodulatory arm separable from its pro-inflammatory signaling.

    Evidence Recombinant MIC4 macrophage stimulation with Dynasore endocytosis blockade and cytokine ELISA in T. gondii

    PMID:33912181

    Open questions at the time
    • Pharmacological blockade only, no genetic confirmation of endocytic route
    • In vivo relevance of tolerance not established
  12. 2025 Medium

    Identifying carbohydrate-dependent T-cell binding and ROS/MAPK-driven apoptosis positioned MIC4 as a potential immune-evasion effector acting on adaptive lymphocytes.

    Evidence Flow cytometry binding/apoptosis assays, ROS detection and MAPK inhibitor assays with recombinant MIC4 on Jurkat cells

    PMID:40333130

    Open questions at the time
    • In vitro cell line model only
    • Physiological relevance during infection not tested
    • Host T-cell glycan target not identified

Open questions

Synthesis pass · forward-looking unresolved questions
  • The in vivo host glycan targets of the MIC4 fifth-domain CRD during invasion, and the mechanism coupling galactose binding to host cell entry, remain undefined.
  • No structure of the MIC4-MIC1 interface
  • In vivo relevance of immunomodulatory functions untested
  • Relative in vivo contributions of MIC4 vs MIC1 signaling unresolved

Mechanism profile

Synthesis pass · controlled-vocabulary classification
Partners
HOST TLR2 (VIA N-GLYCANS)HOST TLR4 (VIA N-GLYCANS)TGMIC1 (DIRECT BINDING PARTNER WITHIN THE TRIMERIC COMPLEX)TGMIC6 (ESCORTER WITHIN THE COMPLEX)TGSUB1 (GPI-ANCHORED SUBTILISIN THAT PROCESSES MIC4 AFTER SECRETION)
Complex memberships
MIC4-MIC1-MIC6 trimeric micronemal adhesin complex

Evidence

Reading pass · 13 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2000 MIC4 is a 61-kDa protein secreted by Toxoplasma gondii tachyzoites that localizes to the micronemes of all invasive forms (tachyzoites, bradyzoites, sporozoites, merozoites). It is proteolytically processed at both N- and C-termini only upon release from the organelle. The adhesive motif maps to the most C-terminal (fifth/sixth) apple domain, and MIC4 binds efficiently to host cells. Protein purification and microsequencing, gene cloning, immunolocalization, deletion/domain mapping assays, host cell binding assays The Journal of biological chemistry High 11053441
2001 MIC6 functions as an escorter for MIC1 and MIC4, forming a trimeric complex (MIC4-MIC1-MIC6) that is required for accurate targeting of MIC1 and MIC4 to the micronemes. MIC4 binds directly to MIC1 and behaves as a passive cargo molecule. MIC6 carries a sorting signal in its cytoplasmic tail, while its association with MIC1 involves a lumenal EGF-like domain. MIC1 is linked to a quality control system and is absolutely required for the complex to leave early secretory compartments. Gene disruption (knockouts of MIC1, MIC4, MIC6), domain mapping, Co-IP, sorting/trafficking assays The Journal of cell biology High 11157983
2001 MIC1 and MIC4 co-purify from tachyzoite soluble antigen by affinity chromatography on immobilized lactose, consistent with their physical association. The lectin activity (hemagglutination inhibited by beta-galactosides) was attributed to MIC1, not to native MIC4. Affinity chromatography (lactose column), N-terminal microsequencing, hemagglutination inhibition assay, blot overlay Glycobiology Medium 11447133
2000 MIC4, or a MIC4-like protein, is expressed in macrogametes and associated with oocyst wall formation. By immuno-electron microscopy, MIC4 antigen was detected in a subset of wall forming body type 1 (WFB1a) dense granules, which sequentially release their contents to form an anti-MIC4-positive outer veil on the developing oocyst wall. Immunocytochemistry, immuno-electron microscopy International journal for parasitology Medium 11027789
2005 The N-terminal TSR1-like domains of TgMIC1 promote association with TgMIC4. The C-terminal domain of TgMIC1 adopts a novel Galectin-like fold (without carbohydrate-binding properties) that interacts with and stabilizes MIC6, providing the structural basis for a quality-control mechanism enabling the MIC4-MIC1-MIC6 complex to exit early secretory compartments and traffic to micronemes. X-ray crystallography (3D structure of TgMIC1 C-terminal domain), NMR, interaction assays, mutagenesis The Journal of biological chemistry High 16166092
2010 The microneme subtilisin protease TgSUB1, anchored via a GPI moiety, is responsible for proteolytic trimming of MIC4 (and MIC2, M2AP) after secretion onto the parasite surface. Loss of TgSUB1 abolishes this processing, decreases cell attachment and gliding efficiency, and reduces invasion rates in vitro and virulence in mice. Gene knockout (Δsub1), complementation with full-length and ΔGPI-SUB1 constructs, surface processing assays, invasion and gliding assays, mouse virulence assay Cellular microbiology High 20678172
2012 The crystal structure of TgMIC4 reveals six apple domains; the fifth domain displays potent galactose-binding activity and is cleaved from the complex during parasite invasion. TgMIC4 binds a variety of galactose-containing carbohydrate ligands. X-ray crystallography, carbohydrate-binding assays (glycan array), structural mutagenesis The Journal of biological chemistry High 22399295
2009 The cytoplasmic tail of MIC6 interacts with parasite aldolase, connecting the MIC4-MIC1-MIC6 adhesive complex to the parasite cytoskeleton during host cell invasion. Co-immunoprecipitation, pulldown assays Parasitology research Medium 19308454
2019 MIC4's galactose-recognition activity (via its carbohydrate recognition domain, CRD) enables it to interact physically with N-glycans on TLR2 and TLR4 on host innate immune cells, triggering NF-κB activation and pro-inflammatory cytokine (IL-12, TNF-α, IL-6) production. Point mutations in the CRD of MIC4 abolish innate immune cell activation. Parasites expressing CRD-mutant MIC4 fail to induce wild-type IL-12 levels in vitro. Recombinant protein stimulation of dendritic cells/macrophages, CRD point mutants, HEK cell TLR glycomutant transfections, Co-IP, in vitro infection with MIC4-knockout or CRD-mutant parasites, cytokine measurement PLoS pathogens High 31226171
2019 TLR2 heterodimerization (with TLR1 or TLR6) augments but is not critical for MIC4-induced cell activation; CD14 and CD36 enhance the response. MIC4 (and MIC1) activate macrophages through a TAK1-, p38 MAPK-, and NF-κB-dependent signaling pathway. The interaction of MIC4 with TLR2 N-glycans alone is sufficient to induce cell activation. Macrophage stimulation with recombinant MICs, HEK293T transfection with TLR2/TLR1/TLR6/CD14/CD36 constructs, NF-κB reporter assay, cytokine ELISA, pathway inhibitor assays International journal of molecular sciences Medium 31658592
2021 MIC4 (and MIC1) trigger IL-10 production from macrophages through TLR4 endocytosis/internalization. Blocking endocytosis with Dynasore prevents IL-10 but not TNF-α production in response to MIC4. MIC4-stimulated macrophages acquire transient tolerance to LPS. Macrophage stimulation with recombinant MIC4, Dynasore-mediated endocytosis blockade, cytokine ELISA, in vitro T. gondii infection Frontiers in immunology Medium 33912181
2025 Recombinant MIC4 binds Jurkat T lymphocytes in a carbohydrate-dependent manner and induces apoptosis through activation of both extrinsic and intrinsic caspase pathways, driven by reactive oxygen species (ROS) production via the NADPH oxidase complex and activation of p38 and JNK MAPK signaling. Flow cytometry (cell binding, apoptosis/caspase assays), ROS detection, MAPK inhibitor assays, viability assays Pathogens (Basel, Switzerland) Medium 40333130
2015 TgMIC4 binds desialylated (galactose-exposing) glycoproteins (asialofetuin) preferentially over sialylated glycoproteins (fetuin), with a conformational change accompanying galactose binding, as detected by a slower kinetic step in QCM-D measurements. This galactose-induced conformational change may contribute to host cell internalization. Quartz crystal microbalance with dissipation monitoring (QCM-D) using recombinant TgMIC4 immobilized on sensor surface, binding kinetics with fetuin vs. asialofetuin Langmuir : the ACS journal of surfaces and colloids Low 26488670

Source papers

Stage 0 corpus · 34 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2001 Identification and characterization of an escorter for two secretory adhesins in Toxoplasma gondii. The Journal of cell biology 178 11157983
2001 Microneme proteins: structural and functional requirements to promote adhesion and invasion by the apicomplexan parasite Toxoplasma gondii. International journal for parasitology 178 11566297
2002 A family of transmembrane microneme proteins of Toxoplasma gondii contain EGF-like domains and function as escorters. Journal of cell science 135 11861763
2000 The toxoplasma micronemal protein MIC4 is an adhesin composed of six conserved apple domains. The Journal of biological chemistry 123 11053441
2005 A combination of antigenic regions of Toxoplasma gondii microneme proteins induces protective immunity against oral infection with parasite cysts. The Journal of infectious diseases 73 15655789
2001 Toxoplasma gondii micronemal protein MIC1 is a lactose-binding lectin. Glycobiology 64 11447133
2006 Immunization with MIC1 and MIC4 induces protective immunity against Toxoplasma gondii. Microbes and infection 62 16616574
2005 A novel galectin-like domain from Toxoplasma gondii micronemal protein 1 assists the folding, assembly, and transport of a cell adhesion complex. The Journal of biological chemistry 62 16166092
2010 Toxoplasma gondii protease TgSUB1 is required for cell surface processing of micronemal adhesive complexes and efficient adhesion of tachyzoites. Cellular microbiology 58 20678172
2015 Vaccination with Recombinant Microneme Proteins Confers Protection against Experimental Toxoplasmosis in Mice. PloS one 53 26575028
2005 Use of recombinant antigens for early postnatal diagnosis of congenital toxoplasmosis. Journal of clinical microbiology 43 16333076
2019 The lectin-specific activity of Toxoplasma gondii microneme proteins 1 and 4 binds Toll-like receptor 2 and 4 N-glycans to regulate innate immune priming. PLoS pathogens 39 31226171
2012 Galactose recognition by the apicomplexan parasite Toxoplasma gondii. The Journal of biological chemistry 39 22399295
2008 Complete resonance assignment of the first and second apple domains of MIC4 from Toxoplasma gondii, using a new NMRView-based assignment aid. Biomolecular NMR assignments 30 19636884
2017 Targeting and Recognition of Toll-Like Receptors by Plant and Pathogen Lectins. Frontiers in immunology 28 29326706
2009 MIC6 associates with aldolase in host cell invasion by Toxoplasma gondii. Parasitology research 26 19308454
2021 Multi-epitope vaccine expressed in Leishmania tarentolae confers protective immunity to Toxoplasma gondii in BALB/c mice. Microbial pathogenesis 25 33933602
2009 Toxoplasma gondii: protective effect of an intranasal SAG1 and MIC4 DNA vaccine in mice. Experimental parasitology 25 19366622
2000 The microneme protein MIC4, or an MIC4-like protein, is expressed within the macrogamete and associated with oocyst wall formation in Toxoplasma gondii. International journal for parasitology 21 11027789
2018 Galectin-3 and Galectin-9 May Differently Regulate the Expressions of Microglial M1/M2 Markers and T Helper 1/Th2 Cytokines in the Brains of Genetically Susceptible C57BL/6 and Resistant BALB/c Mice Following Peroral Infection With Toxoplasma gondii. Frontiers in immunology 13 30108583
2021 Microneme Proteins 1 and 4 From Toxoplasma gondii Induce IL-10 Production by Macrophages Through TLR4 Endocytosis. Frontiers in immunology 12 33912181
2019 Receptor Heterodimerization and Co-Receptor Engagement in TLR2 Activation Induced by MIC1 and MIC4 from Toxoplasma gondii. International journal of molecular sciences 12 31658592
2021 Immunoinformatic analysis of immunogenic B- and T-cell epitopes of MIC4 protein to designing a vaccine candidate against Toxoplasma gondii through an in-silico approach. Clinical and experimental vaccine research 11 33628756
2021 Diverse Roles of TgMIC1/4/6 in the Toxoplasma Infection. Frontiers in microbiology 11 34220751
2019 Secretory Microneme Proteins Induce T-Cell Recall Responses in Mice Chronically Infected with Toxoplasma gondii. mSphere 11 30814319
2022 Enhancement of immune responses by vaccine potential of three antigens, including ROP18, MIC4, and SAG1 against acute toxoplasmosis in mice. Experimental parasitology 5 36379272
2022 Immunological evaluation of a novel multi-antigenic DNA vaccine encoding SAG1, SAG3, MIC4, GRA5, GRA7, AMA1and BAG1 against Toxoplasma gondii in BALB/c mice. Experimental parasitology 5 36403800
2019 Identification of candidate antigens by 2-DE Immunoblotting for diagnosis of Toxoplasma gondii infection in chickens and rabbits. Experimental parasitology 5 31299265
2015 Evidence for Conformational Mechanism on the Binding of TgMIC4 with β-Galactose-Containing Carbohydrate Ligand. Langmuir : the ACS journal of surfaces and colloids 5 26488670
2021 Proteomics analysis reveals that the proto-oncogene eIF-5A indirectly influences the growth, invasion and replication of Toxoplasma gondii tachyzoite. Parasites & vectors 3 34039408
2025 Structural Prediction and Antigenic Analysis of ROP18, MIC4, and SAG1 Proteins to Improve Vaccine Design against Toxoplasma gondii: An In silico Approach. Infectious disorders drug targets 2 39350555
2022 Protective immunity induced with a DNA vaccine encoding B- and T-cells multi-epitope SAG1, ROP16, MIC4, GRA12, M2AP, and multi-epitope ROP8 against acute and chronic toxoplasmosis in BALB/c mice. Experimental parasitology 2 36162598
2025 Recombinant Microneme Proteins MIC1 and MIC4 from Toxoplasma gondii Cause Cytotoxic Effects in the Human Jurkat T-Lymphocyte Cell Line. Pathogens (Basel, Switzerland) 1 40333130
2020 MIC4 from Toxoplasma gondii: A Lectin Acting as a Toll-Like Receptor Agonist. Methods in molecular biology (Clifton, N.J.) 1 32306345