Checkpoints of apicomplexan cell division identified in Toxoplasma gondii
Fig 8
A roadmap of putative checkpoints of the tachyzoite cell cycle.
Cartoon summaries findings of the current study. The tachyzoite cell cycle consists of G1, S-phase, and mitosis that overlaps and is coordinated with cytokinesis (Black and white circle). Binary division of tachyzoites continues for 5–6 rounds within the same vacuole, and then, parasites lyse the host cell and egress. We identified several potential stopping points (checkpoints) regulated by Cdk-related kinases of T. gondii. Red hexagons indicate the timing of the synchronized growth arrest or retardation caused by dis-regulation of the major cellular pathway. It appears that T. gondii retained three conserved stop points in the cell cycle. The restriction point in G1 that is related to cell differentiation and dormancy and is regulated by TgCrk2 kinase in complex with TgPHO80 cyclin (orange arrow). Licensing of DNA replication is likely under control of atypical TgCrk5 (blue arrow) (Naumov and White, personal communication), while novel TgCrk6 (dark green arrow) might operate the spindle assembly checkpoint in metaphase. Specialized mitosis of apicomplexan parasites seems to acquire two additional points of cell cycle control. We showed that coccidian-specific kinase TgCrk4 was required to maintain proper stoichiometry of the novel bipartite centrosome (light green arrow). And distantly related to higher eukaryotic Cdk11, TgCrk1 appears to control a vital parasite-specific process of the zoite assembly (purple arrow).