Unraveling the Elusive Rhoptry Exocytic Mechanism of Apicomplexa

Rhoptries are connected to an apical vesicle which is docked the plasma membrane (PM) and extends a rosette of PM-embedded intramembranous particles via electron-dense connecting material. Rhoptry secretion thus involves multiple membrane-fusion events. The essential for rhoptry but dispensable microneme exocytosis. Rosette formation exocytosis require conserved proteins that restricted Alveolata. Apicomplexan parasites unicellular eukaryotes invade cells in they proliferate. development genetic tools Toxoplasma, then Plasmodium, 1990s allowed first description molecular machinery used motility invasion, revealing crucial role two different secretory organelles, micronemes rhoptries. injected directly into host cytoplasm not only promote invasion also manipulate functions. Nonetheless, injection has remained mysterious, major conundrum field. Here we review recent progress uncovering structural components implicated explain how revisiting early findings considering evolutionary origins Apicomplexa contributed some these discoveries. alveolates constitute clade within arose ~850 million years ago. group includes several thousand organisms (protists) with very forms environmental niches. They may have free-living or parasitic life styles. three phyla Ciliata, Dinoflagellata, Apicomplexa, it contains Perkinsozoa Chromerida. entire unified by presence flattened sacs underlying evolved specific functions each taxon. In anchor key required parasites. this phylum diverse intracellular vertebrates infect nonvertebrate hosts. typically sexual cycles defined host, though species. closest relatives Chrompodellids, include Chromerids predatory Colpodellids. ciliates heterotrophic ecologically important free-swimming estimated 30 000 morphologically owe their name numerous motile cilia partially completely cover cell surface. characterized dimorphic organization, transcriptionally silent micronucleus carrying germline genome, macronucleus somatic genome sustains all gene expression during vegetative growth. emerging powerful electron microscopy imaging technique can reveal structure intact frozen-hydrated state 3D macromolecular (∼4 nm) resolution. cryo-ET, plunge-frozen onto microscope grids imaged at temperature liquid nitrogen angles, as tilted. This generates series projection images allows reconstruction part 3D, essentially native state. dinoflagellates mostly marine plankton freshwater organisms. possess flagella. Dinoflagellata – flagella parasitize mollusks. visualizing internal organization membranes. It consists freezing breaking biological sample, membranes visualized vacuum-deposition platinum–carbon observed transmission microscopy. seen freeze-fractured cellular membranes, presumed be formed integral proteins. membrane-bounded organelle Tetrahymena, homologous trichocyst. Like trichocysts, stably membrane, almost store newly synthesized cargo secreted when extracellular signals stimulate exocytic fusion. club-shaped organelle, parasite's end through elongated neck. content made lipidic membranous parasite interacts cell. term 'secretion' associated trafficking pathway common eukaryotes. translocation from endoplasmic reticulum Golgi apparatus, vesicle-mediated transport towards membrane. 'exocytosis' represents last step, fuses releases its space. often refer step itself. use 'rhoptry secretion' full process cell, fusion occurring Paramecium. named according Greek words sqίvός (hair) jύrsg (vesicle, cyst). Trichocysts were cytoplasmic organelles described, decades before mitochondria chloroplasts.