Unique defense strategy by the endoplasmic reticulum body in plants

Molecular cloning, characterization and analysis of the intracellular localization of a water-soluble chlorophyll-binding protein (WSCP) from Virginia pepperweed (Lepidium virginicum), a unique WSCP that preferentially binds chlorophyll b in vitro

Various plants possess non-photosynthetic, hydrophilic chlorophyll (Chl) proteins called water-soluble Chl-binding proteins (WSCPs). WSCPs are categorized into two classes; Class I (photoconvertible type) and Class II (non-photoconvertible type). Among Class II WSCPs, only Lepidium virginicum WSCP (LvWSCP) exhibits a low Chl a/b ratio compared with that found in the leaf. Although the physicochemical properties of LvWSCP have been characterized, its molecular properties have not yet been documented. Here, we report the characteristics of the LvWSCP gene, the biochemical properties of a recombinant LvWSCP, and the intracellular localization of LvWSCP. The cloned LvWSCP gene possesses a 669-bp open reading frame. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis revealed that the precursor of LvWSCP contains both N- and C-terminal extension peptides. RT-PCR analysis revealed that LvWSCP was transcribed in various tissues, with the levels being higher in developing tissues. A recombinant LvWSCP and hexa-histidine fusion protein (LvWSCP-His) could remove Chls from the thylakoid in aqueous solution and showed an absorption spectrum identical to that of native LvWSCP. Although LvWSCP-His could bind both Chl a and Chl b, it bound almost exclusively to Chl b when reconstituted in 40 % methanol. To clarify the intracellular targeting functions of the N- and C-terminal extension peptides, we constructed transgenic Arabidopsis thaliana lines expressing the Venus protein fused with the LvWSCP N- and/or C-terminal peptides, as well as Venus fused at the C-terminus of LvWSCP. The results showed that the N-terminal peptide functioned in ER body targeting, while the C-terminal sequence did not act as a trailer peptide.

A prodrug approach to the use of coumarins as potential therapeutics for superficial mycoses

Superficial mycoses are fungal infections of the outer layers of the skin, hair and nails that affect 20–25% of the world's population, with increasing incidence. Treatment of superficial mycoses, predominantly caused by dermatophytes, is by topical and/or oral regimens. New therapeutic options with improved efficacy and/or safety profiles are desirable. There is renewed interest in natural product-based antimicrobials as alternatives to conventional treatments, including the treatment of superficial mycoses. We investigated the potential of coumarins as dermatophyte-specific antifungal agents and describe for the first time their potential utility as topical antifungals for superficial mycoses using a prodrug approach. Here we demonstrate that an inactive coumarin glycone, esculin, is hydrolysed to the antifungal coumarin aglycone, esculetin by dermatophytes. Esculin is hydrolysed to esculetin β-glucosidases. We demonstrate that β-glucosidases are produced by dermatophytes as well as members of the dermal microbiota, and that this activity is sufficient to hydrolyse esculin to esculetin with concomitant antifungal activity. A β-glucosidase inhibitor (conduritol B epoxide), inhibited antifungal activity by preventing esculin hydrolysis. Esculin demonstrates good aqueous solubility (<6 g/l) and could be readily formulated and delivered topically as an inactive prodrug in a water-based gel or cream. This work demonstrates proof-of-principle for a therapeutic application of glycosylated coumarins as inactive prodrugs that could be converted to an active antifungal in situ. It is anticipated that this approach will be applicable to other coumarin glycones.

Pathogen infection trial increases the secretion of proteins localized in the endoplasmic reticulum body of Arabidopsis

Endoplasmic reticulum structures facilitate the increased secretion of proteins during the plant immune response.

ML3 is a NEDD8- and ubiquitin-modified protein

NEDD8 (NEURAL PRECURSOR CELL-EXPRESSED, DEVELOPMENTALLY DOWN-REGULATED PROTEIN8) is an evolutionarily conserved 8-kD protein that is closely related to ubiquitin and that can be conjugated like ubiquitin to specific lysine residues of target proteins in eukaryotes. In contrast to ubiquitin, for which a broad range of substrate proteins are known, only a very limited number of NEDD8 target proteins have been identified to date. Best understood, and also evolutionarily conserved, is the NEDD8 modification (neddylation) of cullins, core subunits of the cullin-RING-type E3 ubiquitin ligases that promote the polyubiquitylation of degradation targets in eukaryotes. Here, we show that Myeloid differentiation factor-2-related lipid-recognition domain protein ML3 is an NEDD8- as well as ubiquitin-modified protein in Arabidopsis (Arabidopsis thaliana) and examine the functional role of ML3 in the plant cell. Our analysis indicates that ML3 resides in the vacuole as well as in endoplasmic reticulum (ER) bodies. ER bodies are Brassicales-specific ER-derived organelles and, similar to other ER body proteins, ML3 orthologs can only be identified in this order of flowering plants. ML3 gene expression is promoted by wounding as well as by the phytohormone jasmonic acid and repressed by ethylene, signals that are known to induce and repress ER body formation, respectively. Furthermore, ML3 protein abundance is dependent on NAI1, a master regulator of ER body formation in Arabidopsis. The regulation of ML3 expression and the localization of ML3 in ER bodies and the vacuole is in agreement with a demonstrated importance of ML3 in the defense to herbivore attack. Here, we extend the spectrum of ML3 biological functions by demonstrating a role in the response to microbial pathogens.

Unconventional pathways of secretory plant proteins from the endoplasmic reticulum to the vacuole bypassing the Golgi complex

Studies on the basic mechanisms that regulate vacuolar delivering of proteins synthesized in the endoplasmic reticulum (ER) have a great importance in plant cell biology. Indeed, many aspects of plant physiology are affected by this intracellular traffic, for example, germination or reaction to biotic stresses due to the accumulation of storage proteins in seeds or enzymes in vegetative tissues, respectively. Up to now, the Golgi complex has been considered the main hub in the sorting of vacuolar secretory proteins; those polypeptides able to reach their final destination without the aid of this organelle are regarded as exceptions to an established route. This mini-review aims to emphasize the existence of several Golgi-independent pathways involved in the trafficking of different types of vacuolar proteins.

Label-free quantitative proteomic analysis of systemic responses to local wounding and virus infection in Arabidopsis thaliana

Plants are continuously exposed to changing environmental conditions and must, as sessile organisms, possess sophisticated acclimative mechanisms. To gain insight into systemic responses to local virus infection or wounding, we performed comparative LC-MS/MS protein profiling of distal, virus-free leaves four and five days after local inoculation of Arabidopsis thaliana plants with either Oilseed rape mosaic virus (ORMV) or inoculation buffer alone. Our study revealed biomarkers for systemic signaling in response to wounding and compatible virus infection in Arabidopsis, which should prove useful in further addressing the trigger-specific systemic response network and the elusive systemic signals. We observed responses common to ORMV and mock treatment as well as protein profile changes that are specific to local virus infection or mechanical wounding (mock treatment) alone, which provides evidence for the existence of more than one systemic signal to induce these distinct changes. Comparison of the systemic responses between time points indicated that the responses build up over time. Our data indicate stress-specific changes in proteins involved in jasmonic and abscisic acid signaling, intracellular transport, compartmentalization of enzyme activities, protein folding and synthesis, and energy and carbohydrate metabolism. In addition, a virus-triggered systemic signal appears to suppress antiviral host defense.

Set-point control of RD21 protease activity by AtSerpin1 controls cell death in Arabidopsis

Programmed cell death (PCD) in plants plays a key role in defense response and is promoted by the release of compartmentalized proteases to the cytoplasm. Yet the exact identity and control of these proteases is poorly understood. Serpins are an important group of proteins that uniquely curb the activity of proteases by irreversible inhibition; however, their role in plants remains obscure. Here we show that during cell death the Arabidopsis serpin protease inhibitor, AtSerpin1, exhibits a pro-survival function by inhibiting its target pro-death protease, RD21. AtSerpin1 accumulates in the cytoplasm and RD21 accumulates in the vacuole and in endoplasmic reticulum bodies. Elicitors of cell death, including the salicylic acid agonist benzothiadiazole and the fungal toxin oxalic acid, stimulated changes in vacuole permeability as measured by the changes in the distribution of marker dye. Concomitantly, a covalent AtSerpin1-RD21 complex was detected indicative of a change in protease compartmentalization. Furthermore, mutant plants lacking RD21 or plants with AtSerpin1 over-expression exhibited significantly less elicitor-stimulated PCD than plants lacking AtSerpin1. The necrotrophic fungi Botrytis cinerea and Sclerotina sclerotiorum secrete oxalic acid as a toxin that stimulates cell death. Consistent with a pro-death function for RD21 protease, the growth of these necrotrophs was compromised in plants lacking RD21 but accelerated in plants lacking AtSerpin1. The results indicate that AtSerpin1 controls the pro-death function of compartmentalized protease RD21 by determining a set-point for its activity and limiting the damage induced during cell death.

Molecular cloning and functional expression of a water-soluble chlorophyll-binding protein from Japanese wild radish

Hydrophilic chlorophyll (Chl)-binding proteins have been isolated from various Brassicaceae plants and are categorized into Class II water-soluble Chl-binding proteins (WSCPs). Although the molecular properties of class II WSCPs including Brassica-type (e.g., cauliflower WSCP, Brussels sprouts WSCP and BnD22, a drought- and salinity-stress-induced 22 kDa protein of rapeseed), a Lepidium-type, and an Arabidopsis-type WSCPs have been well characterized, those of Raphanus-type WSCPs are poorly understood. To gain insight into the molecular diversity of Class II WSCPs, we cloned a novel cDNA encoding a Raphanus sativus var. raphanistroides (Japanese wild radish called 'Hamadaikon') WSCP (RshWSCP). Sequence analysis revealed that the open reading frame of the RshWSCP gene consisted of 666 bp encoding 222 aa residues, including 23 residues of a deduced signal peptide. Functional recombinant RshWSCP was expressed in Escherichia coli as a hexa-histidine fusion protein (RshWSCP-His). Although the RshWSCP-His was expressed as a soluble protein in E. coli, the apo-protein was highly unstable and tended to aggregate during a series of purification steps. When the soluble fraction of RshWSCP-His-expressing E. coli was mixed immediately with homogenate of spinach leaves containing thylakoid, RshWSCP-His was able to remove Chl molecules from the thylakoid and formed a stable Chl-WSCP complex with high hydrophilicity. UV-visible absorption spectra of the reconstituted RshWSCP-His revealed that RshWSCP-His is one of the Class IIA WSCP with the highest Chl a/b ratio analyzed thus far. A semi-quantitative reverse transcription-polymerase chain reaction analysis revealed that RshWSCP was transcribed in buds and flowers but not in roots, stems and various leaves.

Identification of two novel endoplasmic reticulum body-specific integral membrane proteins

The endoplasmic reticulum (ER) body, a large compartment specific to the Brassicales, accumulates β-glucosidase and possibly plays a role in the defense against pathogens and herbivores. Although the ER body is a subdomain of the ER, it is unclear whether any ER body-specific membrane protein exists. In this study, we identified two integral membrane proteins of the ER body in Arabidopsis (Arabidopsis thaliana) and termed them MEMBRANE PROTEIN OF ENDOPLASMIC RETICULUM BODY1 (MEB1) and MEB2. In Arabidopsis, a basic helix-loop-helix transcription factor, NAI1, and an ER body component, NAI2, regulate ER body formation. The expression profiles of MEB1 and MEB2 are similar to those of NAI1, NAI2, and ER body β-glucosidase PYK10 in Arabidopsis. The expression of MEB1 and MEB2 was reduced in the nai1 mutant, indicating that NAI1 regulates the expression of MEB1 and MEB2 genes. MEB1 and MEB2 proteins localize to the ER body membrane but not to the ER network, suggesting that these proteins are specifically recruited to the ER body membrane. MEB1 and MEB2 physically interacted with ER body component NAI2, and they were diffused throughout the ER network in the nai2 mutant, which has no ER body. Heterologous expression of MEB1 and MEB2 in yeast (Saccharomyces cerevisiae) suppresses iron and manganese toxicity, suggesting that MEB1 and MEB2 are metal transporters. These results indicate that the membrane of ER bodies has specific membrane proteins and suggest that the ER body is involved in defense against metal stress as well as pathogens and herbivores.

Molecular cloning, characterization and analysis of the intracellular localization of a water-soluble Chl-binding protein from Brussels sprouts (Brassica oleracea var. gemmifera)

A water-soluble Chl-binding protein from Brussels sprouts (Brassica oleracea var. gemmifera), hereafter termed BoWSCP, is categorized into the Class II WSCPs (non-photoconvertible WSCPs). Previous studies on BoWSCP have focused mainly on its biochemical characterization. In this study, we cloned the cDNA encoding BoWSCP. Sequence analysis revealed that the BoWSCP gene was composed of a single exon corresponding to 654 bp of an open reading frame encoding 218 amino acid residues, including 19 residues of a deduced signal peptide targeted to the endoplasmic reticulum (ER). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis of native BoWSCP revealed that the molecular mass of the subunit was 19,008.523 Da, corresponding to a mature protein of 178 amino acids, indicating the removal of 21 residues in the C-terminal region. Functional BoWSCP was expressed in Escherichia coli as a hexa-histidine fusion protein (BoWSCP-His). When BoWSCP-His was mixed with thylakoid membranes in aqueous solution, BoWSCP-His was able to remove Chls from the thylakoid membranes. The absorption spectrum of the reconstituted BoWSCP-His was identical to that of the native BoWSCP. Chl binding analyses of BoWSCP-His revealed that the BoWSCP-His bound both Chl a and Chl b with almost the same affinity in 40% methanol solution, although the native BoWSCP had a higher content of Chl a. To reveal the intracellular localization of BoWSCP, we constructed a transgenic plant expressing the fluorescent protein fused with the N-terminal deduced signal peptide of BoWSCP. The fluorescence emitted from the chimeric protein was detected in the ER body, an ER-derived compartment observed only in Brassicaceae plants.