title: Structural Analysis of an Avr4 Effector Ortholog Offers Insight into Chitin Binding and Recognition by the Cf-4 Receptor.
abstract: Chitin is a key component of fungal cell walls and a potent inducer of innate immune responses. Consequently, fungi may secrete chitin-binding lectins, such as the Cf-Avr4 effector protein from the tomato pathogen Cladosporium fulvum, to shield chitin from host-derived chitinases during infection. Homologs of Cf-Avr4 are found throughout Dothideomycetes, and despite their modest primary sequence identity, many are perceived by the cognate tomato immune receptor Cf-4. Here, we determined the x-ray crystal structure of Pf-Avr4 from the tomato pathogen Pseudocercospora fuligena, thus providing a three-dimensional model of an Avr4 effector protein. In addition, we explored structural, biochemical, and functional aspects of Pf-Avr4 and Cf-Avr4 to further define the biology of core effector proteins and outline a conceptual framework for their pleiotropic recognition by single immune receptors. We show that Cf-Avr4 and Pf-Avr4 share functional specificity in binding (GlcNAc)6 and in providing protection against plant- and microbial-derived chitinases, suggesting a broader role beyond deregulation of host immunity. Furthermore, structure-guided site-directed mutagenesis indicated that residues in Pf-Avr4 important for binding chitin do not directly influence recognition by Cf-4 and further suggested that the property of recognition is structurally separated or does not fully overlap with the virulence function of the effector.
authors.affiliation: Department of Plant Pathology, University of California Davis, Davis, California 95616.
authors.forename: Amanda C
authors.initials: AC
authors.lastname: Kohler
authors.affiliation: Department of Plant Pathology, University of California Davis, Davis, California 95616.
authors.forename: Li-Hung
authors.initials: LH
authors.lastname: Chen
authors.affiliation: Department of Chemistry, University of California Davis, Davis, California 95616.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Hurlburt
authors.affiliation: Department of Plant Pathology, University of California Davis, Davis, California 95616.
authors.forename: Anthony
authors.initials: A
authors.lastname: Salvucci
authors.affiliation: Department of Plant Pathology, University of California Davis, Davis, California 95616.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Schwessinger
authors.affiliation: Department of Chemistry, University of California Davis, Davis, California 95616 Department of Molecular and Cellular Biology, University of California Davis, Davis, California 95616.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Fisher
authors.affiliation: Department of Plant Pathology, University of California Davis, Davis, California 95616 istergiopoulos@ucdavis.edu.
authors.forename: Ioannis
authors.identifier: 0000-0002-2368-6119
authors.initials: I
authors.lastname: Stergiopoulos
mesh_terms: D002686:Chitin / Q000378:metabolism; D002963:Cladosporium / Q000472:pathogenicity; D018551:Solanum lycopersicum / Q000235:genetics / Q000378:metabolism* / Q000382:microbiology; D016297:Mutagenesis, Site-Directed; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding
chemical_list: D010940:Plant Proteins; D002686:Chitin