Towards an understanding on how RxLR-effector proteins are translocated from oomycetes into host cells

The most notorious oomycetes, such as Phytophthora infestans, are pathogens of higher plants, although numerous other species of these fungal-like microorganisms infect algae, crustacea, nematodes, fish and mammals. While there is now ample evidence that oomycetes and fungi deliver effector proteins inside the host cell during infection, like bacteria using the well characterised type III secretion system, the mechanism employed by eukaryotic pathogens remains unclear. In oomycetes this process depends on an N-terminal motif defined by a short conserved amino acid sequence (RxLR) located near the signal peptide of many secreted proteins. This motif resembles the host-cell targeting signal found in virulence proteins from the malaria parasite Plasmodium falciparum (RxLxE/D/Q).This review will focus on the recent findings contributing to the understanding of the delivery of oomycete effector molecules into the host cells, with emphasis on how they compare with various models proposed for filamentous fungi and the malaria parasite.