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. Microbial interactions with host cells frequently involve utilization of pathogenic effectors that cause virulent infection and disease in the host. How these eukaryotic pathogenic effectors appear in the host cell is largely unknown. Recent studies have identified the first host-targeting (HT) signal for a eukaryotic pathogen in the human malaria parasite Plasmodium falciparum. Bhattacharjee, Haldar, and colleagues show that the Plasmodium HT-signal is conserved in the biotrophic oomycete Phytophthora infestans that caused the Irish potato famine. Like its malarial counterpart, the Phytophthora HT signal is present in major, known, virulence proteins, and predicts a pathogenic “host-targeted-secretome” of hundreds of putative effectors to colonize the host cell. Since Plasmodium and Phytophthora belong to distinct evolutionary groups, the study establishes for the first time that different eukaryotic microbes can share similar strategies in delivering toxic proteins to their hosts. This may present shared targets for controlling vastly different infections of both animals and plants. The present work has implications for agriculture and human health, since Phytophthora species devastate a wide range of food and commercial crops and Plasmodium species cause malaria, which kills more than one million children each year.