Microtubule stability and dynamics in Plasmodium sporozoites

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Microtubules are long filamentous structures formed from dimers of alpha- and ß-tubulin. Microtubules form and disassemble dynamically and these dynamics are essential for many cellular processes. Post-translational modifications and microtubule binding proteins play key roles in modulating microtubule dynamics. Malaria causing Plasmodium parasites belong to the apicomplexa within the chromalveolata and are divergent life forms with a cellular biology distinct to opisthokont model organisms such as yeast. Like in other organisms, microtubules in Plasmodia are important for cell division, vesicular transport, motility and parasite morphogenesis. Some inhibitors of microtubule dynamics have been shown to kill parasites in their growth phase but once formed the microtubules are remarkably stable. We recently showed that deletion of one of the two alpha-tubulin genes blocks formation of Plasmodium sporozoites, the forms transmitted by the Anopheles mosquito. In these α1-tubulin(-) parasites, no microtubules formed inside the parasitic oocysts where sporozoites are formed. Remarkably, when complementing this deletion with the alpha2-tubulin gene sporozoites formed that showed fewer and shorter microtubules and had an aberrant shape. Strikingly, sporozoites were shorter and showed a stronger curvature, which lead to different movement paths of these highly motile parasites. These alpha2-tubulin complemented parasites were less infectious than the wild type or alpha1-tubulin complemented control lines but could complete the parasite life cycle. Detailed microscopic analysis of microtubule length during the formation of the sporozoites revealed a surprising growth of microtubules to over 12 micrometer during sporozoite formation followed by a shrinkage to about 6 micrometer after parasite formation is completed for the wild type and to 4.5 micrometer in the mutant. We now want to address the exact molecular basis for the different behaviors of the two alpha-tubulin isotypes and generate parasites with longer microtubules to test how these impact on sporozoite shape and infectivity. To do so we will generate a series of parasite lines expressing chimeric alpha-tubulin genes. In addition we will investigate the functionality of an acetylation site that could impact microtubule stability.