Targeted deletion of MIC5 enhances trimming proteolysis of Toxoplasma invasion proteins

Limited proteolysis of proteins transiently expressed on the surface of the opportunistic pathogen Toxoplasma gondii accompanies cell invasion and facilitates parasite migration across cell barriers during infection. However, little is known about what factors influence this specialized proteolysis or how these proteolytic events are regulated. Here we show that genetic ablation of the micronemal protein MIC5 enhances the normal proteolytic processing of several micronemal proteins secreted by Toxoplasma tachyzoites. Restoring MIC5 expression by genetic complementation reversed this phenotype, as did treatment with the protease inhibitor ALLN, which was previously shown to block the activity of a hypothetical parasite surface protease called MPP2. We show that, despite its lack of obvious membrane association signals, MIC5 occupies the parasite surface during invasion in the vicinity of the proteins affected by enhanced processing. Proteolysis of other secretory proteins, including GRA1, was also enhanced in MIC5 knockout parasites, indicating that the phenotype is not strictly limited to proteins derived from micronemes. Together, our findings suggest that MIC5 either directly regulates MPP2 activity or it influences MPP2's ability to access substrate cleavage sites on the parasite surface.