Pattern recognition receptors: from the cell surface to intracellular dynamics

Detection of potentially infectious microorganisms is essential for plant immunity. Microbial communities growing on plant surfaces are constantly monitored according to their conserved microbe-associated molecular patterns (MAMPs). In recent years, several pattern-recognition receptors, including receptor-like kinases and receptor-like proteins, and their contribution to disease resistance have been described. MAMP signaling must be carefully controlled and seems to involve receptor endocytosis. As a further surveillance layer, plants are able to specifically recognize microbial effector molecules via nucleotide-binding site leucine-rich repeat receptors (NB-LRR). A number of recent studies show that NB-LRR translocate to the nucleus in order to exert their activity. In this review, current knowledge regarding the recognition of MAMPs by surface receptors, receptor activation, signaling, and subcellular redistribution are discussed.     

Plant pattern-recognition receptors controlling innate immunity

Plants are exposed to numerous potential pathogenic microbes. To counter the threat, plants have evolved diverse patternrecognition receptors(PRRs), which are receptor kinases(RKs) and receptor proteins(RPs) specialized to detect conserved pathogen/microbe-associated molecular patterns(PAMPs/MAMPs). Although only a handful of RKs and RPs are known PRRs,they belong to the receptor-like kinase(RLK) and receptor-like protein(RLP) superfamilies that undergo lineage-specific expansion, suggesting that many of these RLKs and RLPs are potential PRRs. Analyses of existing PRRs have uncovered ligand-induced RLK-RK or RLK-RP oligomerization as a common mechanism for immune activation. PRRs can recruit additional components to form dynamic receptor complexes, which mediate specific cellular responses. Detailed analyses of these components are shedding light on molecular mechanisms underlying the regulation of PRR activity and downstream signaling.     

Erratum to: Plant pattern-recognition receptors controlling innate immunity

Plants are exposed to numerous potential pathogenic microbes. To counter the threat, plants have evolved diverse patternrecognition receptors(PRRs), which are receptor kinases(RKs) and receptor proteins(RPs) specialized to detect conserved pathogen/microbe-associated molecular patterns(PAMPs/MAMPs). Although only a handful of RKs and RPs are known PRRs, they belong to the receptor-like kinase(RLK) and receptor-like protein(RLP) superfamilies that undergo lineage- specific expansion, suggesting that many of these RLKs and RLPs are potential PRRs. Analyses of existing PRRs have uncovered ligand-induced RLK-RK or RLK-RP oligomerization as a common mechanism for immune activation. PRRs can recruit additional components to form dynamic receptor complexes, which mediate specific cellular responses. Detailed analyses of these components are shedding light on molecular mechanisms underlying the regulation of PRR activity and downstream signaling.     

The Dictyostelium repertoire of seven transmembrane domain receptors

The availability of fully sequenced genomes allows the in silico analysis of whole gene families in a given genome. A particularly large and interesting gene family is the G-protein-coupled receptor family. These receptors detect a variety of extracellular signals and transduce them, generally via heterotrimeric G-proteins, to effector proteins inside the cell and thus elicit a physiological response. G-protein-coupled receptors are found in all eukaryotes and constitute in vertebrates 3-5% of all genes. They are also very important drug targets and approximately 25 of the top 100 selling drugs are directed against these receptors. The Dictyostelium discoideum genome contains a surprisingly high number of 55 such receptors, approximately 0.5% of the encoded genes. Besides the four well-studied cAMP receptors the genome encodes eight additional cAMP receptor-like proteins and one of these is distinguished by a novel domain structure, one secretin-like receptor, 17 GABA(B)-like and 25 Frizzled-like receptors. The existence of the latter three types of receptors in D. discoideum was surprising because they had not been observed outside the animal kingdom before. Their presence suggests unprecedentedly complex and so far unknown signaling activities in this lower eukaryote.     

Glutamate receptors in plants

Ionotropic glutamate receptors function in animals as glutamate-gated non-selective cation channels. Numerous glutamate receptor-like (GLR) genes have been identified in plant genomes, and plant GLRs are predicted, on the basis of sequence homology, to retain ligand-binding and ion channel activity. Non-selective cation channels are ubiquitous in plant membranes and may function in nutrient uptake, signalling and intra-plant transport. However, there is little evidence for amino acid gating of plant ion channels. Recent evidence suggests that plant GLRs do encode non-selective cation channels, but that these channels are not gated by amino acids. The functional properties of these proteins and their roles in plant physiology remain a mystery. The problems surrounding characterization and assignation of function to plant GLRs are discussed in this Botanical Briefing, and potential roles for GLR proteins as non-selective cation channels involved in metabolic signalling are described.     

Phylogenetic analysis of 277 human G-protein-coupled receptors as a tool for the prediction of orphan receptor ligands

Background

G-protein-coupled receptors (GPCRs) are the largest and most diverse family of transmembrane receptors. They respond to a wide range of stimuli, including small peptides, lipid analogs, amino-acid derivatives, and sensory stimuli such as light, taste and odor, and transmit signals to the interior of the cell through interaction with heterotrimeric G proteins. A large number of putative GPCRs have no identified natural ligand. We hypothesized that a more complete knowledge of the phylogenetic relationship of these orphan receptors to receptors with known ligands could facilitate ligand identification, as related receptors often have ligands with similar structural features.

Results

A database search excluding olfactory and gustatory receptors was used to compile a list of accession numbers and synonyms of 81 orphan and 196 human GPCRs with known ligands. Of these, 241 sequences belonging to the rhodopsin receptor-like family A were aligned and a tentative phylogenetic tree constructed by neighbor joining. This tree and local alignment tools were used to define 19 subgroups of family A small enough for more accurate maximum-likelihood analyses. The secretin receptor-like family B and metabotropic glutamate receptor-like family C were directly subjected to these methods.

Conclusions

Our trees show the overall relationship of 277 GPCRs with emphasis on orphan receptors. Support values are given for each branch. This approach may prove valuable for identification of the natural ligands of orphan receptors as their relation to receptors with known ligands becomes more evident.     

The CHASE domain: a predicted ligand-binding module in plant cytokinin receptors and other eukaryotic and bacterial receptors

A novel, 200-230 amino acid extracellular domain was identified in the plant cytokinin receptor Cre1, in the receptor-histidine kinase DhkA and the adenylyl cyclase Acg from the slime mold Dictyostelium discoideum, and in a variety of other receptor-like proteins from bacteria and eukaryotes. The domain is predicted to bind diverse low molecular weight ligands, such as the cytokinin-like adenine derivatives or peptides, and mediate signal transduction through the respective receptors.     

The Dictyostelium repertoire of seven transmembrane domain receptors

The availability of fully sequenced genomes allows the in silico analysis of whole gene families in a given genome. A particularly large and interesting gene family is the G-protein-coupled receptor family. These receptors detect a variety of extracellular signals and transduce them, generally via heterotrimeric G-proteins, to effector proteins inside the cell and thus elicit a physiological response. G-protein-coupled receptors are found in all eukaryotes and constitute in vertebrates 3–5% of all genes. They are also very important drug targets and approximately 25 of the top 100 selling drugs are directed against these receptors. The Dictyostelium discoideum genome contains a surprisingly high number of 55 such receptors, approximately 0.5% of the encoded genes. Besides the four well-studied cAMP receptors the genome encodes eight additional cAMP receptor-like proteins and one of these is distinguished by a novel domain structure, one secretin-like receptor, 17 GABA_B-like and 25 Frizzled-like receptors. The existence of the latter three types of receptors in D. discoideum was surprising because they had not been observed outside the animal kingdom before. Their presence suggests unprecedentedly complex and so far unknown signaling activities in this lower eukaryote.     

Role of LysM receptors in chitin-triggered plant innate immunity

Recent research findings clearly indicate that lysin motif (LysM)-containing cell surface receptors are involved in the recognition of specific oligosaccharide elicitors (chitin and peptidoglycan), which trigger an innate immunity response in plants. These receptors are either LysM-containing receptor-like kinases (LYKs) or LysM-containing receptor proteins (LYPs). In Arabidopsis, five LYKs (AtCERK1/AtLYK1 and AtLYK2–5) and three LYPs (AtLYP1–3) are likely expressed on the plasma membrane. In this review, we summarize recent research results on the role of these receptors in plant innate immunity, including the recent structural characterization of AtCERK1 and composition of the various receptor complexes in Arabidopsis.     

RAMPs and G protein coupled receptors

RAMPs (receptor activity-modifying proteins) were discovered in 1998 as accessory proteins needed to the functionnal activity of CGRP (calcitonin gene-related peptide) receptors. Three RAMPs generated by three different genes are known in human, rat and mice. The coding sequences of such genes are described, but as yet, regulation sequences are unknown. RAMPs interact with GPCR (G protein-coupled receptors) of class II. In the case of the calcitonin/CGRP peptide family, RAMPs determine the functionnal specificity of the receptor, glycosylate and translocate the receptor to the cell surface. CGRP receptors are observed in presence of the RAMP1/calcitonin receptor-like receptor (CRLR), but the association of RAMP2 or RAMP3 with CRLR generates an adrenomedullin receptor. The calcitonin receptor (CTR) is translocated alone to the cell surface, but interactions of RAMPs with CTR forms amylin receptors. If RAMPs can interact with glucagon, parathyroid hormone and VIP/PACAP (vasoactive intestinal peptide/pituitary adenylate cyclase activating polypeptide (VPACR1)) receptors, the functionnal specificity of these receptors remains unaltered. However, the complex VPACR1/RAMP2 enhances specifically the phosphoinoside signaling pathway.     

Functional calcitonin gene-related peptide receptors are formed by the asymmetric assembly of a calcitonin receptor-like receptor homo-oligomer and a monomer of receptor activity-modifying protein-1

In addition to their interactions with hetero-trimeric G proteins, seven-transmembrane domain receptors are now known to form multimeric complexes that can include receptor homo- or hetero-oligomers and/or accessory proteins that modulate their activity. The calcitonin gene-related peptide (CGRP) receptor requires the assembly of the seven-transmembrane domain calcitonin receptor-like receptor with the single-transmembrane domain receptor activity-modifying protein-1 to reach the cell surface and be active. However, the relative stoichiometric arrangement of these two proteins within a receptor complex remains unknown. Despite recent advances in the development of protein-protein interactions assays, determining the composition and stoichiometric arrangements of such signaling complexes in living cells remains a challenging task. In the present study, we combined bimolecular fluorescence complementation (BiFC) with bioluminescence resonance energy transfer (BRET) to probe the stoichiometric arrangement of the CGRP receptor complex. Together with BRET competition assays, co-immunoprecipitation experiments, and BiFC imaging, dual BRET/BiFC revealed that functional CGRP receptors result from the association of a homo-oligomer of the calcitonin receptor-like receptor with a monomer of the accessory protein receptor activity-modifying protein-1. In addition to revealing the existence of an unexpected asymmetric oligomeric organization for a G protein-coupled receptor, our study illustrates the usefulness of dual BRET/BiFC as a powerful tool for analyzing constitutive and dynamically regulated multiprotein complexes.     

Adrenomedullin and related peptides: receptors and accessory proteins.

Adrenomedullin (AM), alpha- and beta-calcitonin gene-related peptide (CGRP), amylin and calcitonin (CT) are structurally and functionally related peptides. The structure of a receptor for CT (CTR) was elucidated in 1991 through molecular cloning, but the structures of the receptors for the other three peptides had yet to be elucidated. The discovery of receptor-activity-modifying proteins (RAMP) 1 and -2 and their co-expression with an orphan receptor, calcitonin receptor-like receptor (CRLR) has led to the elucidation of functional CGRP and AM receptors, respectively. RAMP1 and -3 which are co-expressed with CTR revealed two amylin receptor isotypes. Molecular interactions between CRLR and RAMPs are involved in their transport to the cell surface. Heterodimeric complexes between CRLR or CTR and RAMPs are required for ligand recognition.     

Adrenomedullin receptors: molecular identity and function.

Since its discovery in 1993 adrenomedullin (AM) has been the subject over 600 published articles. This multifunctional peptide has powerful vasodilator actions and recent evidence from AM gene-deleted mice suggest that AM plays an essential role in vascular development. However the lack of valid AM receptor clones and non-peptide receptor ligands has considerably slowed research progress on this important peptide. In this review we have focused on the proposition that the calcitonin receptor-like receptor (CRLR) is a receptor both for AM and the related vasoactive peptide calcitonin gene-related peptide (CGRP). The receptor activity modifying proteins (RAMPs) that are essential for defining CRLR pharmacology will also be discussed. We will describe how AM receptors have been reported to signal and be regulated and to consider whether further receptors for AM beyond CRLR/RAMP combinations might exist.     

Cutting edge: human FcRL4 and FcRL5 are receptors for IgA and IgG

Fc receptor-like (FcRL) proteins are a family of cellular receptors homologous to FcγRI and are predominantly expressed by B cells. They function to costimulate or inhibit BCR signaling through consensus ITAMs and ITIMs; however, the extracellular ligands of these receptors remain unknown or controversial. In this study, we tested the ability of human FcRL proteins to bind Igs and found FcRL4 and FcRL5 to be bona fide Fc receptors. In cellular binding assays, FcRL4 bound efficiently to IgA and FcRL5 binds all IgG isotypes with varied efficiency. Additionally, we generated mAbs capable of specifically blocking these interactions. Given their expression on activated B cells and potential for inhibitory signaling, FcRL4 and FcRL5 are likely to be important for immune complex-dependent human B cell regulation, and they represent novel therapeutic targets for receptor blockade therapies.     

Arabidopsis plasmodesmal proteome

The multicellular nature of plants requires that cells should communicate in order to coordinate essential functions. This is achieved in part by molecular flux through pores in the cell wall, called plasmodesmata. We describe the proteomic analysis of plasmodesmata purified from the walls of Arabidopsis suspension cells. Isolated plasmodesmata were seen as membrane-rich structures largely devoid of immunoreactive markers for the plasma membrane, endoplasmic reticulum and cytoplasmic components. Using nano-liquid chromatography and an Orbitrap ion-trap tandem mass spectrometer, 1341 proteins were identified. We refer to this list as the plasmodesmata- or PD-proteome. Relative to other cell wall proteomes, the PD-proteome is depleted in wall proteins and enriched for membrane proteins, but still has a significant number (35%) of putative cytoplasmic contaminants, probably reflecting the sensitivity of the proteomic detection system. To validate the PD-proteome we searched for known plasmodesmal proteins and used molecular and cell biological techniques to identify novel putative plasmodesmal proteins from a small subset of candidates. The PD-proteome contained known plasmodesmal proteins and some inferred plasmodesmal proteins, based upon sequence or functional homology with examples identified in different plant systems. Many of these had a membrane association reflecting the membranous nature of isolated structures. Exploiting this connection we analysed a sample of the abundant receptor-like class of membrane proteins and a small random selection of other membrane proteins for their ability to target plasmodesmata as fluorescently-tagged fusion proteins. From 15 candidates we identified three receptor-like kinases, a tetraspanin and a protein of unknown function as novel potential plasmodesmal proteins. Together with published work, these data suggest that the membranous elements in plasmodesmata may be rich in receptor-like functions, and they validate the content of the PD-proteome as a valuable resource for the further uncovering of the structure and function of plasmodesmata as key components in cell-to-cell communication in plants.     

Cloning and subcellular location of an Arabidopsis receptor-like protein that shares common features with protein-sorting receptors of eukaryotic cells.

Many receptors involved in clathrin-mediated protein transport through the endocytic and secretory pathways of yeast and animal cells share common features. They are all type I integral membrane proteins containing cysteine-rich lumenal domains and cytoplasmic tails with tyrosine-containing sorting signals. The cysteine-rich domains are thought to be involved in ligand binding, whereas the cytoplasmic tyrosine motifs interact with clathrin-associated adaptor proteins during protein sorting along these pathways. In addition, tyrosine-containing signals are required for the retention and recycling of some of these membrane proteins to the trans-Golgi network. Here we report the characterization of an approximately 80-kD epidermal growth factor receptor-like type I integral membrane protein containing all of these functional motifs from Arabidopsis thaliana (called AtELP for A. thaliana Epidermal growth factor receptor-Like Protein). Biochemical analysis indicates that AtELP is a membrane protein found at high levels in the roots of both monocots and dicots. Subcellular fractionation studies indicate that the AtELP protein is present in two membrane fractions corresponding to a novel, undefined compartment and a fraction enriched in vesicles containing clathrin and its associated adaptor proteins. AtELP may therefore serve as a marker for compartments involved in intracellular protein trafficking in the plant cell.     

小麦受体样蛋白激酶及其衍生蛋白的研究进展

受体样蛋白激酶及其衍生蛋白(受体样胞内激酶和受体样蛋白)在调控植物生长、发育和不良环境反应等过程中发挥重要作用。该文简要介绍了近年来获得的对植物受体样蛋白激酶及其衍生蛋白的主要认识, 着重总结了小麦(Triticum aestivum)受体样蛋白激酶及其衍生蛋白的研究进展与不足, 并展望了该领域的发展趋势, 对在小麦中进一步有效开展受体样蛋白激酶及其衍生蛋白的结构和功能研究具参考意义。     

Esr proteins are secreted by the cells of the embryo surrounding region

Three highly homologous Esr genes are expressed specifically in the embryo surrounding region at the micropylar end of the maize endosperm. The proteins belong to a family of small hydrophilic proteins that share a conserved motive with Clv3, the ligand of the receptor-like kinase Clv1. In this study, co-localization of Esr proteins with their mRNAs in the embryo surrounding region was shown with polyclonal antibodies recognizing all three Esr proteins. On a subcellular level the secretion of Esr proteins and their association with the cell wall was shown independently by cell fractionation, immunohistochemistry and transient expression of Gfp fusion proteins. Furthermore, a possible interaction of Esr proteins with a 35 kDa protein present in the lower half of maize kernels was suggested by in vitro affinity chromatography. Therefore Esr proteins share two characteristics with ligands of receptor-like kinases: they are released in the extracellular space and they have the capacity to form protein-protein interactions.