Abstract
The survival of Plasmodium spp. within the host red blood cell (RBC) depends on the function of a membrane protein complex, termed the Plasmodium translocon of exported proteins (PTEX), that exports certain parasite proteins, collectively referred to as the exportome, across the parasitophorous vacuolar membrane (PVM) that encases the parasite in the host RBC cytoplasm. The core of PTEX consists of three proteins: EXP2, PTEX150, and the HSP101 ATPase; of these three proteins, only EXP2 is a membrane protein. Studying the PTEX-dependent transport of members of the exportome, we discovered that exported proteins, such as ring-infected erythrocyte surface antigen (RESA), failed to be transported in parasites in which the parasite rhoptry protein RON3 was conditionally disrupted. RON3-deficient parasites also failed to develop beyond the ring stage, and glucose uptake was significantly decreased. These findings provide evidence that RON3 influences two translocation functions, namely, transport of the parasite exportome through PTEX and the transport of glucose from the RBC cytoplasm to the parasitophorous vacuolar (PV) space where it can enter the parasite via the hexose transporter (HT) in the parasite plasma membrane.IMPORTANCE The malarial parasite within the erythrocyte is surrounded by two membranes. Plasmodium translocon of exported proteins (PTEX) in the parasite vacuolar membrane critically transports proteins from the parasite to the erythrocytic cytosol and membrane to create protein infrastructure important for virulence. The components of PTEX are stored within the dense granule, which is secreted from the parasite during invasion. We now describe a protein, RON3, from another invasion organelle, the rhoptry, that is also secreted during invasion. We find that RON3 is required for the protein transport function of the PTEX and for glucose transport from the RBC cytoplasm to the parasite, a function thought to be mediated by PTEX component EXP2.
Highlights
The survival of Plasmodium spp. within the host red blood cell (RBC) depends on the function of a membrane protein complex, termed the Plasmodium translocon of exported proteins (PTEX), that exports certain parasite proteins, collectively referred to as the exportome, across the parasitophorous vacuolar membrane (PVM) that encases the parasite in the host red blood cells (RBCs) cytoplasm
We describe the surprising finding that a conditional knockout (KO) of the misnamed rhoptry neck protein 3 (RON3) caused the early death of the parasite with an inability of ring-infected erythrocyte surface antigen (RESA), a protein in the dense granule, to be transported across the PVM into the RBC cytoplasm
Two forms of RON3loxP were created by clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein-9 (Cas9)-mediated gene editing, whereby the locus of X-over P1 sites were inserted into different segments of the RON3 gene
Summary
The survival of Plasmodium spp. within the host red blood cell (RBC) depends on the function of a membrane protein complex, termed the Plasmodium translocon of exported proteins (PTEX), that exports certain parasite proteins, collectively referred to as the exportome, across the parasitophorous vacuolar membrane (PVM) that encases the parasite in the host RBC cytoplasm. ® mbio.asm.org 1 functions, the parasite must export a subset of its own proteins, termed the exportome, across the PVM into the RBC cytoplasm and to the RBC membrane These transported proteins are needed to build the protein infrastructure required for parasite functions essential for their pathological survival in the vertebrate host, including remodeling of the RBC membrane to express receptors for endothelial ligands that sequester infected RBCs in host tissue, thereby avoiding elimination by the spleen. This purified PTEX complex, in both an “engaged” state and a “resetting” state, shows EXP2 forming a transmembrane funnel-shaped protein-conducting channel tethered to the HSP101 ATPase that powers protein translocation by a ratchet-like translocation mechanism These findings elegantly confirmed that PTEX was the site of parasite proteins passing from the parasitophorous vacuolar (PV) space to the erythrocyte cytoplasm. It is possible that EXP2 functions alone, in a complex with PTEX, or with non-PTEX components to transport nutrients
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