The Malarial Host-Targeting Signal Is Conserved in the Irish Potato Famine Pathogen

Abstract

Animal and plant eukaryotic pathogens, such as the human malaria parasite Plasmodium falciparum and the potato late blight agent Phytophthora infestans, are widely divergent eukaryotic microbes. Yet they both produce secretory virulence and pathogenic proteins that alter host cell functions. In P. falciparum, export of parasite proteins to the host erythrocyte is mediated by leader sequences shown to contain a host-targeting (HT) motif centered on an RxLx (E, D, or Q) core: this motif appears to signify a major pathogenic export pathway with hundreds of putative effectors. Here we show that a secretory protein of P. infestans, which is perceived by plant disease resistance proteins and induces hypersensitive plant cell death, contains a leader sequence that is equivalent to the Plasmodium HT-leader in its ability to export fusion of green fluorescent protein (GFP) from the P. falciparum parasite to the host erythrocyte. This export is dependent on an RxLR sequence conserved in P. infestans leaders, as well as in leaders of all ten secretory oomycete proteins shown to function inside plant cells. The RxLR motif is also detected in hundreds of secretory proteins of P. infestans, Phytophthora sojae, and Phytophthora ramorum and has high value in predicting host-targeted leaders. A consensus motif further reveals E/D residues enriched within ~25 amino acids downstream of the RxLR, which are also needed for export. Together the data suggest that in these plant pathogenic oomycetes, a consensus HT motif may reside in an extended sequence of ~25–30 amino acids, rather than in a short linear sequence. Evidence is presented that although the consensus is much shorter in P. falciparum, information sufficient for vacuolar export is contained in a region of ~30 amino acids, which includes sequences flanking the HT core. Finally, positional conservation between Phytophthora RxLR and P. falciparum RxLx (E, D, Q) is consistent with the idea that the context of their presentation is constrained. These studies provide the first evidence to our knowledge that eukaryotic microbes share equivalent pathogenic HT signals and thus conserved mechanisms to access host cells across plant and animal kingdoms that may present unique targets for prophylaxis across divergent pathogens.

Translocation ofMagnaporthe oryzaeEffectors into Rice Cells and Their Subsequent Cell-to-Cell Movement

“…Although translocation motifs have been suggested, fungal effectors do not contain highly conserved, easily recognizable amino acid motifs (Ellis et al, 2007;Kamoun, 2007) such as the RxLxE/Q/D motif in P. falciparum effectors or the RXLR-dEER motif in oomycete plant pathogens (Bhattacharjee et al, 2006;Dou et al, 2008). Using live-cell imaging of fluorescent reporter proteins in rice leaf sheaths, we developed a robust assay for cytoplasmic effector translocation that will facilitate mutational approaches for motif identification.…”

“…With this assay, our top priority is to identify the still elusive translocation motif, if any, for rice blast effectors. Such a motif would become a valuable resource for identifying sets of putative effectors by bioinformatic analyses, as has been true for the malarial and oomycete pathogens (Bhattacharjee et al, 2006;Whisson et al, 2007;Jiang et al, 2008).…”

“…To achieve biotrophic colonization, these pathogens deliver a subset of effector proteins, termed cytoplasmic effectors, into host cells to dampen defenses and assume control (Bhattacharjee et al, 2006;Ellis et al, 2006;Bhavsar et al, 2007;Kamoun, 2007;Whisson et al, 2007). For the eukaryotic pathogens, which are less-studied than prokaryotic pathogens, this raises the major question of how effectors are delivered across the plasma membrane to reach the host cytoplasm.…”

“…In some compatible infection sites, generally characterized by nonuniform BAS4 outlining of IH, BAS4 appeared to have spilled into the host cell cytoplasm. Therefore, the EIHM compartment containing blast IH appears analogous to extrahaustorial compartments of rusts that are separated from the host apoplast by a neckband (Mendgen and Hahn, 2002;O'Connell and Panstruga, 2006) or to the parasitophorous vacuole that completely encloses the malarial pathogen, Plasmodium falciparum, inside invaded red blood cells (Bhattacharjee et al, 2006(Bhattacharjee et al, , 2008.…”

A Single Binding Site Mediates Resistance- and Disease-Associated Activities of the Effector Protein NIP1 from the Barley Pathogen Rhynchosporium secalis

“…Likewise, expression of the Avr3a gene from Phytophthora infestans in its host plant potato carrying the R gene R3a, as well as coexpression of both genes in Nicotiana benthamiana, resulted in a resistance response after removal of the signal peptide-encoding sequence, suggesting cytoplasmic interaction (Armstrong et al, 2005). Recently, a host-targeting signal with an RxLx(E, D, or Q) core motif was identified in the leader sequence of the Avr3a protein, as well as of other secretory oomycete proteins (Bhattacharjee et al, 2006). This motif was originally defined to enable translocation of effector proteins from the Malaria parasite Plasmodium falciparum across mammalian membranes (Hiller et al, 2004;Marti et al, 2004).…”