Affinage · Toxoplasma gondii

PCNA1

proliferating cell nuclear antigen PCNA1 · UniProt A0A125YTT1

Annotated
2026-09-08
4 papers in source corpus 3 papers cited in narrative 5 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

TgPCNA1 is the primary replisomal proliferating cell nuclear antigen of Toxoplasma gondii, functioning as the essential DNA replication clamp in the tachyzoite stage (PMID:10960171, PMID:15878790). It is one of two PCNA paralogs (TgPCNA1 and TgPCNA2) that arose from an ancestral apicomplexan gene duplication (PMID:10960171). Heterologous expression of TgPCNA1, but not TgPCNA2, rescues a cold-sensitive yeast POL30 mutant, establishing it as the functional equivalent of canonical eukaryotic PCNA and the major replisomal factor (PMID:15878790). TgPCNA1 is constitutively nuclear across all cell cycle phases and reorganizes into punctate replication foci during S-phase, with DNA replication initiating at or near centromeric regions (PMID:15878790, PMID:40590555). The gene cannot be disrupted in tachyzoites, indicating it is essential for growth (PMID:15878790), and its S-phase-specific dynamics have been exploited to build it into a FUCCI cell cycle probe (PMID:40590555). Beyond its role as a replication clamp, the interacting partners and post-translational regulation of TgPCNA1 have not been characterized in the available corpus.

Mechanistic history

Synthesis pass · year-by-year structured walk · 4 steps
  1. 2000 Medium

    Establishing that T. gondii carries two distinct PCNA genes defined the genetic architecture of the parasite's replication clamp machinery and raised the question of functional division between the paralogs.

    Evidence cDNA library screening with Southern, Northern, and Western blotting plus phylogenetic analysis of TgPCNA1/TgPCNA2 in tachyzoites

    PMID:10960171

    Open questions at the time
    • Did not determine which paralog is the functional replisomal clamp
    • No localization or essentiality data
    • Roles of the two paralogs left undefined
  2. 2005 High

    Yeast complementation resolved which paralog is the canonical clamp by showing TgPCNA1, but not TgPCNA2, restores POL30 function, assigning TgPCNA1 the role of major replisomal PCNA.

    Evidence Complementation of a cold-sensitive POL30 yeast mutant with heterologously expressed TgPCNA1 vs TgPCNA2

    PMID:15878790

    Open questions at the time
    • Does not identify the T. gondii replication partners loaded by the clamp
    • Complementation in yeast does not resolve native replisome composition
  3. 2005 Medium

    Localization and knockout attempts tied TgPCNA1's molecular identity to cell biology by showing it is constitutively nuclear, forms S-phase replication foci, and is refractory to disruption, marking it as essential.

    Evidence TgPCNA1-GFP immunofluorescence across cell cycle stages and gene disruption attempts in tachyzoites

    PMID:15878790

    Open questions at the time
    • Essentiality inferred from failure to disrupt, a negative genetic result
    • Mechanism of continuous nuclear retention unknown
    • Foci not mapped to specific replication origins
  4. 2025 Medium

    Quantitative live-cell imaging refined the spatiotemporal picture, showing TgPCNA1-marked replication initiates near centromeres with bell-shaped dynamics and validating it as an S-phase FUCCI reporter.

    Evidence TgPCNA1-NeonGreen live-cell imaging and immunofluorescence in the ToxoFUCCIS auxin-degron background

    PMID:40590555

    Open questions at the time
    • Molecular basis of centromere-proximal replication initiation not defined
    • Does not identify clamp-loading or interacting factors

Open questions

Synthesis pass · forward-looking unresolved questions
  • The interactome, clamp-loading mechanism, and post-translational regulation that distinguish TgPCNA1 function and control its centromere-proximal replication dynamics remain undefined.
  • No replisomal or PIP-box partners identified
  • No structural model of the T. gondii clamp
  • Functional division of labor between TgPCNA1 and TgPCNA2 not mechanistically resolved

Mechanism profile

Synthesis pass · controlled-vocabulary classification
No controlled-vocabulary terms were assigned to this entry.

Evidence

Reading pass · 5 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2000 TgPCNA1 encodes a ~40 kDa protein expressed in T. gondii tachyzoites; two distinct PCNA genes (TgPCNA1 and TgPCNA2) exist in T. gondii genomic DNA, likely arising from gene duplication in the common ancestor of apicomplexans. TgPCNA1 and PfPCNA1 share 49% amino acid identity, higher than intraspecies PCNA pairs (27–30%). cDNA library screening, Southern blot, Northern blot, Western blot with specific antibodies, phylogenetic analysis Molecular and biochemical parasitology Medium 10960171
2005 TgPCNA1 is constitutively concentrated in the nucleus throughout all cell cycle phases in tachyzoites, including during S-phase where it displays a punctate pattern consistent with assembly into replication foci. Indirect immunofluorescence with TgPCNA1-GFP fusion protein and cell cycle staging Molecular and biochemical parasitology Medium 15878790
2005 Heterologous expression of TgPCNA1, but not TgPCNA2, complemented a POL30 (PCNA) cold-sensitive yeast mutant, indicating TgPCNA1 is functionally equivalent to canonical eukaryotic PCNA and likely serves as the major replisomal factor in T. gondii. Yeast complementation assay using cold-sensitive POL30 mutant strain Molecular and biochemical parasitology High 15878790
2005 The TgPCNA1 gene could not be disrupted in tachyzoites, indicating it is essential for tachyzoite stage growth, consistent with its role as the primary replisomal PCNA. Gene disruption/knockout attempts in transgenic parasites Molecular and biochemical parasitology Medium 15878790
2025 TgPCNA1 tagged with NeonGreen exhibits varied, dynamic subcellular localization across cell cycle phases, with DNA replication (detected by TgPCNA1 signal) initiating at or near centromeric regions and following a bell-shaped dynamic. TgPCNA1 was validated as a functional FUCCI probe for marking the S-phase in T. gondii tachyzoites. Quantitative live-cell imaging and immunofluorescence microscopy using TgPCNA1-NeonGreen in the auxin-induced degradation parental strain (ToxoFUCCIS system) mBio Medium 40590555

Source papers

Stage 0 corpus · 4 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2000 Two genes encoding unique proliferating-cell-nuclear-antigens are expressed in Toxoplasma gondii. Molecular and biochemical parasitology 21 10960171
2005 Biochemical and genetic analysis of the distinct proliferating cell nuclear antigens of Toxoplasma gondii. Molecular and biochemical parasitology 20 15878790
2025 Deciphering cell cycle organization of Toxoplasma endodyogeny. mBio 5 40590555
2026 Assessing the Toxoplasma Tachyzoite Cell Cycle Phases Using Fluorescent Ubiquitination-Based Cell Cycle Indicator. Bio-protocol 0 41607704