Pattern recognition receptors confer plant resistance to pathogen infection by recognizing the conserved pathogen-associated molecular patterns. The cell surface receptor chitin elicitor receptor kinase 1 of Arabidopsis (AtCERK1) directly binds chitin through its lysine motif (LysM)-containing ectodomain (AtCERK1-ECD) to activate immune responses. The crystal structure that we solved of an AtCERK1-ECD complexed with a chitin pentamer reveals that their interaction is primarily mediated by a LysM and three chitin residues. By acting as a bivalent ligand, a chitin octamer induces AtCERK1-ECD dimerization that is inhibited by shorter chitin oligomers. A mutation attenuating chitin-induced AtCERK1-ECD dimerization or formation of nonproductive AtCERK1 dimer by overexpression of AtCERK1-ECD compromises AtCERK1-mediated signaling in plant cells. Together, our data support the notion that chitin-induced AtCERK1 dimerization is critical for its activation.

Brassinosteroids (BRs) are essential phytohormones that play crucial roles in plant growth and development. Perception of BRs requires an active complex of brassinosteroid-insensitive 1 (BRI1) and BRI1-associated kinase 1 (BAK1). Recognized by the extracellular leucine-rich repeat (LRR) domain of BRI1, BRs induce a phosphorylation-mediated cascade to regulate gene expression. Here we present the crystal structures of BRI1-LRR in free and brassinolide (BL)-bound forms. BRI1-LRR exists as a monomer in crystals and solution independent of BL. It comprises a helical solenoid structure that accommodates a separate insertion domain at its concave surface. Sandwiched between them, BL binds to a hydrophobicity-dominating surface groove on BRI1-LRR. BL recognition by BRI1-LRR is through an induced-fit mechanism involving stabilization of two inter-domain loops that creates a pronounced non-polar surface groove for the hormone binding. Together, our results define the molecular mechanisms by which BRI1 recognizes BRs and provide insight into BR-induced BRI1 activation.

Mitochondrial calcium uptake is crucial to the regulation of eukaryotic Ca 2+ homeostasis and is mediated by the mitochondrial calcium uniporter (MCU). While MCU alone can transport Ca 2+ in primitive eukaryotes, metazoans require an essential single membrane-spanning auxiliary component called EMRE to form functional channels; however, the molecular mechanism of EMRE regulation remains elusive. Here, we present the cryo-EM structure of the human MCU-EMRE complex, which defines the interactions between MCU and EMRE as well as pinpoints the juxtamembrane loop of MCU and extended linker of EMRE as the crucial elements in the EMRE-dependent gating mechanism among metazoan MCUs. The structure also features the dimerization of two MCU-EMRE complexes along an interface at the N-terminal domain (NTD) of human MCU that is a hotspot for post-translational modifications. Thus, the human MCU-EMRE complex, which constitutes the minimal channel components among metazoans, provides a framework for future mechanistic studies on MCU.

TRPM4 is a calcium-activated, phosphatidylinositol bisphosphate (PtdInsP2) modulated, non-selective cation channel, and belongs to the family of melastatin-related transient receptor potential (TRPM) channels. Here we present the cryo-EM structures of the mouse TRPM4 channel with and without ATP. TRPM4 consists of multiple transmembrane and cytosolic domains, which assemble into a three-tiered architecture. The N-terminal nucleotide binding domain (NBD) and the C-terminal coiled coil participate in the tetrameric assembly of the channel; ATP binds at NBD and inhibits channel activity. TRPM4 has an exceptionally wide filter but is only permeable to monovalent cations; filter residue Gln973 is essential in defining monovalent selectivity. S1-S4 domain and post-S6 TRP domain form the central gating apparatus that likely house the Ca2+ and PtdInsP2 binding sites. These structures provide an essential starting point for elucidating the complex gating mechanisms of TRPM4 and also reveal the molecular architecture of the TRPM family for the first time.