Identification of divergent Toxoplasma Nuclear Pore Complex components highlights speciation of mRNA export machinery

A hallmark of the eukaryotic cell is the regulated transport between the nucleus and cytoplasm, which is mediated by a multi-subunit protein assembly called the nuclear pore complex (NPC). While its overall architecture has been preserved across eukaryotes, variations in NPC structure appear to have tuned its function in different organisms. Outside of a handful of model systems, the NPC has not been comprehensively studied. This is particularly true of species that are not closely related to well-studied models, such as apicomplexan parasites. Indeed, the evolutionary divergence of Apicomplexa has complicated facile prediction of these proteins in these organisms. Because of this, the NPC components remain largely unidentified, and therefore NPC cellular function in Apicomplexa is poorly understood.

Here we identified, experimentally validated, and functionally characterized protein components of the NPC in the apicomplexan parasite Toxoplasma gondii. By combining proximity biotinylation with careful bioinformatic analysis we identified 15 previously uncharacterized proteins that localize to the Toxoplasma NPC. We demonstrated 7 of these proteins are essential to parasite replication. Importantly, we defined components of the mRNA export machinery, as well as subunits required for the stability and/or assembly of specific NPC subcomplexes. Consistent with the evolutionary distance between Toxoplasma and well-studied models, the majority of our newly validated NPC components show no clear homology to NPC proteins in yeast, animals, or plants. Moreover, we demonstrated that the Toxoplasma mRNA export machinery has a distinct composition from other well-established systems. Intriguingly, several well-defined domains the TREX-2 complex that are essential in other systems are missing from the Toxoplasma genome. In contrast, others, such as Centrin-3, have been conserved in Toxoplasma, but are not required for mRNA export in the parasite.

Our work highlights the distinct composition of multiple subcomplexes of the Toxoplasma NPC and paves the way for future studies to provide high-resolution structural information on the parasite's unusual NPC architecture.