Ultrastructure expansion microscopy of Plasmodium gametocytes reveals the molecular architecture of a microtubule organisation centre coordinating mitosis with axoneme assembly

Abstract

The transmission of malaria-causing parasites to mosquitoes relies on the production of the gametocyte stages and their development into gametes upon a blood feed. These stages display various microtubule cytoskeletons and the architecture of the corresponding microtubule organisation centres (MTOC) remains elusive. Combining ultrastructure expansion microscopy (U-ExM) with bulk proteome labelling, we first reconstructed in 3D the subpellicular microtubule network and its associated actin cytoskeleton, which confer cell rigidity to Plasmodium falciparum gametocytes. Upon activation, as the microgametocyte undergoes three rounds of endomitosis, it simultaneously assembles axonemes to form eight flagellated microgametes. Here, U-ExM combined with Pan-ExM revealed the molecular architecture of a single bipartite MTOC coordinating mitosis with axoneme formation. This MTOC spans the nuclear membrane linking acentriolar mitotic plaques to cytoplasmic basal bodies by proteinaceous filaments. The eight basal bodies are concomitantly de novo assembled from a deuterosome-like structure, where centrin, {gamma}-tubulin, SAS4/CPAP and SAS6 form distinct subdomains. Once assembled, the basal bodies show a fusion of the proximal and central cores where colocalised centrin and SAS6 are surrounded by a SAS4/CPAP-toroid in the lumen of the microtubule wall. Sequential nucleation of axonemes and mitotic spindles is associated with a dynamic movement of {gamma}-tubulin from the basal bodies to the acentriolar plaques. We finally show that this atypical MTOC architecture relies on two non-canonical MTOC regulators, the calcium-dependent protein kinase 4 and the serine/arginine-protein kinase 1. Altogether, these results provide insights into the molecular organisation of a bipartite MTOC that may reflect a functional transition of a basal body to coordinate axoneme formation with mitosis.