Protein ADP-ribosylation in plant stress signaling
Protein ADP-ribosylation, i. e. the covalent attachment of one or several ADP-ribose molecules onto amino acid side chains, is a poorly characterized post-translational modification in plants. Protein ADP-ribosylation is catalyzed by members of the Poly(ADP-ribose) Polymerase (PARP) family that localize to the plant cell nucleus. Genetic evidence suggests that PARPs influence signal transduction in response to both, attack by plant pathogens and adverse abiotic conditions such as drought, heat or high light stress. However, it remains largely unknown which plant proteins are modified by ADP-ribosylation and how this alters their biological function. Notably, several plant pathogens use the same post-translational mechanism to influence the outcome of an attempted infection. They employ host-targeted effector proteins with ADP-ribosyl-transferase activity to manipulate plant immune signaling [1]. In this project we use protein mass-spectrometry and structural methods to obtain a better understanding of protein ADP-ribosylation in plants exposed to abiotic and biotic stress conditions. Ultimately, we aim to uncover how protein ADP-ribosylation alters the biological function of key proteins in plant stress signaling and how this allows plants to withstand unfavourable environmental conditions.
Role of SRO proteins in plant stress signaling
In addition to canonical PARPs, plant genomes encode a family of sequence-related proteins referred to as SRO proteins. SROs share a central PARP-like domain with canonical PARPs, but differ in their N- and C-terminal domains. SRO proteins appear to function as transcriptional co-regulators and influence redox homeostasis and stress responses to drought and elevated salinity in dicot and monocot plants. We have solved a crystal structure of a PARP-like domain from an Arabidopsis SRO protein and demonstrated that amino acids contributing to the catalytic center of canonical PARPs are dispensable for SRO protein function [2]. This suggests a non-canonical function of SRO proteins in plant stress signaling that is independent of PARP activity. We found that SRO proteins are phosphorylated and isolated photoregulatory protein kinases (PPKs) as novel interactors. Several phosphorylation sites map to the N-terminal WWE domain of SRO proteins that might constitute a poly(ADP-ribose) binding module. Notably, some pathogen effectors also bind to the WWE domain of SRO proteins. In this project, we aim to obtain a better understanding of SRO proteins and their role as central mediators under abiotic and biotic stress conditions. Furthermore, we will investigate if binding of pathogen effectors alters the transcriptional co-regulator function of SRO proteins.
