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.

Dense granules: are they key organelles to help understand the parasitophorous vacuole of all apicomplexa parasites?

Together with micronemes and rhoptries, dense granules are specialised secretory organelles of Apicomplexa parasites. Among Apicomplexa, Plasmodium represents a model of parasites propagated by way of an insect vector, whereas Toxoplasma is a model of food borne protozoa forming cysts. Through comparison of both models, this review summarises data accumulated over recent years on alternative strategies chosen by these parasites to develop within a parasitophorous vacuole and explores the role of dense granules in this process. One of the characteristics of the Plasmodium erythrocyte stages is to export numerous parasite proteins into both the host cell cytoplasm and/or plasma membrane via the vacuole used as a step trafficking compartment. Whether this feature can be correlated to few storage granules and a restricted number of dense granule proteins, is not yet clear. By contrast, the Toxoplasma developing vacuole is decorated by abundantly expressed dense granule proteins and is characterised by a network of membranous nanotubes. Although the exact function of most of these proteins remains currently unknown, recent data suggest that some of these dense granule proteins could be involved in building the intravacuolar membranous network. Conserved expression of the Toxoplasma dense granule proteins throughout most of the parasite stages suggests that they could also be key elements of the cyst formation.

GRA9, a new Toxoplasma gondii dense granule protein associated with the intravacuolar network of tubular membranes

Important components of the parasitophorous vacuole in which the intracellular protozoan parasite Toxoplasma gondii develops, comprise proteins secreted from apicomplexan specific secretory organelles named the dense granules. Here, we confirm by immunofluorescence and by cryo-electron microscopy that the recently isolated B10 protein (318 amino acids, 41kDa) is a new dense granule protein that should now be referred to as GRA9. Within the vacuolar compartment, GRA9, like GRA2, GRA4 and GRA6, associates with the network of tubular membranes connected to the parasitophorous vacuole delimiting membrane. Like the other GRA proteins, GRA9 is secreted into the vacuole from the anterior end of the parasite. However, unlike GRA2 or GRA6, GRA9 does not transit by the posterior invaginated pocket of the parasite where the network first assembles. Within the dense granules, GRA9 exists in both a soluble and an insoluble state. Like the other GRA proteins, GRA9 is secreted as a soluble form only and like most of the GRA proteins, two forms of GRA9 of the similar molecular weight are detected within the vacuolar space: a soluble form and a membrane associated form. The dual properties of GRA9 are not only ascribed by the presence of amphipathic and hydrophobic alpha-helices but also by the fact that the protein is mainly hydrophilic.

Nitric oxide-mediated regulation of gamma interferon-induced bacteriostasis: inhibition and degradation of human indoleamine 2,3-dioxygenase

Tryptophan depletion resulting from indoleamine 2,3-dioxygenase (IDO) activity within the kynurenine pathway is one of the most prominent gamma interferon (IFN-gamma)-inducible antimicrobial effector mechanisms in human cells. On the other hand, nitric oxide (NO) produced by the inducible isoform of NO synthase (iNOS) serves a more immunoregulatory role in human cells and thereby interacts with tryptophan depletion in a number of ways. We investigated the effects of NO on IDO gene transcription, protein synthesis, and enzyme activity as well as on IDO-mediated bacteriostasis in the human epithelial cell line RT4. IFN-gamma-stimulated RT4 cells were able to inhibit the growth of Staphylococcus aureus in an IDO-mediated fashion, and this bacteriostatic effect was abolished by endogenously produced NO. These findings were supported by experiments which showed that IDO activity in extracts of IFN-gamma-stimulated cells is inhibited by the chemical NO donors diethylenetriamine diazeniumdiolate, S-nitroso-L-cysteine, and S-nitroso-N-acetyl-D,L-penicillamine. Furthermore, we found that both endogenous and exogenous NO strongly reduced the level of IDO protein content in RT4 cells. This effect was not due to a decrease in IDO gene transcription or mRNA stability. By using inhibitors of proteasomal proteolytic activity, we showed that NO production led to an accelerated degradation of IDO protein in the proteasome. This is the first report, to our knowledge, that demonstrates that the IDO is degraded by the proteasome and that NO has an effect on IDO protein stability.