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Formin-2 drives intracellular polymerisation of actin filaments enabling correct segregation of apicoplasts in Plasmodium falciparum and Toxoplasma gondii

Pathogenic obligate-intracellular apicomplexan parasites possess an essential chloroplast-like organelle called the apicoplast that undergoes division and segregation during replication. Parasite actin is essential during intracellular development, implicated in vesicular transport, parasite replication and apicoplast inheritance. However, the inability to visualise live actin dynamics in apicomplexan parasites limited functional characterisation of both filamentous-actin (F-actin) and actin regulatory factors. Apicomplexans possess at least two distinct formins, Formin-1 and Formin-2, predicted to serve as actin-nucleating factors, and previously implicated in regulating gliding motility and host cell invasion. Here, we expressed chromobodies and validated them as F-actin-binding sensors in Plasmodium falciparum and characterised the in vivo dynamics of the F-actin network. The F-actin network could be modulated chemically and disrupted by conditionally deleting the actin-1 gene. In a comparative approach, we demonstrate that Formin-2 is closely associated with apicoplasts and with the F-actin network in P. falciparum and Toxoplasma gondii. Consequently, disruption of Formin-2 resulted not only in an apicoplast segregation defect, but also in complete abrogation of F-actin dynamics in intracellular parasites. Together, our results strongly indicate that Formin-2-mediated filament formation is the common primary mechanism for F-actin nucleation during apicomplexan intracellular growth effecting apicoplast segregation.