Characterization of a conserved structural and functional module in Phytophthora effectors

Human population is projected to reach 9.7 billion by 2050. The looming challenge of feeding the rapidly growing population is threatened by crop losses from plant diseases. An average of 13% annual crop yield is lost to pathogens and pests. Developing crops with resistance to pathogens is a major mission in agriculture but it can only be accomplished after the establishment of a deep understanding of the governing principles in pathogenesis.

Co-evolution is a central concept in host-pathogen interactions. Plants have evolved a myriad of defense mechanisms to prevent infection by potential pathogens, while pathogens employ effector proteins to overcome the immunity and cause disease. An important feature of effectors is their fast evolution. Pathogens often produce a diverse complement of effectors, reflecting constant arms race with their hosts. Understanding the molecular mechanisms that underlie effector evolution is critically important for designing innovative approaches to achieve sustainable resistance.

In this project, we will focus on Phytophthora pathogens to investigate effector evolution. Phytophthora are filamentous eukaryotes that morphologically similar, but evolutionarily divergent, to fungi. All Phytophthora species are pathogens of plants; among them, the most notorious is Phytophthora infestans, which causes the potato late blight disease that triggered the Irish Famine. Remarkably, each Phytophthora species encodes hundreds to over one thousand effectors, many of which contain tandem repeats of a conserved structural module (called WY/LWY). It has been proposed that the WY/LWY tandem repeats facilitate the evolution of novel virulence activities, leading to diversification of the effector repertoire in Phytophthora.

We aim to investigate how WY/LWY module contributes to the diversification of effector functions. For this purpose, we recently solved the crystal structure of a protein complex formed by a Phytophthora effector PSR2, which contains seven WY/LWY units, and the host Ser/Thr protein phosphatase PP2A. This interaction is mediated by a specific combination of two N-terminal WY/LWY units in PSR2 and required for the virulence activity of PSR2. Intriguingly, the same interaction interface is adopted by multiple LWY effectors on their N-terminal region; however, these effectors have different subcellular localizations in plant cells and contain diverse LWY units in their C-terminus. Based on these observations, we hypothesize that a subset of LWY effectors perform their virulence functions by hijacking the host PP2A phosphatase. We will investigate the mechanisms by which different PP2A-associating effectors recruit distinct sets of substrates to the PP2A enzyme for dephosphorylation and examine the role of PP2A as a novel susceptibility gene and a hub targeted by multiple pathogen effectors. This research will determine how the LWY serves as a functional module to enable diversification of the effectors.

The outcome of this project will advance a fundamental understanding of Phytophthora pathogenesis and effector evolution. This knowledge will offer new opportunities to develop novel disease control strategies.

This project aims to understand the mechanisms that underlie rapid evolution of effectors. We will investigate how tandem repeats facilitate the functional diversification of effectors in the devastating Phytophthora pathogens.

Many Phytophthora effectors consist tandem repeats of the WY/LWY module, which forms conserved alpha-helix folds. Recent structural analysis of the effector PSR2, containing one WY and six LWY units, revealed a conserved concatenation mechanism enabled by the LWY module. PSR2 and other LWY effectors form an overall linear structure, making it possible to assign specific activities and host targets to distinct LWY units or unit combinations. As such, LWY effectors offer an excellent system to investigate functional diversification facilitated by tandem repeats.

This project is built on our unpublished structural analysis of PSR2 in a complex with the plant Ser/Thr phosphatase PP2A. PSR2 interacts with PP2A core enzyme through a specific interface formed by LWY2-LWY3. A search in Phytophthora infestans identified similar interaction interfaces formed by two adjacent N-terminal WY/LWY units in multiple LWY effectors. Intriguingly, these effectors have distinct C-terminal LWY units and locate to different subcellular compartments in plant cells. In this way, the LWY units facilitate functional diversification of the effectors, which exert their virulence functions through a common mechanism.

We will employ quantitative phosphoproteomics, coupled with proximity labelling, to test the hypothesis that a subset of LWY effectors hijack the host PP2A phosphatase and recruit different sets of substrates for dephosphorylation. We will use a genetics approach to confirm that the host principle phosphatase is required for the virulence activities of the effectors. Finally, we will use domain shuffling to determine the LWY units that is responsible for substrate-binding specificity of the effectors.