UZH

Plant receptor kinases are major pattern recognition receptors (PRRs) that function as part of dynamic multimeric complexes to perceive pathogen-associated molecular patterns or host-derived damage-associated molecular patterns at the plasma membrane (PM). Our long-term objective is to decipher the molecular basis of plant innate immunity and to understand how plant receptor kinases work. Our recent findings point to an important role of endogenous peptides in the regulation of plant innate immune signaling. The main aim of this proposal is to understand how these endogenous peptides and their corresponding receptors regulate plant innate immune signaling. The central hypotheses of this research are that: (i) a subset of plant endogenous peptides are perceived by receptor kinases to fine-tune dynamically plant innate immune signaling, and thus act as ‘phytocytokines’; (ii) these phytocytokines and their receptors regulate the formation of active immune-signaling nanoclusters at the PM; and (iii) phytocytokine receptors participate in the sensory continuum represented by the plant PM and the cell wall to respond dynamically to environmental challenges. We will pursue the following specific objectives: (1) decipher the regulation, function, and perception of RALF peptides by malectin-like receptor kinases during immunity; (2) elucidate the formation, composition, and function of PM immune receptor nanoclusters; (3) reveal the function of the receptor kinase MIK2 and its ligand(s) in immunity. Through a balanced combination of straight-forward and high risk/high gain biochemical, biophysical, bioimaging, and genetics approaches, this project will provide groundbreaking insights into the molecular mechanisms underlying the establishment and regulation of plant innate immune signaling, but also into the general mechanisms that control plant receptor kinase functions and by which the myriad endogenous peptides encoded by plant genomes control environmental sensing.

To infect humans, the devastating pathogen Mycobacterium tuberculosis critically depends on two closely related siderophores – soluble carboxymycobactin and membrane-bound mycobactin – which capture iron with high affinity inside the host cell. Despite their undisputed importance for virulence, little is known about how these siderophores are exported and imported across the two mycobacterial membranes. Building on my lab’s experience in elucidating transport processes of pathogenic bacteria, we will unravel the molecular mechanism of an unusual ABC exporter which is thought to import iron-loaded siderophores across the inner mycobacterial membrane and to release iron in the cytoplasm by virtue of its attached siderophore interacting domain. Further, we will investigate two proton-driven transporters responsible for the efflux of empty siderophores, exhibiting an unknown protein fold. We will determine atomic structures by combining X-ray crystallography and cryo-EM and thoroughly investigate active in- and efflux of siderophores in liposomes as well as in cells. Siderophore transport across the outer mycobacterial membrane is a terra incognita. By combining high-density transposon mutagenesis with deep sequencing (Tn-Seq), we aim to discover novel receptors, carriers and channels involved in siderophore transport, which are subsequently characterized at the biochemical and structural level. Siderophore-mediated iron acquisition offers a vulnerable attacking point of M. tuberculosis. Using protein engineering, we will develop a human siderocalin exhibiting low affinity binding for carboxymycobactin into a therapeutic agent able to efficiently capture mycobacterial siderophores and thereby starve M. tuberculosis for iron. In summary, we will discover novel proteins involved in iron acquisition, gain mechanistic insights into poorly understood siderophore transport processes at the molecular level and explore novel strategies to treat tuberculosis.