Awshah, D. | Search bioRxiv

bioRxiv2024

A FUCCI sensor reveals complex cell cycle organization of Toxoplasma endodyogeny

In this study, we report the atypical cell cycle organization of the unicellular eukaryotic pathogen Toxoplasma gondii. The remarkably flexible cell division of T. gondii and other apicomplexan parasites differs considerably from the cell division modes employed by other model eukaryotes. Additionally, there is a lack of recognizable cell cycle regulators, which have contributed to the difficulties in deciphering the order of events in the apicomplexan cell cycle. To aid in studies of the cell cycle organization of the T. gondii tachyzoite, we have created the Fluorescent Ubiquitination-based Cell Cycle Indicator (FUCCI) probes, ToxoFUCCIS and ToxoFUCCISC. We introduced a DNA replication factor TgPCNA1 tagged with NeonGreen that can be used alone or in conjunction with an mCherry-tagged budding indicator TgIMC3 in the auxin-induced degradation (AID) parental strain. The varied localization and dynamic cell cycle oscillation have confirmed TgPCNA1 to be a suitable T. gondii FUCCI probe. The ToxoFUCCIS analysis showed that tachyzoite DNA replication starts at or near centromeric regions, has a bell-shaped dynamic and a significant degree of the cell cycle asynchrony within the vacuoles. Quantitative live and immunofluorescence microscopy analyses of ToxoFUCCIS and its derivatives co-expressing epitope-tagged cell cycle markers have revealed an unusual composite cell cycle phase that incorporates overlapping S, G2, mitosis and cytokinesis (budding). We identified five intervals of the composite phase and their approximate duration: S (19%), S/G2/C (3%), S/M/C (9%), M/C (18%) and C/G1 (<1%). The ToxoFUCCIS probe efficiently detected G2/M and Spindle Assembly Checkpoints, as well as the SB505124-induced TgMAPK1 dependent block. Altogether, our findings showed an unprecedented complexity of the cell cycle in apicomplexan parasites.

cell biology↗

bioRxiv2023

The Crk4-Cyc4 complex regulates G2 phase of apicomplexan endodyogeny

Division of apicomplexan parasites differs drastically from the division of their host cells. A fraction of apicomplexans divides in the traditional binary mode, such as Toxoplasma gondii in asexual stages, whereas the vast majority instead divide in a multinuclear fashion. Such variety of replication modes and a dearth of conserved conventional regulators have hindered the progress of apicomplexan cell cycle studies. We previously identified five Cdk-related kinases (Crk) involved in endodyogenic division of T. gondii tachyzoites. The current study investigates the roles of a novel essential cell cycle kinase TgCrk4. We identified this kinase cyclin partner and demonstrated that TgCrk4 regulates processes carried out during conventional G2 phase, such as repression of chromosome rereplication and centrosome re-duplication. Accumulation of TgCyc4 in the nucleus and on the centrosomes supported the role of TgCrk4-TgCyc4 complex as a coordinator of chromosome and centrosome cycles in T. gondii. Examination of the TgCrk4-deficient tachyzoites confirmed a cell cycle stop prior to the TgCrk6-regulated spindle assembly checkpoint. Furthermore, we identified an ortholog of the DNA replication licensing factor Cdt1 that was a dominant interactor of the TgCrk4-TgCyc4 complex. T. gondii Cdt1 is highly divergent but preserved critical signature domains and appeared to play a minimal or no role in licensing DNA replication in G1 phase. Functional analyses indicated the primary role of TgCdt1 is in controlling chromosome rereplication and centrosome reduplication. Global phosphoproteome analyses identified immediate TgCrk4 substrates, such as DNA replication licensing factor TgORC4, component of the anaphase-promoting complex TgCdc20, {gamma}-tubulin nucleation factor TgGCP2, and the catalytic subunit of cell cycle phosphatase TgPP2ACA. Importantly, our phylogenetic and structural analyses revealed that the functional TgCrk4-TgCyc4 complex was encoded in the limited group of apicomplexans dividing in a binary fashion. Together with the minimal representation of binary division in Apicomplexa phylum, our findings support the novel view of apicomplexans acquiring binary division to repress ancestral multinuclear mechanisms.

cell biology↗