title: Structures of Phytophthora RXLR effector proteins: a conserved but adaptable fold underpins functional diversity.
abstract: Phytopathogens deliver effector proteins inside host plant cells to promote infection. These proteins can also be sensed by the plant immune system, leading to restriction of pathogen growth. Effector genes can display signatures of positive selection and rapid evolution, presumably a consequence of their co-evolutionary arms race with plants. The molecular mechanisms underlying how effectors evolve to gain new virulence functions and/or evade the plant immune system are poorly understood. Here, we report the crystal structures of the effector domains from two oomycete RXLR proteins, Phytophthora capsici AVR3a11 and Phytophthora infestans PexRD2. Despite sharing <20% sequence identity in their effector domains, they display a conserved core α-helical fold. Bioinformatic analyses suggest that the core fold occurs in ∼44% of annotated Phytophthora RXLR effectors, both as a single domain and in tandem repeats of up to 11 units. Functionally important and polymorphic residues map to the surface of the structures, and PexRD2, but not AVR3a11, oligomerizes in planta. We conclude that the core α-helical fold enables functional adaptation of these fast evolving effectors through (i) insertion/deletions in loop regions between α-helices, (ii) extensions to the N and C termini, (iii) amino acid replacements in surface residues, (iv) tandem domain duplications, and (v) oligomerization. We hypothesize that the molecular stability provided by this core fold, combined with considerable potential for plasticity, underlies the evolution of effectors that maintain their virulence activities while evading recognition by the plant immune system.
authors.affiliation: Department of Biological Chemistry, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, United Kingdom.
authors.forename: Laurence S
authors.initials: LS
authors.lastname: Boutemy
authors.affiliation: Department of Biological Chemistry, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, United Kingdom.
authors.forename: Stuart R F
authors.initials: SRF
authors.lastname: King
authors.affiliation: Sainsbury Laboratory, Norwich Research Park, Norwich NR4 7UH, United Kingdom.
authors.forename: Joe
authors.initials: J
authors.lastname: Win
authors.affiliation: Department of Biological Chemistry, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, United Kingdom.
authors.forename: Richard K
authors.initials: RK
authors.lastname: Hughes
authors.affiliation: Centre for Molecular and Structural Biochemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom.
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Clarke
authors.affiliation: Centre for Molecular and Structural Biochemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom.
authors.forename: Tharin M A
authors.initials: TMA
authors.lastname: Blumenschein
authors.affiliation: Sainsbury Laboratory, Norwich Research Park, Norwich NR4 7UH, United Kingdom. Electronic address: sophien.kamoun@tsl.ac.uk.
authors.forename: Sophien
authors.initials: S
authors.lastname: Kamoun
authors.affiliation: Department of Biological Chemistry, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, United Kingdom. Electronic address: mark.banfield@jic.ac.uk.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Banfield
mesh_terms: D005656:Fungal Proteins / Q000737:chemistry* / Q000378:metabolism; D055750:Phytophthora infestans / Q000737:chemistry* / Q000472:pathogenicity; D010935:Plant Diseases / Q000382:microbiology; D017510:Protein Folding; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013045:Species Specificity; D037521:Virulence Factors / Q000737:chemistry* / Q000378:metabolism
chemical_list: D005656:Fungal Proteins; D037521:Virulence Factors