Structures of the flax-rust effector AvrM reveal insights into the molecular basis of plant-cell entry and effector-triggered immunity

• Published in: Proceedings of the National Academy of Sciences - PNAS 2013-10, Vol.110 (43), p.17594-17599
• Main Authors: Ve, Thomas, Williams, Simon J., Catanzariti, Ann-Maree, Rafiqi, Maryam, Rahman, Motiur, Ellis, Jeffrey G., Hardham, Adrienne R., Jones, David A., Anderson, Peter A., Dodds, Peter N., Kobe, Bostjan
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animal cells; Basidiomycota - genetics; Basidiomycota - immunology; Basidiomycota - physiology; Binding sites; Biological Sciences; Cells; Crystal structure; Crystallography, X-Ray; Crystals; Dimers; Flax - cytology; Flax - immunology; Flax - microbiology; Flowers & plants; Fungal Proteins - chemistry; Fungal Proteins - immunology; Fungal Proteins - metabolism; Host-Pathogen Interactions - immunology; Immunoblotting; Internalization; Linum usitatissimum; Melampsora lini; Microscopy, Confocal; Models, Molecular; Molecular Sequence Data; Molecules; Mutation; Nicotiana - genetics; Nicotiana - metabolism; Oomycetes; Pathogens; Phosphatidylinositols - immunology; Phosphatidylinositols - metabolism; Plant cells; Plant Cells - immunology; Plant Cells - microbiology; Plant Diseases - genetics; Plant Diseases - immunology; Plant Diseases - microbiology; Plant immunity; Plant Leaves - genetics; Plant Leaves - metabolism; Plants, Genetically Modified; Protein Binding - immunology; Protein Multimerization; Protein Structure, Secondary; Protein Structure, Tertiary; Proteins; Sequence Homology, Amino Acid
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1307614110

Fungal and oomycete pathogens cause some of the most devastating diseases in crop plants, and facilitate infection by delivering a large number of effector molecules into the plant cell. AvrM is a secreted effector protein from flax rust (Melampsora lini) that can internalize into plant cells in the absence of the pathogen, binds to phosphoinositides (PIPs), and is recognized directly by the resistance protein M in flax (Linum usitatissimum), resulting in effector-triggered immunity. We determined the crystal structures of two naturally occurring variants of AvrM, AvrM-A and avrM, and both reveal an L-shaped fold consisting of a tandem duplicated four-helix motif, which displays similarity to the WY domain core in oomycete effectors. In the crystals, both AvrM variants form a dimer with an unusual nonglobular shape. Our functional analysis of AvrM reveals that a hydrophobic surface patch conserved between both variants is required for internalization into plant cells, whereas the C-terminal coiled-coil domain mediates interaction with M. AvrM binding to PIPs is dependent on positive surface charges, and mutations that abrogate PIP binding have no significant effect on internalization, suggesting that AvrM binding to PIPs is not essential for transport of AvrM across the plant membrane. The structure of AvrM and the identification of functionally important surface regions advance our understanding of the molecular mechanisms underlying how effectors enter plant cells and how they are detected by the plant immune system.