Subproject D03: The BIR family of LRR-RLKs – negative regulators and determinants of specificity in receptor complexes?

Principal investigator:

Dr. Birgit Kemmerling

Universität Tübingen

ZMBP, Pflanzenbiochemie

Auf der Morgenstelle 5, 72076 Tübingen

Tel 07071 - 29 76654

Fax 07071 - 29 5226

birgit.kemmerlingspam prevention@zmbp.uni-tuebingen.de

Summary:

The first line of defense against microbial pathogens in plants is based on the perception of microbe associated molecular patterns (MAMP) by surface exposed receptors called pattern recognition receptors (PRRs). Many of these receptors belong to the family of leucine-rich repeat receptor kinases (LRR-RKs). The best studied PRRs are FLS2 and EFR, the receptors for bacterial flagellin and the elongation factor TU, respectively. After ligand binding to the receptor, FLS2 and EFR form stable complexes with BAK1, another LRR-RLK. This LRR-RK BAK1 interacts with multiple ligand-binding receptor kinases and thereby positively controls multiple signal transduction pathways. Together with the brassinosteroid receptor BRI1 BAK1 regulates growth and shape of plants, and with FLS2 or EFR BAK1 regulates plant immunity. Furthermore, BAK1 is involved in cell death control. BAK1 is a general regulator of LRR-RKs and regulates not only autonomous but also adaptive developmental processes; therefore, BAK1 is a central node for the plasticity of plants. FLS2 and BRI1 reside in different nanodomains of the plasma membrane, indicating that receptor specific nanoclusters are formed with BAK1. In our lab, two novel LRR-RKs have been identified that interact with BAK1 – BIR2 and BIR3 (BAK1 interacting RK). Both proteins interact constitutively with BAK1 and influence BAK1-dependent signaling. Both have a negative regulatory impact on BAK1 that functions by the interference with BAK1 complex formation with ligand binding receptors. However, the pathway specificities of the BIR proteins are distinct. BIR2 is involved in MAMP signaling and cell death control. BIR3 is predominantly involved in plant immunity and brassinosteroid signaling, while it has only a minor role in cell death control. The BIR protein family shows that evolutionary diversification leads to gradual differences in the function of the BIR proteins, with overlapping and distinct functions within this protein family.

How BAK1- and BIR-mediated cell death is regulated and which signaling components are involved is still unclear. We performed interactome screens for BIR2 and BIR3 and identified two components, an NLR-type resistance protein and an adapter protein of membrane receptors that might help clarifying the cell death pathway and how the receptor complexes are residing in the membrane. In this project, we will investigate: 1. How BAK1- and BIR-mediated cell death is initiated and which components are involved, 2. What determines the localization of receptor complexes in nanodomains of the plasma membrane, and 3. if there is a link between nanocluster integrity and cell death initiation. Biochemical approaches as well as superresolution microscopy will help elucidating how the nanoclusters formed by BAK1, BIR proteins and ligand binding receptors are constituted, how their integrity is sensed, and how the molecular specificity is determined and maintained.