Co-dependent formation of the Toxoplasma gondii subpellicular microtubules and inner membrane skeleton

One of the defining features of apicomplexan parasites is their cytoskeleton composed of alveolar vesicles, known as the inner membrane complex (IMC) undergirded by an intermediate filament-like protein network and an array of subpellicular microtubules (SPMTs). In Toxoplasma gondii, this specialized cytoskeleton is involved in all aspects of the disease-causing lytic cycle and notably acts as a scaffold for parasite offspring in the internal budding process. Despite advances in our understanding of the architecture and molecular composition, insights pertaining to the coordinated assembly of the scaffold are still largely elusive. Here, T. gondii tachyzoites were dissected by advanced, iterative expansion microscopy (pan-expansion microscopy), revealing new insights into the very early sequential formation steps of the tubulin and IMC scaffold. A comparative study of the related parasite Sarcocystis neurona revealed that different MT bundling organizations of the nascent SPMTs correlate with the number of central and basal alveolar vesicles. In the absence of a so-far identified MT nucleation mechanism, we genetically dissected T. gondii γ-tubulin and γ-tubulin complex proteins 4, 5, and 6 (GCP4/5/6). While γ-tubulin depletion abolished the formation of the tubulin scaffold, a set of MTs still formed that suggests SPMTs are nucleated at the outer core of the centrosome. Depletion of GCP4/5/6 interfered with the correct assembly of nascent SPMTs into the forming daughter buds, further indicating that the parasite utilizes the γ-tubulin ring complex in tubulin scaffold formation.IMPORTANCEApicomplexan protozoan parasites rely on their specialized cytoskeleton to form offspring. The cytoskeleton serves as an essential scaffold for the emerging daughter cells and is formed by the inner membrane complex (IMC) and underlying subpellicular microtubules (SPMTs). In Toxoplasma gondii, the IMC is composed of several membranous sacks and supported by 22 SPMTs, the latter are evenly spaced around the apical end of mature parasites. Although many advances have been made, little is known about the earliest steps of scaffold formation. Here, we gain unprecedented insights into IMC and SPMT establishment via iterative expansion microscopy and comparative cell biology. We show that at the onset of division, SPMTs are grouped and reveal that the number of groups determines the number of IMC sacks that are assembled. We further dissect the parasite's γ-tubulin ring complex and show that it is critically involved in scaffold formation.