PTEX is an essential nexus for protein export in malaria parasites

Abstract

During the blood stages of malaria, several hundred parasite-encoded proteins are exported beyond the double-membrane barrier that separates the parasite from the host cell cytosol. These proteins have a variety of roles that are essential to virulence or parasite growth. There is keen interest in understanding how proteins are exported and whether common machineries are involved in trafficking the different classes of exported proteins. One potential trafficking machine is a protein complex known as the Plasmodium translocon of exported proteins (PTEX). Although PTEX has been linked to the export of one class of exported proteins, there has been no direct evidence for its role and scope in protein translocation. Here we show, through the generation of two parasite lines defective for essential PTEX components (HSP101 or PTEX150), and analysis of a line lacking the non-essential component TRX2 (ref. 12), greatly reduced trafficking of all classes of exported proteins beyond the double membrane barrier enveloping the parasite. This includes proteins containing the PEXEL motif (RxLxE/Q/D) and PEXEL-negative exported proteins (PNEPs). Moreover, the export of proteins destined for expression on the infected erythrocyte surface, including the major virulence factor PfEMP1 in Plasmodium falciparum, was significantly reduced in PTEX knockdown parasites. PTEX function was also essential for blood-stage growth, because even a modest knockdown of PTEX components had a strong effect on the parasite's capacity to complete the erythrocytic cycle both in vitro and in vivo. Hence, as the only known nexus for protein export in Plasmodium parasites, and an essential enzymic machine, PTEX is a prime drug target.

In Vivo Function of PTEX88 in Malaria Parasite Sequestration and Virulence

“…The surface antigen labeling of infected erythrocytes was performed using serum from a naive mouse or from a mouse immunized with blood-stage parasites (15,19). A single drop of tail blood was taken from a WT (Bergreen)-or ptex88 Ϫ -infected mouse.…”

“…A corresponding Plasmodium translocon of exported proteins (PTEX) (11) is present in all mammalian Plasmodium parasites. This specialized multiprotein complex consists of three essential core components (12,13), two of which have recently been demonstrated to be directly involved in the active export of both PEXEL/VTS-containing proteins and PNEPs (14,15). In the murine malaria model parasite, Plasmodium berghei, two additional PTEX constituents, thioredoxin 2 (TRX2) and PTEX88, appear to fulfill auxiliary roles (12,13).…”

Export of virulence proteins by malaria-infected erythrocytes involves remodeling of host actin cytoskeleton

“…12,13 These proteins move through the parasitophorous vacuole and PVM via a translocator machine. [14][15][16] A number of exported proteins do not contain a PEXEL/VTS motif and are thus called PEXEL-negative exported proteins (PNEPs); their transport pathways are unknown. 5,17 Various proteins play a role in trafficking PfEMP1 to the P falciparum-infected RBC membrane and its assembly into knob structures (reviewed in Maier et al protein 1 (MAHRP1), 21 or ring-exported protein 1 (REX1), 22 PfEMP1 cannot be displayed on the RBC membrane.…”

Host Erythrocyte Environment Influences the Localization of Exported Protein 2, an Essential Component of the Plasmodium Translocon

“…A conserved feature of several exported P. falciparum proteins is the presence of a pentameric recognition sequence (RxLxE/Q/D) called the PEXEL (Plasmodium export element) or HT (host-targeting) motif located ϳ25 amino acids downstream from the endoplasmic reticulum (ER) signal sequence (20,21). Translocation of PEXEL-containing and PEXEL-negative exported proteins across the PVM is mediated by the same essential ATP-dependent, multimeric protein complex, called PTEX (Plasmodium translocon of exported proteins), located in the PVM (22)(23)(24)(25). PTEX is comprised of five known components-heat shock protein HSP101, exported protein 2 (Exp2), thioredoxin 2 (TRX2), and two novel proteins, PTEX150 and PTEX88 (22,26).…”