Steroid hormones are essential for growth, development, and homeostasis of animals, insects, and plants. In plants, one class of polyhydroxylated steroids, called brassinosteroids (BRs), has wide distribution throughout the plant kingdom and unique growth promoting activities. During the past decade, we have analyzed the biosynthesis and cellular functions of BRs, and identified the membrane-localized receptor, BRI1, as well as many of its downstream signaling components. BRI1 is a Toll-like receptor kinase comprised of a large extracellular ligand-binding domain containing 24 leucine-rich repeats, a trans-membrane segment and a cytoplasmic kinase domain. Previous studies indicate that the extracellular domain of BRI1 binds the steroid hormone, which then induces a conformational change in the receptor that in turn leads to auto- phosphorylation of the cytoplasmic kinase domain and the dissociation of the kinase inhibitor protein BKI1. This increases the affinity of BRI1 for its co-receptor, BAK1, a receptor kinase with 5 LRRs. Extensive cross- phosphorylation events between the kinase domains of the receptor and the co-receptor then lead to a fully activated signaling complex. Here, we propose combining structural biology/biochemistry with genetics/cell biology to dissect the molecular mechanism of the receptor. We want to analyze its mode of ligand recognition and activation, its interaction with the co-receptor BAK1, and the regulation of its kinase activity by the novel inhibitor, BKI1. Thus, this proposal has the following specific aims: (1) Determine the detailed mechanism of steroid hormone recognition by the 94 amino acid steroid-binding domain in context of the LRR modules in the extracellular domain of BRI1;(2) Develop a mechanistic model for receptor activation and test this model in vitro and in vivo;(3) Elucidate the molecular mode of action and specificity of BKI1 and 6 related proteins in the negative regulation of BRI1's kinase activity. Our long-term goal is to develop a validated mechanistic model that accurately describes the stimulation and regulation of the BR signaling pathway. BRI1 is the best studied and most understood of any cell surface receptor proposed for steroid hormones in multicellular eukaryotes. The experiments described here will thus inform our mechanistic understanding of a new paradigm for steroid hormone perception and signaling from a cell surface receptor. The strength of Arabidopsis as a genetic model has allowed the identification of dozens of mutant alleles (both gain- and loss-of-function) in both the receptor and its co-receptor, as well as elucidated an entire signaling pathway from the receptor to changes in nuclear gene expression. Thus, we are well-poised to interpret the structural studies. BRI1 is a founding member of the largest family of receptor kinases in plants, and shares significant homology with mammalian innate immunity receptors. As such, our studies will provide mechanistic insight into other plant signaling pathways, and their evolutionary relationship to well-studied mammalian systems.
Despite their divergence from a common ancestor over 1 billion years ago, both plants and animals utilize steroids as hormones to regulate gene expression that controls growth, development, and homeostasis. The pathways of synthesis and turnover of steroids are remarkably conserved, yet steroid receptors appear to have evolved independently in plants and animals, which may be due to the unique predator/prey relationship of animals and plants. The experiments described in this proposal will thus not only inform our understanding of a new paradigm for steroid hormone perception, but they may also influence our thinking concerning the evolution of signaling pathways in humans as a result of our diet.
|Liu, Wanlu; Duttke, Sascha H; Hetzel, Jonathan et al. (2018) RNA-directed DNA methylation involves co-transcriptional small-RNA-guided slicing of polymerase V transcripts in Arabidopsis. Nat Plants 4:181-188|
|Seluzicki, Adam; Burko, Yogev; Chory, Joanne (2017) Dancing in the dark: darkness as a signal in plants. Plant Cell Environ 40:2487-2501|
|Procko, Carl; Burko, Yogev; Jaillais, Yvon et al. (2016) Corrigendum: The epidermis coordinates auxin-induced stem growth in response to shade. Genes Dev 30:2018|
|de Saint Germain, Alexandre; Clavé, Guillaume; Badet-Denisot, Marie-Ange et al. (2016) An histidine covalent receptor and butenolide complex mediates strigolactone perception. Nat Chem Biol 12:787-794|
|Hetzel, Jonathan; Duttke, Sascha H; Benner, Christopher et al. (2016) Nascent RNA sequencing reveals distinct features in plant transcription. Proc Natl Acad Sci U S A 113:12316-12321|
|Simon, Mathilde Laetitia Audrey; Platre, Matthieu Pierre; Marquès-Bueno, Maria Mar et al. (2016) A PtdIns(4)P-driven electrostatic field controls cell membrane identity and signalling in plants. Nat Plants 2:16089|
|Procko, Carl; Burko, Yogev; Jaillais, Yvon et al. (2016) The epidermis coordinates auxin-induced stem growth in response to shade. Genes Dev 30:1529-41|
|Zorzatto, Cristiane; Machado, João Paulo B; Lopes, Kênia V G et al. (2015) NIK1-mediated translation suppression functions as a plant antiviral immunity mechanism. Nature 520:679-82|
|Martins, Sara; Dohmann, Esther M N; Cayrel, Anne et al. (2015) Internalization and vacuolar targeting of the brassinosteroid hormone receptor BRI1 are regulated by ubiquitination. Nat Commun 6:6151|
|Zhong, Xuehua; Du, Jiamu; Hale, Christopher J et al. (2014) Molecular mechanism of action of plant DRM de novo DNA methyltransferases. Cell 157:1050-60|
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