Characterization of Toxoplasma Rack1: Implications for a Novel 'Interacktion'

Receptor for Activated C Kinase 1 (RACK1) has been implicated in multiple protein-protein interactions, although it was first described as the intracellular receptor for protein kinase C (PKC). Significant parasite-specific functions, including roles in resistance to Leishmania major and progression through cytokinesis in Trypanosoma brucei, have also been described. Here the RACK1 homologue in the parasite Toxoplasma gondii is identified and characterized. TgRACK1 displays the anticipated cytoplasmic localization 20-24 hours post infection. Shortly after parasite invasion, however, TgRACK1 concentrates around the nucleus and apicoplast (a parasite-specific organelle). Vesicular trafficking occurs between these organelles in T. gondii, and studies have suggested roles for PKCs in the secretory pathway. Since RACKs target PKCs, an investigation into a link between TgRACK1 and the secretory pathway was initiated. FLAG-epitope-tagged-TgRACK1 colocalized and coprecipitated with the early secretory pathway marker Tgbeta-COP, providing evidence for a novel 'interRACKtion.'