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Crossing the vacuolar Rubicon: Protein export and Pathogenesis in the malaria parasite Plasmodium

Dr Pascal EGEA
University of California Los Angeles, David Geffen School of Medicine, United States

Tuesday, June 11th 2019 - 11 a.m. - Auditorium, IGBMC
Hosted by Integrated structural Biology

 Modern research on malaria seeks to identify and understand biological processes that are unique to the parasite and target them for drug design. Pathogenesis in malaria almost entirely occurs at the red blood cell stage. The ability of the parasite to infect, remodel, and replicate in human red blood cells is central to its pathogenicity. These processes are mediated by a large number of secreted essential parasite proteins transported across the encasing vacuolar membrane via a parasite-derived translocon, the Plasmodium Translocon of Exported Proteins (PTEX), and delivered into the cytoplasm of the red blood cell. PTEX knockdown parasites are nearly avirulent. As the only known nexus for protein export in Plasmodium falciparum, PTEX is a prime drug target.  Until now the molecular mechanisms of this process remained unknown.

Here we show that PTEX is a bona fide translocon by determining structures of its 1.6 MDa core complex at near-atomic resolution using cryo-EM. We isolated the native endogenous core complex containing EXP2, PTEX150 and HSP101/ClpB2 from P. falciparum trapped in the ‘engaged’ and ‘resetting’ states of endogenous cargo translocation using epitope tags inserted by CRISPR–Cas9 gene-editing of parasites. In our two structures, EXP2 and PTEX150 interdigitate to form a static, funnel-shaped and pseudo-seven-fold-symmetric protein-conducting channel that spans the vacuolar membrane. The spiral-shaped AAA+ HSP101/ClpB2 hexamer is tethered atop this funnel, and undergoes pronounced compaction that allows three of six tyrosine-bearing pore loops lining the HSP101/ClpB2 channel to dissociate from the cargo, resetting the translocon for the next threading cycle. The remarkable 6-to-7symmetry mismatch between the ‘moving’ hexameric ATPase and the rigid EXP2/PTEX150 tetradecamer likely provides the flexibility necessary for the complex to translocate substrates across the membrane. Our work reveals the mechanism of P. falciparum effector export, and will inform structure-based design of drugs targeting this unique translocon.

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