The Toxoplasma centrocone houses cell cycle regulatory factors

Our knowledge of cell cycle regulatory mechanisms in apicomplexan parasites is very limited. In this study, we describe a novel Toxoplasma gondii factor, essential for chromosome replication 1 (ECR1), that has a vital role in chromosome replication and the regulation of cytoplasmic and nuclear mitotic structures. ECR1 was discovered by complementation of a temperature sensitive (ts) mutant that suffers lethal, uncontrolled chromosome replication at 40{degrees}C similar to a ts-mutant carrying a defect in topoisomerase. ECR1 is a 52kDa protein containing divergent RING and TRAF-Sina like zinc-binding domains that is dynamically expressed in the tachyzoite cell cycle. ECR1 first appears in the centrocone compartment of the nuclear envelope in early S phase and then in the nucleus in late S phase where it reaches maximum expression. Following nuclear division, but before daughters resolve from the mother, ECR1 is down regulated and is absent in new daughter parasites. The proteomics of ECR1 identified interactions with the ubiquitin-mediated protein degradation machinery and the minichromosome maintenance complex and the loss of ECR1 led to increased stability of a key member of this complex, MCM2. ECR1 also forms a stable complex with the CDK-related kinase, TgCrk5, which shares a similar cell cycle expression and localization during tachyzoite replication. Altogether, the results of this study suggest ECR1 may be a unique E3 ligase that regulates DNA licensing and other mitotic processes. Importantly, the localization of ECR1/TgCrk5 in the centrocone indicates this Apicomplexa-specific spindle compartment houses important regulatory factors that control the parasite cell cycle.\n\nIMPORTANCEParasites of the apicomplexan family are important causes of human disease including malaria, toxoplasmosis, and cryptosporidiosis. Parasite growth is the underlying cause of pathogenesis, yet despite this importance the molecular basis for parasite replication is poorly understood. Filling this knowledge gap cannot be accomplished by mining recent whole genome sequencing because apicomplexan cell cycles differ substantially and lack many of the key regulatory factors of well-studied yeast and mammalian cell division models. We have utilized forward genetics to discover essential factors that regulate cell division in these parasites using the Toxoplasma gondii model. An example of this approach is described here with the discovery of a putative E3 ligase/protein kinase mechanism involved in regulating chromosome replication and mitotic processes of asexual stage parasites.