The Actomyosin Systems in Apicomplexa

Plasmodium falciparum formins are essential for invasion and sexual stage development

The malaria parasite uses actin-based mechanisms throughout its lifecycle to control a range of biological processes including intracellular trafficking, gene regulation, parasite motility and invasion. In this work we assign functions to the Plasmodium falciparum formins 1 and 2 (FRM1 and FRM2) proteins in asexual and sexual blood stage development. We show that FRM1 is essential for merozoite invasion and FRM2 is required for efficient cell division. We also observed divergent functions for FRM1 and FRM2 in gametocyte development. Conditional deletion of FRM1 leads to a delay in gametocyte stage progression. We show that FRM2 controls the actin and microtubule cytoskeletons in developing gametocytes, with premature removal of the protein resulting in a loss of transmissible stage V gametocytes. Lastly, we show that targeting formin proteins with the small molecule inhibitor of formin homology domain 2 (SMIFH2) leads to a multistage block in asexual and sexual stage parasite development.

A PPP-type pseudophosphatase is required for the maintenance of basal complex integrity in Plasmodium falciparum

During its asexual blood stage, P. falciparum replicates via schizogony, wherein dozens of daughter cells are formed within a single parent. The basal complex, a contractile ring that separates daughter cells, is critical for schizogony. In this study, we identify a Plasmodium basal complex protein essential for basal complex maintenance. Using multiple microscopy techniques, we demonstrate that PfPPP8 is required for uniform basal complex expansion and maintenance of its integrity. We characterize PfPPP8 as the founding member of a novel family of pseudophosphatases with homologs in other Apicomplexan parasites. By co-immunoprecipitation, we identify two additional new basal complex proteins. We characterize the unique temporal localizations of these new basal complex proteins (late-arriving) and of PfPPP8 (early-departing). In this work, we identify a novel basal complex protein, determine its specific role in segmentation, identify a new pseudophosphatase family, and establish that the P. falciparum basal complex is a dynamic structure.

Cellularization across eukaryotes: Conserved mechanisms and novel strategies

Many eukaryotes form multinucleated cells during their development. Some cells persist as such during their lifetime, others choose to cleave each nucleus individually using a specialized cytokinetic process known as cellularization. What is cellularization and how is it achieved across the eukaryotic tree of life? Are there common pathways among all species supporting a shared ancestry, or are there key differences, suggesting independent evolutionary paths? In this review, we discuss common strategies and key mechanistic differences in how cellularization is executed across vastly divergent eukaryotic species. We present a number of novel methods and non-model organisms that may provide important insight into the evolutionary origins of cellularization.

The Ringleaders: Understanding the Apicomplexan Basal Complex Through Comparison to Established Contractile Ring Systems

The actomyosin contractile ring is a key feature of eukaryotic cytokinesis, conserved across many eukaryotic kingdoms. Recent research into the cell biology of the divergent eukaryotic clade Apicomplexa has revealed a contractile ring structure required for asexual division in the medically relevant genera Toxoplasma and Plasmodium; however, the structure of the contractile ring, known as the basal complex in these parasites, remains poorly characterized and in the absence of a myosin II homolog, it is unclear how the force required of a cytokinetic contractile ring is generated. Here, we review the literature on the basal complex in Apicomplexans, summarizing what is known about its formation and function, and attempt to provide possible answers to this question and suggest new avenues of study by comparing the Apicomplexan basal complex to well-studied, established cytokinetic contractile rings and their mechanisms in organisms such as S. cerevisiae and D. melanogaster. We also compare the basal complex to structures formed during mitochondrial and plastid division and cytokinetic mechanisms of organisms beyond the Opisthokonts, considering Apicomplexan diversity and divergence.