PCR2

Pcr2 is a component of the preconoidal rings (PCRs) at the apical complex of Toxoplasma gondii that couples apical structural organization to the parasite's invasive machinery (PMID:35994509, PMID:36109645). Genetic ablation of Pcr2 converts normal persistent gliding into spasmodic movement while leaving actomyosin activity (actin chromobody motion) largely intact, placing Pcr2 downstream of or in parallel with the actomyosin motor (PMID:35994509). Beyond motility, Pcr2 knockout parasites fail to properly assemble the moving junction, contributing to defective host-cell invasion (PMID:35994509). Because calcium-induced secretion of the major adhesin MIC2 is unperturbed in the knockout, Pcr2 acts downstream of adhesin secretion in the motility and invasion pathway (PMID:35994509). At the structural level, the PCRs that contain Pcr2 serve as a hub for glideosome components including Formin1, linking this apical ring to glideosome assembly (PMID:36109645). No direct biochemical activity, binding partners, or structural model for Pcr2 itself have been characterized in the available corpus.

1. 2022 Medium

   Establishing where Pcr2 acts in the motility pathway was unknown; showing that knockout parasites glide spasmodically despite intact actin dynamics placed Pcr2 downstream of or in parallel with the actomyosin machinery rather than as part of the motor itself.

   Evidence Knockout generation with calcium-induced egress and live motility imaging using an mEmerald-tagged actin chromobody

   PMID:35994509

   Open questions at the time

   • Molecular mechanism linking Pcr2 to persistent versus spasmodic gliding not defined
   • No direct binding partners identified
   • Whether Pcr2 acts on the motor or on force transmission unresolved
2. 2022 Medium

   Whether Pcr2's role extended to host-cell entry was unknown; the knockout's failure to assemble the moving junction demonstrated an invasion defect distinct from the motility phenotype.

   Evidence Knockout invasion assays and moving junction formation assays

   PMID:35994509

   Open questions at the time

   • Mechanism by which Pcr2 promotes moving junction assembly unknown
   • No identified moving junction component interacting with Pcr2
3. 2022 Medium

   The step at which Pcr2 acts relative to adhesin release was unclear; normal calcium-induced MIC2 secretion in the knockout placed Pcr2 downstream of adhesin secretion.

   Evidence Calcium-induced MIC2 secretion assay in knockout parasites

   PMID:35994509

   Open questions at the time

   • Only MIC2 tested; other secreted effectors not assessed
   • Single method for this sub-finding
4. 2022 Medium

   How the PCR structure containing Pcr2 relates to the glideosome was unknown; high-resolution imaging established the PCRs as a hub for glideosome components including Formin1.

   Evidence Ultrastructure expansion microscopy (U-ExM) localizing glideosome components relative to PCR markers

   PMID:36109645

   Open questions at the time

   • Pcr2 implicated only via broader PCR characterization, not directly resolved
   • Direct interaction between Pcr2 and Formin1 not shown

• The biochemical function of Pcr2, its direct binding partners, and its structural role within the preconoidal rings remain undefined.
• No molecular activity assigned to Pcr2
• No direct interactome or structural model
• Mechanistic link between PCR integrity and persistent gliding unresolved