Essential function of the alveolin network in the subpellicular microtubules and conoid assembly in Toxoplasma gondii

Abstract

The coccidian subgroup of Apicomplexa possesses an apical complex harboring a conoid, made of unique tubulin polymer fibers. This enigmatic organelle extrudes in extracellular invasive parasites and is associated to the apical polar ring (APR). The APR serves as microtubule-organizing center for the 22 subpellicular microtubules (SPMTs) that are linked to a patchwork of flattened vesicles, via an intricate network composed of alveolins. Here, we capitalize on ultrastructure expansion microscopy (U-ExM) to localize the Toxoplasma gondii Apical Cap protein 9 (AC9) and its partner AC10, identified by BioID, to the alveolin network and intercalated between the SPMTs. Parasites conditionally depleted in AC9 or AC10 replicate normally but are defective in microneme secretion and fail to invade and egress from infected cells. Electron microscopy revealed that the mature parasite mutants are conoidless, while U-ExM highlighted the disorganization of the SPMTs which likely results in the catastrophic loss of APR and conoid.

Expansion Microscopy provides new insights into the cytoskeleton of malaria parasites including the conservation of a conoid

“…The relative position of MyoB in merozoite is currently unknown. The orientation of the polyglutamylation profile of the subpellicular microtubules in the merozoite is proposed by analogy with the polyglutamylation profile of subpellicular microtubules in T. gondii tachyzoites 28 . In ookinete the apical section of less polyglutamylated subpellicular microtubules may correspond to the collar.…”

The Riveting Cellular Structures of Apicomplexan Parasites

“…The majority of the SPN is made up of intermediate filament-like proteinsnamed alevolins, based on the presence of characteristic repeatswhich are required for correct localization and assembly into a highly insoluble network [16,87,88]. Assembly of the SPN occurs very early during daughter cell construction [16,89] and interestingly, compared to the maternal SPN, the daughter cell SPN can be extracted more easily by detergent, indicating a more fluid composition [90]. This change in solubility may be mediated either by changes in the SPN composition [11] and/or by post-translational modifications of the alveolins [91].…”

“…One of the stumbling blocks in understanding the roles of the alveolins was imaging the small, densely packed structures. However recent advances in expansion microscopy has allowed unprecedented localization of individual alveolins and microtubules in the cell [69,72,89]. Determining their precise localization will allow functions for individual proteins to be suggested, informing future research.…”

“…In Plasmodium ookinetes, the apically localized protein phosphatase containing kelch-like domains (PPLK) has been identified to mediate the connection of the microtubules to the alveoli, in addition to its role in maintaining the stability of the apical complex [61,101]. Further, the apically restricted alveolins AC9 and AC10 are required to maintain the apical polar ring and the arrangement of cortical microtubules in T. gondii [89]. These results are interesting as they show that alveolins can also convey patterning information, which may have implications in how cells are constructed.…”

The life-cycle of Toxoplasma gondii reviewed using animations

“…Besides, there is a network of filamentous structures with a mean diameter of 8–10 nm and are very labile and located immediately below the inner membrane complex [ 45 ]. They are composed of proteins of the articulin group called alveolins [ 46 – 48 ] and extend throughout the cell ending in a circular structure located at the posterior region, known as the basal complex, where proteins such as the membrane occupation and recognition nexus exist (MORN-1) (Additional file 5 : Figure S5) [ 35 , 49 ].…”