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Nucleotide-binding domain, leucine-rich repeat receptors (NLRs) mediate innate immunity by forming inflammasomes. Activation of the NLR protein NLRP1 requires autocleavage within its function-to-find domain (FIIND).sup.1-7. In resting cells, the dipeptidyl peptidases DPP8 and DPP9 interact with the FIIND of NLRP1 and suppress spontaneous NLRP1 activation.sup.8,9; however, the mechanisms through which this occurs remain unknown. Here we present structural and biochemical evidence that full-length rat NLRP1 (rNLRP1) and rat DPP9 (rDPP9) form a 2:1 complex that contains an autoinhibited rNLRP1 molecule and an active UPA-CARD fragment of rNLRP1. The ZU5 domain is required not only for autoinhibition of rNLRP1 but also for assembly of the 2:1 complex. Formation of the complex prevents UPA-mediated higher-order oligomerization of UPA-CARD fragments and strengthens ZU5-mediated NLRP1 autoinhibition. Structure-guided biochemical and functional assays show that both NLRP1 binding and enzymatic activity are required for DPP9 to suppress NLRP1 in human cells. Together, our data reveal the mechanism of DPP9-mediated inhibition of NLRP1 and shed light on the activation of the NLRP1 inflammasome. The 2:1 complex between the inflammatory mediator NLRP1 and the dipeptidyl peptidase DPP9 functions to sequester an inflammasome-forming fragment of NLRP1 and enhance NLRP1 autoinhibition., Author(s): Menghang Huang [sup.1] , Xiaoxiao Zhang [sup.1] , Gee Ann Toh [sup.2] , Qin Gong [sup.3] , Jia Wang [sup.1] , Zhifu Han [sup.1] , Bin Wu [sup.3] [sup.4] [...]

Receptor kinases of the Catharanthus roseus RLK1-like (CrRLK1L) family have emerged as important regulators of plant reproduction, growth and responses to the environment.sup.1. Endogenous RAPID ALKALINIZATION FACTOR (RALF) peptides.sup.2 have previously been proposed as ligands for several members of the CrRLK1L family.sup.1. However, the mechanistic basis of this perception is unknown. Here we report that RALF23 induces a complex between the CrRLK1L FERONIA (FER) and LORELEI (LRE)-LIKE GLYCOSYLPHOSPHATIDYLINOSITOL (GPI)-ANCHORED PROTEIN 1 (LLG1) to regulate immune signalling. Structural and biochemical data indicate that LLG1 (which is genetically important for RALF23 responses) and the related LLG2 directly bind RALF23 to nucleate the assembly of RALF23-LLG1-FER and RALF23-LLG2-FER heterocomplexes, respectively. A conserved N-terminal region of RALF23 is sufficient for the biochemical recognition of RALF23 by LLG1, LLG2 or LLG3, and binding assays suggest that other RALF peptides that share this conserved N-terminal region may be perceived by LLG proteins in a similar manner. Structural data also show that RALF23 recognition is governed by the conformationally flexible C-terminal sides of LLG1, LLG2 and LLG3. Our work reveals a mechanism of peptide perception in plants by GPI-anchored proteins that act together with a phylogenetically unrelated receptor kinase. This provides a molecular framework for understanding how diverse RALF peptides may regulate multiple processes, through perception by distinct heterocomplexes of CrRLK1L receptor kinases and GPI-anchored proteins of the LRE and LLG family. Uncovering a mechanism of peptide perception by the receptor kinase FER and the LLG1 protein in Arabidopsis thaliana suggests a role for diverse RALF peptides in regulating multiple growth and reproductive processes in plants., Author(s): Yu Xiao [sup.1] , Martin Stegmann [sup.2] [sup.3] , Zhifu Han [sup.1] , Thomas A. DeFalco [sup.2] [sup.4] , Katarzyna Parys [sup.5] , Li Xu [sup.1] , Youssef Belkhadir [...]

Insights derived from the crystal structures of the extracellular domain of PSKR, the receptor for the plant hormone phytosulfokine (PSK) that affects plant growth and development, reveal that PSK interacts with PSKR and enhances PSKR interaction with its co-receptor SERK allosterically. Mode of action of phytosulfokine The plant hormone phytosulfokine (PSK) affects plant growth and development by binding to PSKR, a leucine-rich repeat receptor kinase. How it recognizes and activates PSKR remains unclear. Jijie Chai and colleagues have determined the crystal structures of the extracellular domain of PSKR in free, PSK-bound and co-receptor-bound forms. The structures and supporting evidence reveal that when PSK interacts with PSKR it enhances the interaction between PSKR and its co-receptor SERK. Intriguingly, PSK is not directly involved in the PSKR-SERK interaction; rather it acts allosterically to stabilize a domain within PSKR that can recruit a SERK. This work could provide a basis for the design of PSKR-specific small molecules. Phytosulfokine (PSK) is a disulfated pentapeptide that has a ubiquitous role in plant growth and development.sup.1,2. PSK is perceived by its receptor PSKR.sup.3,4, a leucine-rich repeat receptor kinase (LRR-RK). The mechanisms underlying the recognition of PSK, the activation of PSKR and the identity of the components downstream of the initial binding remain elusive. Here we report the crystal structures of the extracellular LRR domain of PSKR in free, PSK- and co-receptor-bound forms. The structures reveal that PSK interacts mainly with a [beta]-strand from the island domain of PSKR, forming an anti-[beta]-sheet. The two sulfate moieties of PSK interact directly with PSKR, sensitizing PSKR recognition of PSK. Supported by biochemical, structural and genetic evidence, PSK binding enhances PSKR heterodimerization with the somatic embryogenesis receptor-like kinases (SERKs). However, PSK is not directly involved in PSKR-SERK interaction but stabilizes PSKR island domain for recruitment of a SERK. Our data reveal the structural basis for PSKR recognition of PSK and allosteric activation of PSKR by PSK, opening up new avenues for the design of PSKR-specific small molecules., Author(s): Jizong Wang [sup.1] , Hongju Li [sup.2] , Zhifu Han [sup.1] , Heqiao Zhang [sup.1] , Tong Wang [sup.2] , Guangzhong Lin [sup.1] , Junbiao Chang [sup.3] , Weicai [...]

Brassinosteroids are essential phytohormones that have crucial roles in plant growth and development. Perception of brassinosteroids 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, brassinosteroids induce a phosphorylation-mediated cascade to regulate gene expression. Here we present the crystal structures of BRI1(LRR) in free and brassinolide-bound forms. BRI1(LRR) exists as a monomer in crystals and solution independent of brassinolide. It comprises a helical solenoid structure that accommodates a separate insertion domain at its concave surface. Sandwiched between them, brassinolide binds to a hydrophobicity-dominating surface groove on BRI1(LRR). Brassinolide recognition by BRI1(LRR) is through an induced-fit mechanism involving stabilization of two interdomain loops that creates a pronounced non-polar surface groove for the hormone binding. Together, our results define the molecular mechanisms by which BRI1 recognizes brassinosteroids and provide insight into brassinosteroid-induced BRI1 activation. Steroid hormone recognition The brassinolides are potent steroid hormones that are central to plant growth and development. They bind to the plasma membrane receptor BRI1, a member of the leucine-rich repeat (LRR) receptor family that also includes the mammalian Toll-like receptors, which are important in innate immunity. Two independent studies now present the crystal structure of BRI1 from Arabidopsis thaliana, both in the free form and in complex with brassinolide. BRI1 is found to adopt a solenoid-like superhelical structure involving 25 leucine-rich repeats. The structure is strikingly different from that of Toll-like receptors and reveals a novel mechanism of steroid recognition. This work suggests synthetic routes to nonsteroidal mimetics of BRI1 that could have a possible use in agriculture., Author(s): Ji She [sup.1] [sup.2] [sup.3] , Zhifu Han [sup.1] , Tae-Wuk Kim [sup.4] , Jinjing Wang [sup.3] , Wei Cheng [sup.3] , Junbiao Chang [sup.5] , Shuai Shi [sup.5] [...]