A Toxoplasma MORN1 Null Mutant Undergoes Repeated Divisions but Is Defective in Basal Assembly, Apicoplast Division and Cytokinesis

Abstract

The membrane occupation and recognition nexus protein 1 (MORN1) is highly conserved among apicomplexan parasites and is associated with several structures that have a role in cell division. Here we dissected the role of MORN1 using the relatively simple budding process of Toxoplasma gondii as a model. Ablation of MORN1 in a conditional null mutant resulted in pronounced defects suggesting a central role for MORN1 in apicoplast segregation and in daughter cell budding. Lack of MORN1 resulted in double-headed parasites. These Janus-headed parasites form two complete apical complexes but fail to assemble a basal complex. Moreover, these parasites were capable of undergoing several more budding rounds resulting in the formation of up to 16-headed parasites conjoined at the basal end. Despite this segregation defect, the mother's cytoskeleton was completely disassembled in every budding round. Overall this argues that successful completion of the budding is not required for cell cycle progression. None of the known basal complex components, including a set of recently identified inner membrane complex (IMC) proteins, localized correctly in these multi-headed parasites. These data suggest that MORN1 is essential for assembly of the basal complex, and that lack of the basal complex abolishes the contractile capacity assigned to the basal complex late in daughter formation. Consistent with this hypothesis we observe that MORN1 mutants fail to efficiently constrict and divide the apicoplast. We used the null background provided by the mutant to dissect the function of subdomains of the MORN1 protein. This demonstrated that deletion of a single MORN domain already prevented the function of MORN1 whereas a critical role for the short linker between MORN domains 6 and 7 was identified. In conclusion, MORN1 is required for basal complex assembly and loss of MORN1 results in defects in apicoplast division and daughter segregation.

Molecular Characterization of Toxoplasma gondii Formin 3, an Actin Nucleator Dispensable for Tachyzoite Growth and Motility

“…T. gondii MORN1, Centrin2, and DLC1 have been described to be located at the extreme basal end of the parasite (19,24). Disruption of TgMORN1 altered the structure of the parasite posterior end and affected cytokinesis and apicoplast segregation (20,27). However, the absence of TgFRM3 at the basal ends of daughter cells distinguishes it from having a MORN1-related role in parasite division.…”

The HU Protein Is Important for Apicoplast Genome Maintenance and Inheritance in Toxoplasma gondii

“…The elongated organelle is then constricted by a cytokinetic ring unique to the apicomplexan budding process. MORN1 is a key protein of this ring and is required for optimal apicoplast division (26). The final step of apicoplast fission depends on the dynamin-like protein DrpA (49).…”

SPM1 Stabilizes Subpellicular Microtubules in Toxoplasma gondii

“…These microtubule populations are nucleated from distinct microtubule-organizing centers (MTOCs). During cell division, which occurs by a closed mitosis, the poles of intranuclear spindles are associated with a specialized region of the nuclear envelope termed the centrocone or spindle pole plaque, which is labeled by MORN1 (24,30,51,75). The apical polar ring (APR), a circular MTOC found only in apicomplexan parasites, is located at the parasite apex and nucleates the subpellicular microtubules, which extend toward the parasite posterior in close association with the cytosolic face of the parasite pellicle (64,72,77).…”