Molecular Signals in the Trafficking of Toxoplasma gondii Protein MIC3 to the Micronemes

Abstract

The protozoan parasite Toxoplasma gondii is equipped with a sophisticated secretory apparatus, including three distinct exocytic organelles, named micronemes, rhoptries, and dense granules. We have dissected the requirements for targeting the microneme protein MIC3, a key component of T. gondii infection. We have shown that MIC3 is processed in a post-Golgi compartment and that the MIC3 propeptide and epidermal growth factor (EGF) modules contain microneme-targeting information. The minimal requirement for microneme delivery is defined by the propeptide plus any one of the three EGF domains. We have demonstrated that the cleavage of the propeptide, the dimerization of MIC3, and the chitin binding-like sequence, which are crucial for host cell binding and virulence, are dispensable for proper targeting. Finally, we have shown that part of MIC3 is withheld in the secretory pathway in a cell cycle-dependent manner.The apicomplexa are a group of mostly obligate intracellular parasites that are responsible for diseases such as toxoplasmosis, malaria, neosporosis, coccidiosis, and cryptosporidiosis. Host cell invasion is a prerequisite for the establishment and maintenance of infection for these parasites, and although the range of host cell specificity can vary greatly between different apicomplexan species, the machinery they use to invade their host(s) is strikingly conserved. Toxoplasma gondii, the parasite responsible for congenital infections in the developing fetus and for severe neurological complications in immunocompromised individuals, is relatively easy to manipulate genetically, invades virtually all nucleated cells, and, thus, is a good model for studying motility and invasion. Host cell invasion is accompanied by the secretion of parasite proteins from two distinct secretory organelles that are associated with attachment/motility (microneme proteins [MICs]) (23, 26), parasitophorous vacuole biogenesis (rhoptry proteins) (17), and moving junction formation (cooperative role of MICs and rhoptry neck proteins) (2, 27). Exocytosis from dense granules, a third type of specialized secretory granules, occurs during and following the invasion of the host cell and is believed to take part in the maturation of the parasitophorous vacuole, although most of the GRA proteins have not yet been attributed a function (30).The endoplasmic reticulum (ER) is the site of synthesis and the processing of proteins destined for secretion; thus, almost all secretory proteins contain a signal peptide. Most of protein sorting takes place either in the trans-Golgi network (TGN) (or its equivalent) or in immature secretory vesicles beyond the TGN. In T. gondii, soluble foreign reporter proteins, lacking targeting information, are delivered to the parasitophorous vacuole via the dense granules, which are the default route for secreted proteins (22). Similarly to other eukaryotic sorting mechanisms, a system of tyrosine-dependent signals exists in T. gondii that allows for the specific targeting of transmembrane proteins to the rhopt...

A new thrombospondin-related anonymous protein homologue in Neospora caninum (NcMIC2-like1)

“…Here we have demonstrated that NcMIC2-like1 is mainly located in the apical region of the tachyzoite, possibly in the micronemes, such as demonstrated for NcMIC2 (Lovett et al 2000), TgMIC2 , TgMIC 3 (El-Hajj et al 2008) and NcAMA 1 (Zhang et al 2006). To corroborate with microneme localization, the secretion of NcMIC2-like1 was dependent on intracellular Ca 2 + .…”

Toxoplasma gondii secretory proteins bind to sulfated heparin structures

“…Here, we provide evidence that proteins from the tachyzoite secretory organelles, rhoptries and dense granules are heparin-binding lectins. Interestingly, microneme proteins, which are considered to be a major class of T. gondii cellular adhesins (Barragan et al 2005;Carruthers and Tomley 2008;El Hajj et al 2008;Soldati-Favre 2008), were not identified by heparin affinity chromatography, confirming that these proteins have an affinity for sialic acid-containing cell surface molecules. Indeed, the heparin-and sialic acid-based invasion pathways were thought to be simultaneously accessible and independent.…”

“…All these events lead to the creation of the parasitophorous vacuole within the host cell, in which parasites undergo rapid multiplication (Carruthers and Boothroyd 2007;Plattner and Soldati-Favre 2008;Soldati-Favre 2008;Blader and Saeij 2009). Microneme proteins and their proteolytic trimming fragments are considered to be a major class of cellular adhesins involved in recognition and attachment to host cells Cérède et al 2002;Barragan et al 2005;Carruthers and Tomley 2008;El Hajj et al 2008;Soldati-Favre 2008).…”

Identification of Toxoplasma gondii in-vivo induced antigens by cDNA library immunoscreening with chronic toxoplasmosis sera

“…However toxoplasmosis can be harmful in pregnant women and in immuncompromised individuals. Invasion by this parasite involves a large number of proteins such as MIC1-MIC12, RON1-RON5, GRA1-GRA8, which have been characterised using proteomics and genomics methods [2]. Some of these proteins have been used as diagnostic markers and all of them were identified by employing either T. gondii grown in cell culture (in-vitro) or in animals (in-vivo) [3].…”