A Host-Targeting Signal in Virulence Proteins Reveals a Secretome in Malarial Infection

Abstract

Malaria parasites secrete proteins across the vacuolar membrane into the erythrocyte, inducing modifications linked to disease and parasite survival. We identified an 11-amino acid signal required for the secretion of proteins from the Plasmodium falciparum vacuole to the human erythrocyte. Bioinformatics predicted a secretome of >320 proteins and conservation of the signal across parasite species. Functional studies indicated the predictive value of the signal and its role in targeting virulence proteins to the erythrocyte and implicated its recognition by a receptor/transporter. Erythrocyte modification by the parasite may involve plasmodial heat shock proteins and be vastly more complex than hitherto realized.

N-terminal processing of proteins exported by malaria parasites

“…However, the mechanism of how the PEXEL functions within the parasite secretory pathway has not been described [8,9]. The characterization of the conserved cleavage and N-acetylation of the PEXEL in the parasite ER reported here leads to a model for the signals that may be encoded in this unique Plasmodium motif and leads further towards speculation on the machinery responsible for PEXEL processing (Fig.…”

“…6). Starting at the gateway into the parasite's export pathway at the ER membrane, the PEXEL motif can be ruled out as an ER translocation signal because all GFP chimeras containing a mutation in the first, third, or fifth conserved amino acid position in the PEXEL (R, L, or E/Q/D, respectively) are translocated into the parasite ER [8,9,14]. Since the first three PEXEL residues (RxL) are proteolytically removed in the parasite ER, this highly conserved motif possibly represents a novel ER peptidase cleavage site.…”

“…Since the first three PEXEL residues (RxL) are proteolytically removed in the parasite ER, this highly conserved motif possibly represents a novel ER peptidase cleavage site. Mutation of the conserved R or L residue prevents the export of GFP chimeras [8,9,14], thus indicating that PEXEL cleavage by this unknown ER peptidase may be required for trafficking to the host RBC. Since the only known ER peptidase in the classical secretory pathway involved in N-terminal processing is the SPC, the parasite SPC could possibly have an additional role in signal peptide cleavage and also be responsible for PEXEL cleavage.…”

“…A highly conserved 5-amino acid motif (RxLxE/Q/D) termed the Plasmodium export element (PEXEL) or host-targeting signal was recently identified through bioinformatic approaches and experimentally verified to mediate the translocation of parasite-derived proteins across the PVM and into the host RBC [8,9]. Site-directed mutagenesis of the conserved residues R, L, or E/Q/D with alanine abolished the export of green fluorescent protein (GFP) chimeras to the RCC.…”

“…With the discovery of this signal-mediated mechanism in P. falciparum, an updated model for the export of proteins to the host RBC involves recruitment into the parasite endoplasmic reticulum (ER) [11,12], default secretion into the PV lumen [12][13][14][15], and PEXEL-mediated translocation across the PVM [8,9,14,15]. In other eukaryotic cells, proteins destined for export enter the secretory pathway by co-translational translocation across the ER membrane mediated by the recognition of a N-terminal signal peptide and concomitant cleavage of this signal peptide by the signal peptidase complex (SPC) [16].…”