Figure 1.
Ultrastructure of Toxoplasma gondii.
(A) Intracellular T. gondii tachyzoite showing the MICs (Mn), ROPs (Rh), micropore (Mp), Golgi (G), nucleus (N), endoplasmic reticulum (ER), and dense granules (DG). (B) A schematic picture of T. gondii entering into a nucleated mammalian host cell. The apical exocytosis of MICs deploys onto the parasite surface MIC proteins required for parasite motility and the formation of moving junction. ROP secretion provides the ROP proteins that are involved in host cell invasion and modulation of immune responses. The constitutive secretion of dense granules (DG) is involved in the modification of the parasitophorous vacuole (PV). (C) Higher magnification of the single Golgi apparatus of T. gondii. (D) Higher magnification of the T. gondii ER. Scale bars, 1 µm.
Figure 2.
Comparative bioinformatics analysis of genes coding components of vesicle-mediated trafficking and endosomal sorting in apicomplexan parasites, Saccharomyces cerevisiae, and Homo sapiens.
Most of these genes and their corresponding accession numbers were collected from Eupathdb.org (for apicomplexan parasites) and uniprot.org (yeast and human cells). The data from apicomplexan parasites Toxoplasma gondii (T. gondii), Plasmodium falciparum (P. falciparum), Theileria parva (T. parva), and Cryptosporidium parvum (C. parvum) were compared with human (H. sapiens) and the yeast Saccharomyces cerevisiae (S. cerevisiae). AP, adaptor protein; GGAs, Golgi-localized, γ-ear–containing, ADP-ribosylation factor binding protein; COPI, Coatomer complex I (retrograde transport from trans-Golgi apparatus to cis-Golgi and endoplasmic reticulum); COPII, Coatomer complex II (anterograde transport from ER to the cis-Golgi); ESCRT, Endosomal Sorting Complex Required for Transport.
Figure 3.
TgSORTLR co-localizes with TgVsp26 and Tgμ1-adpatin.
(A) Confocal images of tachyzoites expressing endogenously tagged TgVps26-HA (green) that co-localizes with TgSORTLR (red). (B) Confocal images of tachyzoites expressing endogenously tagged Tgμ1adaptin-HA (red) and TgSORTLR (green). White circles indicate the zoomed areas showing co-distribution between TgSORTLR and Tgμ1adaptin-HA or TgVPS26-HA in the Golgi and post-Golgi. Scale bars, 5 µm.
Figure 4.
A model for TgSORTLR functions in protein sorting and the biogenesis of apical secretory organelles.
We propose that TgSORTLR has a distinct role as a type I transmembrane cargo-protein receptor for ROPs and MICs of apicomplexan parasites. We observed TgSORTLR-positive structures that could be transport vesicles destined for the endolysosomal system or they might be integral to the endolysosomal system, i.e., early (EE) and late (LE) endosomes. The model further proposes that the cytoplasmic tail of TgSORTLR binds to AP-1, Sec23/24, clathrin, clathrin-associated adaptor protein, and VPS9, and this defines it as a key receptor involved in the anterograde transport of cargo ROP and MIC proteins. The binding of TgSORTLR to the retromer VPS26/VPS35 also indicates that this receptor is also involved in the retrograde transport of components. T. gondii lysosome-like, acidic vacuolar compartment (VAC), also termed the Plant-Like Vacuole (PLV), contains cathepsin proteases implicated in the proteolytic maturation of proproteins targeted to MICs. Proteolytic maturation likely occurs in the LE where conditions are thought to be more conducive for limited proteolysis.