Functional characterization of Arabidopsis Extra Large G proteins (XLGs) and their role as signal transducers downstream of receptor kinases

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We have previously characterized cerk1-4, a point mutant of the Arabidopsis LysM-RLK CERK1 (Chitin Elicitor Receptor Kinase 1), which causes exaggerated cell death responses and resistance to powdery mildews. In a forward genetic screen, we identified the recessive mutant nole1 (No Lesions 1) as a complete suppressor of the cerk1-4 cell death phenotype. Mapping and complementation revealed nole1 to carry a mutation in XLG2 (Extra Large G-Protein 2). XLG proteins contain a C-terminal domain with significant similarity to the alpha-subunit of heterotrimeric G-proteins (G-alpha), but their function and interaction with the other subunits, G-beta and G-gamma, is poorly understood. G-beta and gamma (but not G-alpha) have been shown to act downstream of RLKs in PAMP and cell death signaling. Thus, we plan to investigate if XLG proteins act as mediators of PAMP signaling and control cell death associated with RLKs other than CERK1. XLG2 interaction with G-beta and gamma in conjunction with PAMP-, pathogen or cell death stimuli will also be addressed. Particular emphasis will be placed on subcellular localization analyses, as we have preliminary evidence for stimulus-dependent nuclear accumulation of XLG2. To date, no information at all is available on the function of XLG N-terminal domains. Also, it is not known if the Ca2+-dependent GTPase activity that has been demonstrated in vitro plays any role in vivo. To address these questions and to functionally characterize XLG2, we will make use of the mutant identified in our suppressor screen (cerk1-4 nole1-1). We will generate a collection of XLG2 mutant alleles by forward genetic approaches as well as targeted mutagenesis. Expression of these in the cerk1-4 nole1-1 background will allow assessment of functionality of the XLG2 alleles based on the cell death phenotype of the transformed plant (nole1-1 complementation). The information gained by these mutational studies will be followed up with in-depth cell biological and biochemical analyses to elucidate the function of the individual XLG2 features and domains. Together, our research will provide novel insights into structure-function relationships, mode of action and subcellular behavior of XLGs within plant innate immunity.