TGME49_318410

TgTCP-1 (TGME49_318410) is a Toxoplasma gondii protein implicated in sialic acid-dependent host cell invasion (PMID:32153542). In the ME49 strain it co-localizes with the microneme protein MIC3, consistent with secretion from tachyzoites into a microneme-like compartment (PMID:32153542). Recombinant TgTCP-1 binds specifically to sialic acid in vitro, and this binding to dendritic and Vero host cells is abolished by competition with free sialic acid or by neuraminidase pretreatment of the host surface, defining sialic acid as its ligand (PMID:32153542). Pre-incubating host cells with recombinant TgTCP-1 reduces parasite invasion, indicating that the protein engages host-surface sialic acid to promote entry (PMID:32153542). Beyond this invasion-associated role, the protein's mechanism is largely uncharacterized in the available corpus; conditional knockdown did not produce a critical growth defect, so TgTCP-1 is not essential for parasite growth under the conditions tested (PMID:21035508).

1. 2010 Low

   Whether TCP-1 is essential for parasite viability was addressed by conditional depletion, establishing that its loss does not critically impair growth and shifting focus away from a housekeeping folding role.

   Evidence Insertional gene trap with Tet-transactivator conditional knockdown and cosmid complementation in T. gondii

   PMID:21035508

   Open questions at the time

   • Negative growth result from a single conditional knockdown, single lab, with complicating genomic rearrangements noted
   • Does not test roles in invasion or stage-specific functions
   • Suppressor gene identified but its relationship to TCP-1 function unresolved
2. 2020 Medium

   The subcellular fate of TgTCP-1 was addressed by imaging, showing co-localization with a microneme marker and implicating it as a secreted, invasion-associated protein rather than a purely cytosolic chaperonin.

   Evidence Co-localization imaging with the microneme protein MIC3 in ME49 tachyzoites

   PMID:32153542

   Open questions at the time

   • Single co-localization experiment in one lab
   • Does not directly demonstrate secretion or microneme discharge kinetics
   • Endogenous localization not confirmed by tagging
3. 2020 Medium

   The molecular ligand of TgTCP-1 was defined by binding assays, showing specific recognition of host-surface sialic acid.

   Evidence Recombinant protein binding to DC and Vero cells with free-sialic-acid competition and neuraminidase controls

   PMID:32153542

   Open questions at the time

   • Recombinant protein assay in a single lab; endogenous protein binding not shown
   • The specific sialylated host glycan/receptor is not identified
   • Binding stoichiometry and affinity not quantified
4. 2020 Medium

   Whether sialic acid binding is functionally relevant to infection was tested by invasion competition, establishing TgTCP-1 as a contributor to sialic acid-dependent host cell invasion.

   Evidence Invasion inhibition assay using recombinant TgTCP-1 pre-treatment of Vero host cells

   PMID:32153542

   Open questions at the time

   • Competition assay with recombinant protein; genetic loss-of-function invasion phenotype not tested
   • Does not define whether TgTCP-1 acts directly at the invasion junction
   • Contribution relative to other adhesins unquantified

• How a putative chaperonin comes to act as a secreted sialic acid-binding adhesin, and the identity of its host receptor, remain unresolved.
• No structural basis for sialic acid recognition characterized
• Host sialylated receptor not identified
• Reconciliation of chaperonin annotation with secreted adhesin function unaddressed