Structure, biochemical function, and signaling mechanism of plant NLRs

To counter pathogen invasion,plants have evolved a large number of immune receptors,including membrane-resident pattern recognition receptors(PRRs)and intracellular nucleotide-binding and leucine-rich repeat receptors(NLRs).Our knowledge about PRR and NLR signaling mechanisms has expanded significantly over the past few years.Plant NLRs form multi-protein complexes called resistosomes in response to pathogen effectors,and the signaling mediated by NLR resistosomes converges on Ca2+-permeable channels.Ca2+-permeable channels important for PRR signaling have also been identified.These findings highlight a crucial role of Ca2+in triggering plant immune signaling.In this review,we first discuss the structural and biochemical mechanisms of non-canonical NLR Ca2+channels and then summarize our knowledge about immune-related Ca2+-permeable channels and their roles in PRR and NLR signaling.We also discuss the potential role of Ca2+in the intricate interaction between PRR and NLR signaling.

Structural insight into chitin perception by chitin elicitor receptor kinase 1 of Oryza sativa

Plants have developed innate immune systems to fight against pathogenic fungi by monitoring pathogenic signals known as pathogen-associated molecular patterns(PAMP)and have established endo symbiosis with arbuscular mycorrhizal(AM)fungi through recognition of mycorrhizal(Myc)factors.Chitin elicitor receptor kinase 1 of Oryza sativa subsp.Japonica(OsC ERK1)plays a bifunctional role in mediating both chitin-triggered immunity and symbiotic relationships with AM fungi.However,it remains unclear whether OsC ERK1 can directly recognize chitin molecules.In this study,we show that OsC ERK1 binds to the chitin hexamer((NAG)6)and tetramer((NAG)4)directly and determine the crystal structure of the OsC ERK1-(NAG)6complex at 2?.The structure shows that one OsC ERK1 is associated with one(NAG)6.Upon recognition,chitin hexamer binds OsC ERK1 by interacting with the shallow groove on the surface of LysM 2.These structural findings,complemented by mutational analyses,demonstrate that LysM 2 is crucial for recognition of both(NAG)6and(NAG)4.Altogether,these findings provide structural insights into the ability of OsC ERK1 in chitin perception,which will lead to a better understanding of the role of OsCERK1 in mediating both immunity and symbiosis in rice.

SERK Family Receptor-like Kinases Function as Co-receptors with PXY for Plant Vascular Development

In Arabidopsis, the CLAVATA3/EMBRYO SURROUNDING REGION-RELATED （CLE） peptides play important roles in regulating proliferation and differentiation of plant-specific stem cells. Although receptors of CLEs are reported to be leucine-rich repeat receptor kinases, the mechanisms underlying CLE-induced receptor activation remain largely unknown. Here we show that SOMATIC EMBRYOGENESIS RECEPTOR KINASEs （SERKs） serve as co-receptors in CLE41/TDIF-PXY signaling to regulate plant vascular development. TDIF induces interaction of its receptor PXY with SERKs in vitro and in vivo. Furthermore, the serk1-1 serk2-1 bakl-5 mutant plants are less sensitive to TDIF, phenocopying the pxy mutant with a compromised promotion of procambial cell proliferation. Crystal structure of the PXY-TDIF-SERK2 complex reveals that the last amino acid of TDIF conserved among CLEs and other evolutionary-related peptides is important for the interaction between SERK2 and PXY. Taken together, our current study identifies SERKs as signaling components of the TDIF-PXY pathway and suggests a conserved activation mechanism of CLE receptors.

Molecular actions of NLR immune receptors in plants and animals

NLRs constitute intracellular immune receptors in both plants and animals. Direct or indirect ligand recognition results in formation of oligomeric NLR complexes to mediate immune signaling. Over the past 20 years, rapid progress has been made in our understanding of NLR signaling. Structural and biochemical studies provide insight into molecular basis of autoinhibition,ligand recognition, and resistosome/inflammasome formation of several NLRs. In this review, we summarize these studies focusing on the structural aspect of NLRs. We also discuss the analogies and differences between plant and animal NLRs in their mechanisms of action and how the available knowledge may shed light on the signaling mechanisms of other NLRs.

Structural basis for differential recognition of brassinolide by its receptors

Brassinosteroids,a group of plant steroid hormones,reg-ulate many aspects of plant growth and development.We and other have previously solved the crystal structures of BRI1(LRR)in complex with brassinolide,the most active brassinosteroid identifi ed thus far.Although these studies provide a structural basis for the recognition of brassi-nolide by its receptor BRI1,it still remains poorly under-stood how the hormone differentiates among its con-served receptors.Here we present the crystal structure of the BRI1 homolog BRL1 in complex with brassinolide.The structure shows that subtle differences around the brassinolide binding site can generate a striking effect on its recognition by the BRI1 family of receptors.Structural comparison of BRL1 and BRI1 in their brassinolide-bound forms reveals the molecular basis for differential binding of brassinolide to its different receptors,which can be used for more effi cient design of plant growth regulators for agricultural practice.On the basis of our structural studies and others’data,we also suggest possible mech-anisms for the activation of BRI1 family receptors.

Structural Insight into Recognition of Plant Peptid Hormones by Receptors

Secreted signaling peptides or peptide hormones play crucial roles in plant growth and development through coordination of cell-cell communication. Perception of peptide hormones in plants generally relies on membrane-localized receptor kinases （RKs）. Progress has recently been made in structural elucidation of interactions between posttranslationally modified peptide hormones and RKs. The structural studies suggest conserved receptor binding and activation mechanisms of this type of peptide hormones involving their conserved C-termini. Here, we review these structural data and discuss how the conserved mecha- nisms can be used to match peptide-RK pairs.

Structural basis for a homodimeric ATPase subunit of an ECF transporter

The transition metal cobalt,an essential cofactor for many enzymes in prokaryotes,is taken up by several specifi c transport systems.The CbiMNQO protein complex be-longs to type-1 energy-coupling factor(ECF)transporters and is a widespread group of microbial cobalt transport-ers.CbiO is the ATPase subunit(A-component)of the cobalt transporting system in the gram-negative thermo-philic bacterium Thermoanaerobacter tengcongensis.Here we report the crystal structure of a nucleotide-free CbiO at a resolution of 2.3Å.CbiO contains an N-terminal canonical nucleotide-binding domain(NBD)and C-termi-nal helical domain.Structural and biochemical data show that CbiO forms a homodimer mediated by the NBD and the C-terminal domain.Interactions mainly via conserved hydrophobic amino acids between the two C-terminal do-mains result in formation of a four-helix bundle.Structural comparison with other ECF transporters suggests that non-conserved residues outside the T-component bind-ing groove in the A component likely act as a specifi city determinant for T components.Together,our data provide information on understanding of the structural organiza-tion and interaction of the CbiMNQO system.

A loop matters for FTO substrate selection

Recent studies have unequivocally established the link between FTO and obesity.FTO was biochemically shown to belong to the AlkB-like family DNA/RNA demethylase.However,FTO differs from other AlkB members in that it has unique substrate specificity and contains an extended C-terminus with unknown functions.Insight into the substrate selection mechanism and a functional clue to the C-terminus of FTO were gained from recent structural and biochemical studies.These data would be valuable to design FTO-specific inhibitors that can be potentially translated into therapeutic agents for treatment of obesity or obesity-related diseases.

Atomic-resolution view of complete TCR-CD3 revealed

Antigen-specific T-cell responses are triggered via interaction of T cell receptors(TCR)with pathogen-or tumorderived peptides presented on major histocompatibility complexes(pMHC)by antigen-presenting cells(APCs).The mechanisms of TCR signaling are fundamentally important to our understanding of adaptive immunity and TCR-based therapies.This can be exemplified by the fact that studies of TCR have led to clinical development of chimeric antigen receptor T(CAR-T)cells(Srivastava and Riddell,2015).TCR is a multiprotein complex consisting of variable TCR receptorαandβchains(TCRα/β)associated with the dimeric signaling modules CD3γ/ε,δ/ε,andζ/ζ.Over the past two decades,many models have been proposed on the arrangement of the receptor subunits,stoichiometry of the TCR-CD3 complex and the mechanisms of TCR triggering(Rudolph et al.,2006).Our knowledge of TCR signaling,however,is far from being complete partly due to the lack of structural information of a complete TCR-CD3 complex.In one recent remarkable study published in Nature(Dong et al.2019),an atomic-resolution view of a full TCR-CD3 complex has been revealed.The structure significantly advanced our understanding of the mechanism of TCR-CD3 assembly and offered unprecedented insight into TCR triggering.

Door to the cell for COVID-19 opened,leading way to therapies

A very recent study by Lan et al.1 published in Nature determined the crystal structure of the severe acute respiratory syndrome coronavirus(SARS-CoV)-2 receptor-binding domain(RBD)bound to angiotensin-converting enzyme 2(ACE2).The structure reveals the mechanism of SARS-CoV-2 RBD recognition by its receptor ACE2,which is highly conserved in ACE2 recognition of SARS-CoV RBD.The study provides structural information on developing small molecules targeting SARS-CoV-2 RBD/ACE2 and implies the existence of other mechanisms than receptor binding for the markedly different infection activity of the two evolutionarily close viruses.