CRK2

TgCrk2 is a cytoplasmic CDK-related kinase that governs G1 phase progression in Toxoplasma gondii tachyzoites (PMID:28671988). It is essential for tachyzoite growth: conditional knockdown arrests parasites in G1, establishing TgCrk2 as a control point for cell cycle commitment (PMID:28671988). Rather than acting through a canonical cyclin, TgCrk2 assembles context-dependent complexes with atypical P-type cyclins — forming alternative complexes with TgCycP1, TgCycP2, and TgCyc5 that constitute the machinery of the G1 restriction checkpoint governing the switch between rapid tachyzoite replication and bradyzoite differentiation (PMID:35652638). Within this network, TgCycP2 regulates G1 duration in developmentally competent ME49 tachyzoites while TgCyc5 is critical for tissue cyst maturation (PMID:35652638). TgCrk2 also engages the cyclin H-class cyclin TgCYC1, a functional cyclin partner able to rescue a G1/S cyclin defect in yeast (PMID:11897124). Beyond these cyclin partnerships and the G1-arrest phenotype, its direct substrates and catalytic mechanism have not been characterized in the available corpus.

1. 1998 Low

   Establishing whether Toxoplasma encodes a bona fide cell-cycle kinase, the isolation of a cdc2-related kinase gene provided the molecular entry point for studying CDK control in the parasite.

   Evidence genomic cloning and molecular characterisation of T. gondii CRK2

   PMID:9719519

   Open questions at the time

   • no functional in vitro kinase assay reported
   • no phenotype or cell-cycle role established
   • no cyclin partner identified
2. 2002 Medium

   To determine whether TgCRK2 has a functional cyclin partner, a yeast two-hybrid screen identified the cyclin H-class cyclin TgCYC1, which complemented a yeast G1/S cyclin defect, showing TgCRK2 can engage a functional cyclin.

   Evidence yeast two-hybrid screen with heterologous complementation in S. cerevisiae

   PMID:11897124

   Open questions at the time

   • interaction not confirmed in Toxoplasma cells
   • no demonstration that TgCYC1-TgCRK2 acts at a specific cell-cycle stage in the parasite
3. 2017 High

   To define the cell-cycle role of TgCrk2 in the parasite, conditional knockdown demonstrated it is essential and that its loss arrests parasites in G1, placing TgCrk2 as a cytoplasmic controller of G1 progression through an atypical P-type cyclin partner.

   Evidence conditional tet-OFF knockdown, cell cycle analysis, co-immunoprecipitation, and immunofluorescence in tachyzoites

   PMID:28671988

   Open questions at the time

   • direct kinase substrates not identified
   • molecular basis of cytoplasmic localization unresolved
   • identity and number of P-type cyclin partners not fully defined at this stage
4. 2022 High

   To resolve how a single kinase can regulate the growth-versus-differentiation decision, reciprocal co-IP and conditional knockdown showed TgCrk2 forms alternative complexes with TgCycP1, TgCycP2, and TgCyc5 constituting the G1 restriction checkpoint, with distinct cyclins controlling G1 duration versus cyst maturation.

   Evidence reciprocal co-immunoprecipitation, conditional knockdown, cell cycle and differentiation assays (alkaline stress and rat embryonic brain cell models) in RH and ME49

   PMID:35652638

   Open questions at the time

   • biochemical basis of cyclin selectivity not defined
   • phosphorylation targets that execute the R-point decision unknown
   • how upstream signals partition TgCrk2 among alternative cyclins unresolved

• The direct phosphorylation substrates of TgCrk2 and the structural basis of its selective assembly with atypical P-type versus cyclin H-class cyclins remain unknown.
• no in vitro or in vivo substrate identified
• no structural model of TgCrk2-cyclin complexes
• mechanism linking cyclin choice to the growth/differentiation switch not defined