Sequence elements within the PEXEL motif and its downstream region
modulate PTEX dependent protein export in Plasmodium falciparum .
Abstract
The parasite Plasmodium falciparum causes the most severe form of
malaria and to invade and replicate in red blood cells (RBCs), it
exports hundreds of proteins across the encasing parasitophorous vacuole
membrane (PVM) into this host cell. The exported proteins help modify
the RBC to support rapid parasite growth and avoidance of the human
immune system. Most exported proteins possess a conserved
Plasmodium Export Element (PEXEL) motif with the consensus
RxLxE/D/Q amino acid sequence, which acts as a proteolytic cleavage
recognition site within the parasite’s endoplasmic reticulum (ER).
Cleavage occurs after the P 1 L residue and is thought
to help release the protein from the ER so it can be putatively escorted
by the HSP101 chaperone to the parasitophorous vacuole space surrounding
the intraerythrocytic parasite. HSP101 and its cargo are then thought to
assemble with the rest of a Plasmodium Translocon for Exported
proteins (PTEX) complex, that then recognises the xE/D/Q capped N
terminus of the exported protein and translocates it across the vacuole
membrane into the RBC compartment. Here, we present evidence that
supports a dual role for the PEXEL’s conserved P 2’
position E/Q/D residue, firstly, for plasmepsin V cleavage in the ER,
and secondly, for efficient PTEX mediated export across the PVM into the
RBC. We also present evidence that the downstream ‘spacer’ region
separating the PEXEL motif from the folded functional region of the
exported protein controls cargo interaction with PTEX as well. The
spacer must be of a sufficient length and permissive amino acid
composition to engage the HSP101 unfoldase component of PTEX to be
efficiently translocated into the RBC compartment.