CPC2

TgCPC2 is a papain-family cathepsin C cysteine exopeptidase of Toxoplasma gondii that removes dipeptides from the unblocked N-termini of proteins and peptides (PMID:17164247). It localizes to the dense granules, the constitutive secretory vesicles of tachyzoites (PMID:17164247), and contributes to peptide/protein degradation within the parasitophorous vacuole, where cathepsin C inhibition completely blocks turnover of a secreted beta-lactamase reporter (PMID:17164247). TgCPC2 functionally overlaps with the paralog TgCPC1: disruption of TgCPC1 up-regulates TgCPC2 and produces no invasion or growth defect, indicating TgCPC2 can substitute for TgCPC1 during tachyzoite growth (PMID:17164247). Beyond this dense-granule/vacuolar degradative role in tachyzoites, no further mechanistic detail has been characterized in the available corpus.

1. 2006 Medium

   Established the biochemical identity of TgCPC2 as a cathepsin C exopeptidase, defining its mode of substrate processing as dipeptidyl removal from unblocked N-termini.

   Evidence Enzymatic activity assays with the cathepsin C-specific inhibitor Gly-Phe-dimethylketone and defined substrates

   PMID:17164247

   Open questions at the time

   • No structural model or catalytic residue mapping
   • Substrate specificity beyond the dipeptidyl-N-terminal mode not detailed
   • Enzyme characterization from a single lab study
2. 2006 Medium

   Placed TgCPC2 in the dense granule secretory compartment, linking the enzyme to material delivered into the parasitophorous vacuole.

   Evidence Subcellular fractionation/localization in tachyzoites

   PMID:17164247

   Open questions at the time

   • Trafficking route and processing/activation en route to dense granules unknown
   • Whether it is secreted into the vacuole versus retained in granules not resolved
3. 2006 Medium

   Connected cathepsin C activity to protein degradation inside the parasitophorous vacuole, implicating TgCPC2 in vacuolar proteolysis.

   Evidence Transgenic tachyzoites expressing a beta-lactamase reporter in the parasitophorous vacuole, with degradation blocked by cathepsin C inhibitor

   PMID:17164247

   Open questions at the time

   • TgCPC2 contribution not resolved individually from TgCPC1 by genetic knockout
   • Physiological native substrates in the vacuole not identified
   • Inhibitor is not TgCPC2-specific across the two paralogs
4. 2006 Medium

   Demonstrated functional redundancy with TgCPC1, showing TgCPC2 is up-regulated and compensates when TgCPC1 is disrupted, so no growth or invasion defect results.

   Evidence Targeted gene disruption of TgCPC1 with real-time PCR of TgCPC2 and growth/invasion assays

   PMID:17164247

   Open questions at the time

   • Phenotype of a direct TgCPC2 knockout not determined
   • Mechanism of transcriptional up-regulation upon TgCPC1 loss unknown
   • Whether double disruption is lethal not established
5. 2006 Low

   Defined TgCPC2 expression as stage-restricted to tachyzoites, absent in in vivo bradyzoites.

   Evidence Real-time PCR quantification of transcript across tachyzoites and in vivo bradyzoites

   PMID:17164247

   Open questions at the time

   • Expression-only measurement; no functional test of TgCPC2 in bradyzoites
   • Regulatory basis of stage-specific expression unknown

• The individual, non-redundant contribution of TgCPC2 to vacuolar proteolysis and parasite fitness, its native substrates, and its structural basis of catalysis remain undefined.
• No direct TgCPC2 knockout phenotype
• No endogenous substrate identified
• No structural characterization