MIC3

TgMIC3 is a micronemal secretory adhesin of Toxoplasma gondii that couples host-cell attachment during invasion to modulation of host innate immunity (PMID:11207591, PMID:1944419, PMID:24367261). It is synthesized as a 40-kDa precursor and matures into a disulfide-linked homodimer of ~38-kDa polypeptides bearing five partially overlapping EGF-like domains and an N-terminal chitin binding-like (CBL) domain, and it is relocalized from the parasite apex onto the surface during cell entry (PMID:11207591, PMID:1944419, PMID:12925131). Receptor binding requires two prior maturation events: cleavage of the pro-peptide to expose the CBL domain, and C-terminal-domain-mediated dimerization; the dimerization module is potent enough to convert the normally monomeric MIC8 into a receptor-competent dimer (PMID:12032066). The CBL domain engages host cell receptors, including heparan sulfate, and single substitutions that abolish CBL binding attenuate virulence, establishing adhesion as the effector activity relevant to pathogenesis (PMID:15684324, PMID:30664845). Microneme targeting is encoded jointly by the pro-peptide and the EGF modules—the minimal signal being the pro-peptide plus any one EGF domain—and depends on a conserved aliphatic residue near the pro-peptide N-terminus, whereas pro-peptide cleavage, dimerization, and the CBL domain are dispensable for trafficking; the soluble adhesin reaches the micronemes as cargo of the transmembrane escorter MIC8, and is shed from the parasite surface by the rhomboid protease TgROM4 to permit productive invasion (PMID:18390648, PMID:21438967, PMID:11861763, PMID:20421941). Beyond adhesion, secreted MIC3 acts on the host: its EGF domains activate EGFR-Akt signaling to suppress autophagic killing of the parasite, and a MIC3 peptide drives TNF-α production, NF-κB activation, iNOS and Ly6C induction, and M1 macrophage polarization through the TLR11/MyD88/NF-κB pathway (PMID:24367261, PMID:28000706, PMID:36730424). MIC3 acts synergistically with MIC1, and only combined mic1/mic3 disruption severely attenuates virulence (PMID:15684324).

1. 2000 High

   Whether MIC3 was a structural micronemal protein or a functional adhesin was unresolved; biochemical and imaging characterization established it as a disulfide-linked homodimeric adhesin that binds host cells and redistributes to the parasite surface during invasion.

   Evidence adhesion assays with antibody blockade, affinity purification, N-terminal sequencing, invasion IFA, and cDNA cloning in T. gondii

   PMID:11207591 PMID:1944419

   Open questions at the time

   • Host receptor identity not defined
   • Contribution of individual domains to binding not resolved
2. 2001 Medium

   It was unknown what directed MIC3 to the micronemes; live-cell GFP-fusion and deletion analysis showed the cysteine-rich central region is required for proper folding and micronemal targeting, with misfolded protein defaulting to the dense-granule pathway.

   Evidence GFP-fusion imaging and deletion analysis in transfected T. gondii

   PMID:11254953

   Open questions at the time

   • Minimal targeting determinant not yet defined
   • Molecular recognition of the signal not identified
3. 2002 Medium

   How a soluble adhesin with no membrane anchor reaches the micronemes was unclear; MIC8 was shown to act as a transmembrane escorter that sorts MIC3 via cytoplasmic-tail signals and is cleaved upon discharge.

   Evidence co-localization, functional complementation, and processing assays in T. gondii

   PMID:11861763

   Open questions at the time

   • Direct MIC3-MIC8 binding interface not mapped
   • Stoichiometry of the escort complex unknown
4. 2002 High

   The requirements for MIC3 to acquire binding activity were unknown; cell-based reconstitution with truncation and domain-swap mutagenesis showed pro-peptide cleavage exposes the CBL receptor-binding site and C-terminal-domain dimerization is required for host-cell binding.

   Evidence mammalian-cell expression, MIC3-binding assays, and site-directed/domain-swap mutagenesis

   PMID:12032066

   Open questions at the time

   • Host receptor engaged by the CBL domain not identified here
   • Structural basis of dimer-dependent binding not solved
5. 2003 Medium

   Whether MIC3 relocalization was coordinated with regulated microneme exocytosis was untested; quantitative imaging tied MIC3 apical loss to Ca2+-stimulated secretion accompanying invasion, with recovery ~15 min later.

   Evidence in vivo 32P labeling, quantitative IFA, stimulated-exocytosis and immunoelectron microscopy in T. gondii

   PMID:12925131

   Open questions at the time

   • Molecular trigger linking Ca2+ signaling to MIC3 discharge not defined
   • Fate of surface MIC3 during recovery unclear
6. 2005 High

   Whether MIC3 binding activity is functionally important in vivo was unknown; CBL point mutants and single/double knockouts established that CBL-mediated binding contributes to virulence and that MIC1 and MIC3 act synergistically, though MIC3 is dispensable for fibroblast invasion.

   Evidence single/double gene disruption, CBL-domain mutagenesis, invasion assays, and mouse virulence assays in T. gondii RH

   PMID:15684324

   Open questions at the time

   • Molecular basis of MIC1/MIC3 synergy not defined
   • Reason for dispensability in fibroblast invasion unresolved
7. 2008 High

   The precise trafficking determinants and processing compartment were unresolved; systematic deletion and chimera analysis localized pro-peptide cleavage to a post-Golgi compartment and defined the minimal micronemal signal as pro-peptide plus any one EGF domain, distinct from the domains needed for binding.

   Evidence deletion mutants, chimeric proteins, and pulse-chase processing assays in T. gondii

   PMID:18390648

   Open questions at the time

   • Identity of the pro-peptide-cleaving protease not established
   • Basis of cell-cycle-dependent secretory retention unknown
8. 2010 High

   How MIC3 is removed from the surface to allow invasion was unknown; conditional ROM4 knockdown showed the rhomboid protease TgROM4 sheds MIC3, and its loss increases surface MIC3, impairs gliding and invasion despite enhanced binding.

   Evidence conditional ROM4 knockdown with adhesin-release, flow cytometry, motility and invasion assays in T. gondii RH

   PMID:20421941

   Open questions at the time

   • MIC3 cleavage site for ROM4 not mapped
   • Whether MIC3 is a direct vs. indirect ROM4 substrate not resolved
9. 2011 Medium

   It was unknown whether the pro-peptide targeting determinant was conserved across coccidia; mutagenesis showed pro-peptides are interchangeable across MICs and with Eimeria EtMIC5 and that a conserved aliphatic residue near the N-terminus is critical for MIC3 microneme targeting.

   Evidence N-terminal deletion/point mutagenesis and domain-swap chimeras in transfected T. gondii

   PMID:21438967

   Open questions at the time

   • Receptor recognizing the aliphatic-residue signal not identified
   • MIC3-specific data are one component of a broader study
10. 2013 High

    Whether MIC3 has host-signaling functions beyond adhesion was unknown; knockout and recombinant-protein experiments showed its EGF domains activate EGFR-Akt signaling that blocks autophagic targeting and killing of the parasite.

    Evidence mic3 KO infection, recombinant MIC3 treatment, EGFR/Akt phosphorylation, LC3 accumulation and parasite-killing assays

    PMID:24367261

    Open questions at the time

    • Direct MIC3-EGFR contact not demonstrated
    • In vivo relevance of the EGFR-autophagy axis not established
11. 2016 Medium

    Host binding partners of the EGF domains beyond EGFR were unknown; a yeast two-hybrid screen identified Spata3 and Dkk2 as MIC3 interactors mediated by the tandem EGF repeats.

    Evidence yeast two-hybrid screen against a mouse library with confirmatory interaction assays

    PMID:27053129

    Open questions at the time

    • No functional consequence of Spata3/Dkk2 binding demonstrated in the parasite
    • Interactions not validated in an infection context
12. 2016 Medium

    Whether MIC3 abundance and immunostimulatory activity relate to strain virulence was untested; iTRAQ proteomics found MIC3 ~5-fold higher in virulent RH than in TgCtwh3, and recombinant MIC3 induced TNF-α and M1 macrophage polarization.

    Evidence iTRAQ proteomics of secreted antigens and recombinant-protein macrophage stimulation with TNF-α ELISA and M1 assays

    PMID:28000706

    Open questions at the time

    • Receptor/pathway driving TNF-α and M1 polarization not defined here
    • Causal link between MIC3 abundance and strain virulence not proven
13. 2019 Medium

    The identity of a host receptor for the CBL/adhesin activity was still open; recombinant MIC3 was shown to bind heparan sulfate and host cell surfaces, and to competitively block invasion, with higher expression in invading parasites.

    Evidence heparin- and cell-surface binding assays, competitive and antibody-mediated invasion inhibition, and expression analysis in T. gondii RH

    PMID:30664845

    Open questions at the time

    • Whether heparan sulfate is the physiological invasion receptor not established
    • Domain responsible for heparin binding not mapped
14. 2023 High

    The receptor and pathway for MIC3's pro-inflammatory activity were undefined; inhibitor, siRNA and CRISPR-KO approaches showed a MIC3 peptide drives TNF-α, NF-κB, iNOS and Ly6C via TLR11/MyD88/NF-κB in mouse macrophages but not in TLR11-deficient human cells, and it is a TLR11 ligand distinct from profilin (no IL-12p40 induction).

    Evidence MyD88 inhibition, Tlr11 siRNA and CRISPR KO in RAW264.7, NF-κB/TNF-α/iNOS/Ly6C readouts, and mouse-vs-human comparison

    PMID:36730424

    Open questions at the time

    • Direct MIC3-TLR11 binding not demonstrated
    • Human host receptor for MIC3 immunostimulation unknown
    • Role of full-length MIC3 vs. peptide in vivo not resolved

• The physiological host receptor engaged by the CBL domain during invasion, the direct structural basis for EGFR and TLR11 engagement, and the mechanism of MIC1/MIC3 virulence synergy remain unresolved.
• No solved structure of MIC3 in complex with any host receptor
• Direct MIC3-EGFR and MIC3-TLR11 contacts not demonstrated
• Molecular basis of MIC1/MIC3 synergy undefined