Entry of Oomycete and Fungal Effectors into Host Cells

Diverse microbial pathogens and other symbionts, including fungi and oomycetes, utilize effector proteins to promote the microbe's ability to colonize the host. Many of these effectors act inside the host cell. Two basic strategies have evolved by which effectors enter host cells, pathogen-derived machinery, such as bacterial injectisomes, and host-derived machinery, usually one of several pathways of endocytosis. Currently, more than 49 proteins from oomycete and fungal pathogens, most of them effectors, show evidence of the ability to enter host cells. With the possible exception of rice blast effectors, most fungal and oomycete effectors appear to enter host cells via receptor-mediated endocytosis. Oomycete RXLR effectors, and at least some fungal effectors, bind cell surface phosphatidylinositol-3-phosphate prior to entry, while fungal ribotoxins bind the membrane directly. Understanding the mechanisms of effector entry can potentially lead to novel broad-spectrum disease resistance strategies.