ARK1

TgArk1 (Aurora kinase 1) is a serine/threonine kinase that organizes mitotic division in Toxoplasma gondii by serving as the enzymatic core of the chromosomal passenger complex (CPC), which it forms with TgINCENP1 and TgINCENP2 (PMID:30051161). Within this complex, TgINCENP2 acts as the adapter required for TgArk1 nuclear translocation, so that CPC assembly and correct nuclear delivery are coupled (PMID:30051161). TgArk1 displays dynamic, cell cycle-dependent subcellular localization consistent with a role in mitotic spindle organization during endodyogeny (PMID:30051161), and interference with the kinase produces pronounced division and growth defects, establishing it as essential for productive endodyogeny and functional spindle assembly (PMID:30051161). Low-dose hesperadin, an inhibitor of human Aurora kinase B, phenocopies the TgArk1 and TgINCENP2 mutant defects, indicating that TgArk1 catalytic activity is required for its function (PMID:30051161). Beyond CPC composition, nuclear import dependence, localization dynamics, and the loss-of-function phenotype, no direct substrates or structural detail have been characterized in the available corpus.

1. 2018 Medium

   Establishing the subunit composition of the Toxoplasma CPC answered which proteins partner with the Aurora kinase to drive division, defining TgArk1 as the kinase core of a TgINCENP1/TgINCENP2 complex.

   Evidence Co-immunoprecipitation / interaction assays showing TgArk1 binds both TgINCENPs

   PMID:30051161

   Open questions at the time

   • No structural or in vitro reconstitution of the complex
   • Stoichiometry and direct vs bridged interactions among the three subunits not resolved
2. 2018 Medium

   Perturbing TgINCENP2 addressed how TgArk1 reaches its site of action, showing TgINCENP2 is the adapter required for TgArk1 nuclear translocation.

   Evidence Genetic interference with TgINCENP2 combined with TgArk1 localization analysis

   PMID:30051161

   Open questions at the time

   • Molecular mechanism of the import (nuclear localization signal, transport machinery) not defined
   • Single-lab, single-study result
3. 2018 Medium

   Tracking TgArk1 through the cell cycle answered whether its localization is stage-restricted, revealing dynamic redistribution consistent with mitotic spindle organization during endodyogeny.

   Evidence Imaging of tagged TgArk1 across cell cycle stages

   PMID:30051161

   Open questions at the time

   • Precise spindle/kinetochore structures engaged not molecularly defined
   • No independent replication reported
4. 2018 Medium

   Loss-of-function and pharmacological experiments together established that TgArk1 and its catalytic activity are essential for division, since knockdown caused division/growth defects and low-dose hesperadin phenocopied the mutants.

   Evidence Conditional knockdown/dominant-negative interference plus hesperadin inhibition with phenotypic comparison

   PMID:30051161

   Open questions at the time

   • Kinase activity requirement inferred pharmacologically, not by kinase-dead allele or in vitro assay
   • Hesperadin specificity for TgArk1 over other targets not established

• The direct phosphorylation substrates of TgArk1 and the structural basis of CPC assembly and spindle regulation remain unidentified.
• No TgArk1 substrates identified
• No structural or biochemical reconstitution of the CPC
• Mechanism linking CPC activity to spindle organization undefined