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.
Highlights
Many phytopathogens secrete and/or inject “effector” proteins inside host cells to modulate cellular processes [1]
For inclusion of sequences in this updated model, we chose a cut-off of e Ͻ 0.12 from the initial search, which corresponds to the position of P. infestans AVR3a in the list
To address structure/function relationships in oomycete RXLR effectors, we established an E. coli-based expression screen aimed at producing proteins that were amenable to structure determination
Summary
The effector domains of RXLR proteins display extensive sequence diversity and typically share little sequence similarity with proteins with known activities, making functional annotation and prediction of tertiary folds from sequence difficult Despite this diversity, there are recognizable sequence relationships within the C-terminal regions of Phytophthora RXLR effectors, and some repeating sequence motifs termed “W”, “Y”, and “L” have been described [15]. Other oomycete RXLR effectors have been validated based on their ability to modulate defense responses in plants (supplemental Fig. 1) One such effector, P. infestans PexRD2, was identified in a functional screen to promote cell death activity in several susceptible and resistant plants [19, 20]. We propose that the core fold provides both a degree of molecular stability and plasticity that enables development/maintenance of effector virulence activities while allowing evasion of recognition by the plant innate immune system during rapid “arms race” co-evolution
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