Striated biber assemblins in the Apicomplexa
Jordan, Carly Nicole
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The phylum Apicomplexa is a diverse group of intracellular parasites of humans and animals. The invasive stage, the zoite, is found in each species at some point in their life cycle. The zoite is an organized and polar cell with specialized cytoskeletal elements and organelles at the apical end that aid in host cell invasion. Cell division or generation of the zoite occurs using several unconventional mechanisms. Endodyogeny is a form of internal budding, whereby two daughter cells are constructed within the mother following a single round of DNA replication and nuclear division. More complicated forms of Apicomplexan cell division include endopolygeny and schizogony, where dozens of daughters are built within a single mother cell, after multiple rounds of DNA replication with or without consecutive nuclear division. How cells ensure that each daughter receives one copy of the genome and each organelle is a key question that is the focus of this dissertation. Previous work demonstrated that the centrosomes, from which spindle microtubules originate, associate with organelles during division, and that a striated fiber has been observed near the centrosomes in ultrastructural studies of dividing Apicomplexa. We hypothesize that daughter cells construction is organized by the centrosomes via a physical structure composed of striated fiber assemblin (SFA) proteins. SFA genes are encoded in the genomes of many Apicomplexa. We visualized these proteins in T. gondii tachyzoites expressing epitope tags for immunofluorescence assays (IFA), and observed that SFA proteins have a dynamic localization pattern over the cell cycle. SFA first appears as discrete spots on or near duplicated centrosomes. As daughters are built, SFA forms a curved fiber between the centrosome and the apical end of developing daughter cells. Overexpression of the SFAs results in major division defects as well as mislocalization of other proteins involved in daughter cell formation. Antibodies created against recombinant T. gondii SFAs were used for IFA on the related parasite Sarcocystis neurona, and label structures near the spindle and apical end of developing zoites. Taken together, these results suggest that the SFA fibers play an important and conserved role in daughter cell formation in the Apicomplexa.