Crystal Structures of Flax Rust Avirulence Proteins AvrL567-A and -D Reveal Details of the Structural Basis for Flax Disease Resistance Specificity

Ching-I A. Wang ¹, Gregor Gun $c$ ar ², Jade K. Forwood ², Trazel Teh ¹, Ann-Maree Catanzariti ³, Gregory J. Lawrence ³, Fionna E. Loughlin ⁴, Joel P. Mackay ⁴, Horst Joachim Schirra ⁵, Peter A. Anderson ⁶, Jeffrey G. Ellis ³, Peter N. Dodds ³^*, and Bo $s$ tjan Kobe ⁷

¹ School of Molecular and Microbial Sciences, University of Queensland, Brisbane, Queensland 4072, Australia
² School of Molecular and Microbial Sciences, University of Queensland, Brisbane, Queensland 4072, Australia; Institute for Molecular Bioscience, University of Queensland, Brisbane, Queensland 4072, Australia
³ Division of Plant Industry, Commonwealth Scientific and Industrial Research Organization, Canberra ACT 2601, Australia
⁴ School of Molecular and Microbial Biosciences, University of Sydney, NSW 2006, Australia
⁵ Institute for Molecular Bioscience, University of Queensland, Brisbane, Queensland 4072, Australia
⁶ School of Biological Sciences, Flinders University of South Australia, Adelaide 5001, Australia
⁷ School of Molecular and Microbial Sciences, University of Queensland, Brisbane, Queensland 4072, Australia; Institute for Molecular Bioscience, University of Queensland, Brisbane, Queensland 4072, Australia ; Special Research Centre for Functional and Applied Genomics, University of Queensland, Brisbane, Queensland 4072, Australia

^* To whom correspondence should be addressed. E-mail: b.kobe{at}uq.edu.au.

The gene-for-gene mechanism of plant disease resistance involvesdirect or indirect recognition of pathogen avirulence (Avr)proteins by plant resistance (R) proteins. Flax rust (Melampsoralini) AvrL567 avirulence proteins and the corresponding flax(Linum usitatissimum) L5, L6, and L7 resistance proteins interactdirectly. We determined the three-dimensional structures oftwo members of the AvrL567 family, AvrL567-A and AvrL567-D,at 1.4- and 2.3-Å resolution, respectively. The structuresof both proteins are very similar and reveal a ${beta}$ -sandwich foldwith no close known structural homologs. The polymorphic residuesin the AvrL567 family map to the surface of the protein, andpolymorphisms in residues associated with recognition differencesfor the R proteins lead to significant changes in surface chemicalproperties. Analysis of single amino acid substitutions in AvrL567proteins confirm the role of individual residues in conferringdifferences in recognition and suggest that the specificityresults from the cumulative effects of multiple amino acid contacts.The structures also provide insights into possible pathogen-associatedfunctions of AvrL567 proteins, with nucleic acid binding activitydemonstrated in vitro. Our studies provide some of the firststructural information on avirulence proteins that bind directlyto the corresponding resistance proteins, allowing an examinationof the molecular basis of the interaction with the resistanceproteins as a step toward designing new resistance specificities.