Protein retention and localization in the endoplasmic reticulum and the golgi apparatus

Spatiotemporal and Transcriptional Characterization on Tanshinone Initial Synthesis in Salvia miltiorrhiza Roots

Tanshinones are the bioactive constituents of Danshen (Salvia miltiorrhiza Bunge), which is used in Traditional Chinese Medicine to treat cardiovascular and other diseases, and they synthesize and accumulate in the root periderm of S. miltiorrhiza. However, there is no relevant report on the initial stage of tanshinone synthesis, as well as the root structure and gene expression characteristics. The present study aims to provide new insights into how these bioactive principles begin to synthesize by characterizing possible differences in their biosynthesis and accumulation during early root development from both spatial and temporal aspects. The morphological characteristics and the content of tanshinones in roots of S. miltiorrhiza were investigated in detail by monitoring the seedlings within 65 days after germination (DAGs). The ONT transcriptome sequencing was applied to investigate gene expression patterns. The periderm of the S. miltiorrhiza storage taproot initially synthesized tanshinone on about 30 DAGs. Three critical stages of tanshinone synthesis were preliminarily determined: preparation, the initial synthesis, and the continuous rapid synthesis. The difference of taproots in the first two stages was the smallest, and the differentially expressed genes (DEGs) were mainly enriched in terpene synthesis. Most genes involved in tanshinone synthesis were up regulated during the gradual formation of the red taproot. Plant hormone signal transduction and ABC transport pathways were widely involved in S. miltiorrhiza taproot development. Five candidate genes that may participate in or regulate tanshinone synthesis were screened according to the co-expression pattern. Moreover, photosynthetic ferredoxin (FD), cytochrome P450 reductase (CPR), and CCAAT binding transcription factor (CBF) were predicted to interact with the known downstream essential enzyme genes directly. The above results provide a necessary basis for analyzing the initial synthesis and regulation mechanism of Tanshinones.

Hordein Accumulation in Developing Barley Grains

The temporal pattern of accumulation of hordein storage proteins in developing barley grains was studied by enzyme-linked immunosorbent assay (ELISA), western blot and liquid chromatography tandem mass spectrometry (LC-MS/MS). Hordein accumulation was compared to the pattern seen for two abundant control proteins, serpin Z4 (an early accumulator) and lipid transferase protein (LTP1, a late accumulator). Hordeins were detected from 6 days post-anthesis (DPA) and peaked at 30 DPA. Changes in fresh weight indicate that desiccation begins at 20 DPA and by 37 DPA fresh weight had decreased by 35%. ELISA analysis of hordein content, expressed on a protein basis, increased to a maximum at 30 DPA followed by a 17% decrease by 37 DPA. The accumulation of 39 tryptic and 29 chymotryptic hordein peptides representing all classes of hordein was studied by LC-MS/MS. Most peptides increased to a maximum at 30 DPA, and either remained at the maximum or did not decrease significantly. Only five tryptic peptides, members of the related B1- and γ1-hordeins decreased significantly by 21-51% at 37 DPA. Thus, the concentration of some specific peptides was reduced while remaining members of the same family were not affected. The N-terminal signal region was removed by proteolysis during co-translation. In addition to a suite of previously characterized hordeins, two novel barley B-hordein isoforms mapping to wheat low molecular weight glutenins (LMW-GS-like B-hordeins), and two avenin-like proteins (ALPs) sharing homology with wheat ALPs, were identified. These identified isoforms have not previously been mapped in the barley genome. Cereal storage proteins provide significant nutritional content for human consumption and seed germination. In barley, the bulk of the storage proteins comprise the hordein family and the final hordein concentration affects the quality of baked and brewed products. It is therefore important to study the accumulation of hordeins as this knowledge may assist plant breeding for improved health outcomes (by minimizing triggering of detrimental immune responses), nutrition and food processing properties.

Prediction of endoplasmic reticulum resident proteins using fragmented amino acid composition and support vector machine

Background

The endoplasmic reticulum plays an important role in many cellular processes, which includes protein synthesis, folding and post-translational processing of newly synthesized proteins. It is also the site for quality control of misfolded proteins and entry point of extracellular proteins to the secretory pathway. Hence at any given point of time, endoplasmic reticulum contains two different cohorts of proteins, (i) proteins involved in endoplasmic reticulum-specific function, which reside in the lumen of the endoplasmic reticulum, called as endoplasmic reticulum resident proteins and (ii) proteins which are in process of moving to the extracellular space. Thus, endoplasmic reticulum resident proteins must somehow be distinguished from newly synthesized secretory proteins, which pass through the endoplasmic reticulum on their way out of the cell. Approximately only 50% of the proteins used in this study as training data had endoplasmic reticulum retention signal, which shows that these signals are not essentially present in all endoplasmic reticulum resident proteins. This also strongly indicates the role of additional factors in retention of endoplasmic reticulum-specific proteins inside the endoplasmic reticulum.

Methods

This is a support vector machine based method, where we had used different forms of protein features as inputs for support vector machine to develop the prediction models. During training leave-one-out approach of cross-validation was used. Maximum performance was obtained with a combination of amino acid compositions of different part of proteins.

Results

In this study, we have reported a novel support vector machine based method for predicting endoplasmic reticulum resident proteins, named as ERPred. During training we achieved a maximum accuracy of 81.42% with leave-one-out approach of cross-validation. When evaluated on independent dataset, ERPred did prediction with sensitivity of 72.31% and specificity of 83.69%. We have also annotated six different proteomes to predict the candidate endoplasmic reticulum resident proteins in them. A webserver, ERPred, was developed to make the method available to the scientific community, which can be accessed at http://proteininformatics.org/mkumar/erpred/index.html.

Discussion

We found that out of 124 proteins of the training dataset, only 66 proteins had endoplasmic reticulum retention signals, which shows that these signals are not an absolute necessity for endoplasmic reticulum resident proteins to remain inside the endoplasmic reticulum. This observation also strongly indicates the role of additional factors in retention of proteins inside the endoplasmic reticulum. Our proposed predictor, ERPred, is a signal independent tool. It is tuned for the prediction of endoplasmic reticulum resident proteins, even if the query protein does not contain specific ER-retention signal.

Enhancement of α-linolenic acid content in transgenic tobacco seeds by targeting a plastidial ω-3 fatty acid desaturase (fad7) gene of Sesamum indicum to ER

KEY MESSAGE

Expression of sesame plastidial FAD7 desaturase modified with the endoplasmic reticulum targeting and retention signals, enhances the α-linolenic acid accumulation in seeds of Nicotiana tabacum. In plants, plastidial ω-3 fatty acid desaturase-7 (FAD7) catalyzes the formation of C16 and C18 trienoic fatty acids using organellar glycerolipids and participate in the membrane lipid formation. The plastidial ω-3 desaturases (FAD7) share high sequence homology with the microsomal ω-3 desaturases (FAD3) at the amino acid level except the N-terminal organelle transit peptide. In the present study, the predicted N-terminal plastidial signal peptide of fad7 gene was replaced by the endoplasmic reticulum signal peptide and an endoplasmic reticulum retention signal was placed at the C-terminal. The expression of the modified sesame ω-3 desaturase increases the α-linolenic acid content in the range of 4.78-6.77 % in the seeds of transgenic tobacco plants with concomitant decrease in linoleic acid content. The results suggested the potential of the engineered plastidial ω-3 desaturase from sesame to influence the profile of α-linolenic acid in tobacco plant by shifting the carbon flux from linoleic acid, and thus it can be used in suitable genetic engineering strategy to increase the α-linolenic acid content in sesame and other vegetable oils.

A novel cytoplasmic tail motif regulates mouse corin expression on the cell surface

Type II transmembrane serine proteases (TTSPs) are important in many biological processes. Cell surface expression is critical for TTSP activation and function. To date, the mechanism underlying TTSP cell surface expression is poorly understood. Corin is a TTSP and acts as the pro-atrial natriuretic peptide convertase that is essential for sodium homeostasis and normal blood pressure. In this study, we investigated how cytoplasmic tail sequences may regulate corin expression and activation on the cell surface. By site-directed mutagenesis, we made mouse corin proteins with truncations or point-mutations in the cytoplasmic tail. We expressed the mutants in transfected HEK293 cells and analyzed corin cell surface expression and activation by Western blotting and flow cytometry. We found that corin truncation mutants lacking a Lys-Phe-Gln sequence at residues 71-73 had higher levels of cell surface expression and activation compared with that in wild-type corin. When Lys-71, Phe-72 and Gln-73 residues were mutated together, but not individually, in corin with the full-length cytoplasmic tail, increased levels of cell surface expression and zymogen activation were also observed. These results indicate that residues Lys-71, Phe-72 and Gln-73 serve as a novel retention motif in the intracellular pathway to regulate corin cell surface expression and activation.

LocSigDB: a database of protein localization signals

LocSigDB (http://genome.unmc.edu/LocSigDB/) is a manually curated database of experimental protein localization signals for eight distinct subcellular locations; primarily in a eukaryotic cell with brief coverage of bacterial proteins. Proteins must be localized at their appropriate subcellular compartment to perform their desired function. Mislocalization of proteins to unintended locations is a causative factor for many human diseases; therefore, collection of known sorting signals will help support many important areas of biomedical research. By performing an extensive literature study, we compiled a collection of 533 experimentally determined localization signals, along with the proteins that harbor such signals. Each signal in the LocSigDB is annotated with its localization, source, PubMed references and is linked to the proteins in UniProt database along with the organism information that contain the same amino acid pattern as the given signal. From LocSigDB webserver, users can download the whole database or browse/search for data using an intuitive query interface. To date, LocSigDB is the most comprehensive compendium of protein localization signals for eight distinct subcellular locations.

Database URL: http://genome.unmc.edu/LocSigDB/

Expression of recombinant vaccines and antibodies in plants

Plants are able to perform post-translational maturations of therapeutic proteins required for their functional biological activity and suitable in vivo pharmacokinetics. Plants can be a low-cost, large-scale production platform of recombinant biopharmaceutical proteins such as vaccines and antibodies. Plants, however, lack mechanisms of processing authentic human N-glycosylation, which imposes a major limitation in their use as an expression system for therapeutic glycoproducts. Efforts have been made to circumvent plant-specific N-glycosylation, as well as to supplement the plant's endogenous system with human glycosyltransferases for non-immunogenic and humanized N-glycan production. Herein we review studies on the potential of plants to serve as production systems for therapeutic and prophylactic biopharmaceuticals. We have especially focused on recombinant vaccines and antibodies and new expression strategies to overcome the existing problems associated with their production in plants.

Roles for the sarco-/endoplasmic reticulum in cardiac myocyte contraction, protein synthesis, and protein quality control

Although the function of the sarcoplasmic/endoplasmic reticulum (SR/ER) in cardiac contractile calcium handling is well established, its roles in protein synthesis, folding, and quality control in cardiac myocytes are not as clear. This review explores evidence suggesting that, in cardiac myocytes, protein synthesis and contractile calcium handling may be physically and functionally integrated.

Molecular characterization of the fatty alcohol oxidation pathway for wax-ester mobilization in germinated jojoba seeds

Jojoba (Simmondsia chinensis) is the only plant species known to use liquid wax esters (WEs) as a primary seed storage reserve. Upon germination, WE hydrolysis releases very-long-chain fatty alcohols, which must be oxidized to fatty acids by the sequential action of a fatty alcohol oxidase (FAO) and a fatty aldehyde dehydrogenase (FADH) before they can be β-oxidized. Here, we describe the cloning and characterization of genes for each of these two activities. Jojoba FAO and FADH are 52% and 68% identical to Arabidopsis (Arabidopsis thaliana) FAO3 and ALDH3H1, respectively. The genes are expressed most strongly in the cotyledons of jojoba seedlings following germination, but transcripts can also be detected in vegetative tissues. Proteomic analysis indicated that the FAO and FADH proteins can be detected on wax bodies, but they localized to the endoplasmic reticulum when they were expressed as amino-terminal green fluorescent protein fusions in tobacco (Nicotiana tabacum) leaves. Recombinant jojoba FAO and FADH proteins are active on very-long-chain fatty alcohol and fatty aldehyde substrates, respectively, and have biochemical properties consistent with those previously reported in jojoba cotyledons. Coexpression of jojoba FAO and FADH in Arabidopsis enhanced the in vivo rate of fatty alcohol oxidation more than 4-fold. Taken together, our data suggest that jojoba FAO and FADH constitute the very-long-chain fatty alcohol oxidation pathway that is likely to be necessary for efficient WE mobilization following seed germination.

Localization of rabies virus glycoprotein into the endoplasmic reticulum produces immunoprotective antigen

Rabies virus surface glycoprotein (rabies G-protein) with (G+RS) and without (G-RS) endoplasmic reticulum retrieval signal was expressed and characterized in tobacco plants. Transgenically expressed rabies G-protein was estimated at 0.015-0.38 % of total leaf protein. The relative migration of the rabies G-protein on SDS-PAGE was at the position, as anticipated for the viral coat protein (~66 kDa). Immunolocalization by confocal microscopy established that immunoprotective G+RS expressed in tobacco was primarily confined to ER. G+RS showed binding to Con A lectin and was susceptible to N-glycosidase F activity similar to native rabies G-protein. However, the G-RS transgenically expressed in tobacco leaves was glycosylated differently and was resitant to N-glycosidase F. Immunological studies and Rapid Fluorescent Foci Inhibition Test (RFFIT) showed that G+RS was immunogenic and immunoprotective, whereas G-RS was moderately immunogenic but non-protective against live virus challenge. Hence, plants can express the antigenic component of rabies virus with suitable glycosylation, which is important to give protection against rabies virus infection.

The transmembrane domain of the molecular chaperone Cosmc directs its localization to the endoplasmic reticulum

The molecular basis for retention of integral membrane proteins in the endoplasmic reticulum (ER) is not well understood. We recently discovered a novel ER molecular chaperone termed Cosmc, which is essential for folding and normal activity of the Golgi enzyme T-synthase. Cosmc, a type II single-pass transmembrane protein, lacks any known ER retrieval/retention motifs. To explore specific ER localization determinants in Cosmc we generated a series of Cosmc mutants along with chimeras of Cosmc with a non-ER resident type II protein, the human transferrin receptor. Here we show that the 18 amino acid transmembrane domain (TMD) of Cosmc is essential for ER localization and confers ER retention to select chimeras. Moreover, mutations of a single Cys residue within the TMD of Cosmc prevent formation of disulfide-bonded dimers of Cosmc and eliminate ER retention. These studies reveal that Cosmc has a unique ER-retention motif within its TMD and provide new insights into the molecular mechanisms by which TMDs of resident ER proteins contribute to ER localization.

The sunflower plastidial omega3-fatty acid desaturase (HaFAD7) contains the signalling determinants required for targeting to, and retention in, the endoplasmic reticulum membrane in yeast but requires co-expressed ferredoxin for activity

Although plant plastidial omega3-desaturases are closely related to microsomal desaturases, heterologous expression in yeast of the Helianthus annuus FAD7 omega3-desaturase showed low activity in contrast to similar expression of microsomal FAD3 omega3-desaturases. However, the removal of the plastidial transit peptide and the incorporation of a KKNL motif to the C-terminus of HaFAD7 increased the activity by 10-fold compared to the native protein. N-terminal fusion of transmembrane-domains from either the yeast microsomal ELO3, (a type III signal anchor domain), or FAE1, an endoplasmic reticulum membrane anchoring domain, resulted in moderate increases in enzyme activity (5- and 7-fold, respectively), suggesting that the first, most hydrophobic transmembrane domain of HaFAD7 is sufficient to direct targeting to, and insertion into, the endoplasmic reticulum membrane. Furthermore, fusing a hemagglutinin (HA) epitope tag upstream of an endogenous C-terminal KEK motif resulted in a significant loss of activity compared to the un-tagged construct, indicating that the endogenous KEK C-terminal di-lysine motif is capable of directing in yeast the ER-retention of this normally plastidial-located protein. Western blotting analysis of constructs with internal HA epitope revealed that in whole cell extracts, with the exception of the one bound to C-terminal, it did not display a reduced level of protein accumulation. Whilst ferredoxin was shown to be required for HaFAD7 activity in yeast, it appears not necessary for protein stability and accumulation of this plastidial desaturase in the endoplasmic reticulum.

Cytosolic N-terminal arginine-based signals together with a luminal signal target a type II membrane protein to the plant ER

BACKGROUND

In eukaryotic cells, the membrane compartments that constitute the exocytic pathway are traversed by a constant flow of lipids and proteins. This is particularly true for the endoplasmic reticulum (ER), the main "gateway of the secretory pathway", where biosynthesis of sterols, lipids, membrane-bound and soluble proteins, and glycoproteins occurs. Maintenance of the resident proteins in this compartment implies they have to be distinguished from the secretory cargo. To this end, they must possess specific ER localization determinants to prevent their exit from the ER, and/or to interact with receptors responsible for their retrieval from the Golgi apparatus. Very few information is available about the signal(s) involved in the retention of membrane type II protein in the ER but it is generally accepted that sorting of ER type II cargo membrane proteins depends on motifs mainly located in their cytosolic tails.

RESULTS

Here, using Arabidopsis glucosidase I as a model, we have identified two types of signals sufficient for the location of a type II membrane protein in the ER. A first signal is located in the luminal domain, while a second signal corresponds to a short amino acid sequence located in the cytosolic tail of the membrane protein. The cytosolic tail contains at its N-terminal end four arginine residues constitutive of three di-arginine motifs (RR, RXR or RXXR) independently sufficient to confer ER localization. Interestingly, when only one di-arginine motif is present, fusion proteins are located both in the ER and in mobile punctate structures, distinct but close to Golgi bodies. Soluble and membrane ER protein markers are excluded from these punctate structures, which also do not colocalize with an ER-exit-site marker. It is hypothesized they correspond to sites involved in Golgi to ER retrotransport.

CONCLUSION

Altogether, these results clearly show that cytosolic and luminal signals responsible for ER retention could coexist in a same type II membrane protein. These data also suggest that both retrieval and retention mechanisms govern protein residency in the ER membrane. We hypothesized that mobile punctate structures not yet described at the ER/Golgi interface and tentatively named GERES, could be involved in retrieval mechanisms from the Golgi to the ER.

Arabidopsis thaliana GPAT8 and GPAT9 are localized to the ER and possess distinct ER retrieval signals: functional divergence of the dilysine ER retrieval motif in plant cells

Glycerol-3-phosphate acyltransferase (GPAT; EC 2.3.1.15) catalyzes the committed step in the production of glycerolipids, which are major components of cellular membranes, seed storage oils, and epicuticular wax coatings. While the biochemical activities of GPATs have been characterized in detail, the cellular features of these enzymes are only beginning to emerge. Here we characterized the phylogenetic relationships and cellular properties of two GPAT enzymes from the relatively large Arabidopsis thaliana GPAT family, including GPAT8, which is involved in cutin biosynthesis, and GPAT9, which is a new putative GPAT that has extensive homology with a GPAT from mammalian cells involved in storage oil formation and, thus, may have a similar role in plants. Immunofluorescence microscopy of transiently-expressed myc-epitope-tagged GPAT8 and GPAT9 revealed that both proteins were localized to the endoplasmic reticulum (ER), and differential permeabilization experiments indicated that their N- and C-termini were oriented towards the cytosol. However, these two proteins contained distinct types of ER retrieval signals, with GPAT8 possessing a divergent type of dilysine motif (-KK-COOH rather than the prototypic -KKXX-COOH or -KXKXX-COOH motif) and GPAT9 possessing a hydrophobic pentapeptide motif (-phi-X-X-K/R/D/E-phi-; where phi are large hydrophobic amino acid residues). Notably, the divergent dilysine motif in GPAT8 only functioned effectively when additional upstream residues were included to provide the proper protein context. Extensive mutational analyses of the divergent dilysine motif, based upon sequences present in the C-termini of other GPAT8s from various plant species, further expanded the functional definition of this molecular targeting signal, thereby providing insight to the targeting signals in other GPAT family members as well as other ER-resident membrane proteins within plant cells.

Regulation of protein O-glycosylation by the endoplasmic reticulum-localized molecular chaperone Cosmc

Regulatory pathways for protein glycosylation are poorly understood, but expression of branchpoint enzymes is critical. A key branchpoint enzyme is the T-synthase, which directs synthesis of the common core 1 O-glycan structure (T-antigen), the precursor structure for most mucin-type O-glycans in a wide variety of glycoproteins. Formation of active T-synthase, which resides in the Golgi apparatus, requires a unique molecular chaperone, Cosmc, encoded on Xq24. Cosmc is the only molecular chaperone known to be lost through somatic acquired mutations in cells. We show that Cosmc is an endoplasmic reticulum (ER)–localized adenosine triphosphate binding chaperone that binds directly to human T-synthase. Cosmc prevents the aggregation and ubiquitin-mediated degradation of the T-synthase. These results demonstrate that Cosmc is a molecular chaperone in the ER required for this branchpoint glycosyltransferase function and show that expression of the disease-related Tn antigen can result from deregulation or loss of Cosmc function.

Identification of a novel group of putative Arabidopsis thaliana beta-(1,3)-galactosyltransferases

To begin biochemical and molecular studies on the biosynthesis of the type II arabinogalactan chains on arabinogalactan-proteins (AGPs), we adopted a bioinformatic approach to identify and systematically characterise the putative galactosyltransferases (GalTs) responsible for synthesizing the beta-(1,3)-Gal linkage from CAZy GT-family-31 from Arabidopsis thaliana. These analyses confirmed that 20 members of the GT-31 family contained domains/motifs typical of biochemically characterised beta-(1,3)-GTs from mammalian systems. Microarray data confirm that members of this family are expressed throughout all tissues making them likely candidates for the assembly of the ubiquitously found AGPs. One member, At1g77810, was selected for further analysis including location studies that confirmed its presence in the Golgi and preliminary enzyme substrate specificity studies that demonstrated beta-(1,3)-GalT activity. This bioinformatic/molecular study of CAZy GT-family-31 was validated by the recent report of Strasser et al. (Plant Cell 19:2278-2292, 2007) that another member of this family (At1g26810; GALT1) encodes a beta-(1,3)-GalT involved in the biosynthesis of the Lewis a epitope of N-glycans in Arabidopsis thaliana.

Pharming and transgenic plants

Plant represented the essence of pharmacopoeia until the beginning of the 19th century when plant-derived pharmaceuticals were partly supplanted by drugs produced by the industrial methods of chemical synthesis. In the last decades, genetic engineering has offered an alternative to chemical synthesis, using bacteria, yeasts and animal cells as factories for the production of therapeutic proteins. More recently, molecular farming has rapidly pushed towards plants among the major players in recombinant protein production systems. Indeed, therapeutic protein production is safe and extremely cost-effective in plants. Unlike microbial fermentation, plants are capable of carrying out post-translational modifications and, unlike production systems based on mammalian cell cultures, plants are devoid of human infective viruses and prions. Furthermore, a large panel of strategies and new plant expression systems are currently developed to improve the plant-made pharmaceutical's yields and quality. Recent advances in the control of post-translational maturations in transgenic plants will allow them, in the near future, to perform human-like maturations on recombinant proteins and, hence, make plant expression systems suitable alternatives to animal cell factories.

Calcium-dependent enhancement of transcription of p300 by human T-lymphotropic type 1 p12I

Human T-lymphotropic virus type 1 (HTLV-1) p12I localizes to the endoplasmic reticulum and Golgi causing sustained release of calcium, T cell activation, and enhanced expression of several calcium-regulated genes. In recent microarray studies, p300 mRNA was increased in T cells expressing p12I. The co-activator p300 is a key regulator of cellular and viral transcription; however, factors that influence its transcriptional regulation are less well studied. We hypothesized that the transcription of p300 is calcium dependent and that sustained low magnitude increases in intracellular calcium may enhance the transcription of p300. Herein, we report enhanced expression of p300 in T cells by p12I in a calcium-dependent, but calcineurin-independent manner. Sustained low magnitude calcium release induced by ionomycin in T cells was sufficient to increased mRNA and protein levels of p300 resulting in enhanced transcription from a p300-dependent promoter. Promoter analysis of the p300 gene was used to predict calcium-responsive transcription factor binding sites. Using mutant forms of p12I, we demonstrate that ER localization of the viral protein is required to increase p300. In addition, p12I reversed the repression of HTLV-1 LTR-driven transcription by HTLV-1 p30II, a p300-binding protein. HTLV-1 p12I-mediated enhancement of p300 expression represents a novel mechanism of regulation of cellular gene expression by viral proteins. By targeting a ubiquitous second messenger such as calcium, HTLV-1 p12I may regulate the expression of the cellular transcriptional co-activator p300 to modulate viral gene expression and promote lymphocyte survival.

BnNHL18A shows a localization change by stress-inducing chemical treatments

The two genes, named BnNHL18A and BnNHL18B, showing sequence homology with Arabidopsis NDR1/HIN1-like (NHL) genes, were isolated from cDNA library prepared with oilseed rape (Brassica napus) seedlings treated with NaCl. The transcript level of BnNHL18A was increased by sodium chloride, ethephon, hydrogen peroxide, methyl jasmonate, or salicylic acid treatment. The coding regions of BnNHL18A and BnNHL18B contain a sarcolipin (SLN)-like sequence. Analysis of the localization of smGFP fusion proteins showed that BnNHL18A is mainly localized to endoplasmic reticulum (ER). This result suggests that the SLN-like sequence plays a role in retaining proteins in ER membrane in plants. In response to NaCl, hydrogen peroxide, ethephon, and salicylic acid treatments, the protein localization of BnNHL18A was changed. Our findings suggest a common function of BnNHL18A in biotic and abiotic stresses, and demonstrate the presence of the shared mechanism of protein translocalization between the responses to plant pathogen and to osmotic stress.

Biopharmaceutical production in plants: problems, solutions and opportunities

There are major structural differences between plant and mammalian N-linked glycans, with those from plants being immunogenic in most laboratory mammals and eliciting glycan-specific IgE and IgG antibodies in humans, when delivered parenterally. However, because humans are constantly exposed to plant glycoproteins in the diet, glycosylated plant-made pharmaceuticals (PMPs) should be acceptable for topical and oral administration. To exploit fully the potential that plants offer for the production of therapeutic proteins for parenteral administration, it might be necessary to inhibit plant-specific post-translational modifications to obtain "humanized" non-immunogenic N-glycans on PMPs. The benefits that could accrue are lower manufacturing costs, relative to mammalian cell culture, and a reduced risk of transmission of mammalian pathogens.

Modulation of Secretory Granule-targeting Efficiency by Cis and Trans Compounding of Sorting Signals

Several protein domains acting through seemingly different mechanisms have been reported to have the capacity to target proteins to dense core secretory granules. Because proteins enter secretory granules with different efficiencies and because some of these proteins contain more than one granule-targeting motif, we have investigated whether compounding sorting signals could alter the efficiency of protein entry into secretory granules. In the current study we demonstrate that a paired basic cleavage site from human prorenin and an alpha-helix-containing secretory granule-sorting signal from the prohormone convertase PC1/3 can synergize to increase granule-sorting efficiency not only when located on the same protein, but also when located on distinct proteins that associate in the secretory pathway.

Sarcolipin retention in the endoplasmic reticulum depends on its C-terminal RSYQY sequence and its interaction with sarco(endo)plasmic Ca(2+)-ATPases

Sarcolipin (SLN) and phospholamban (PLN) are effective inhibitors of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA). These homologous proteins differ at their N and C termini: the C-terminal Met-Leu-Leu in PLN is replaced by Arg-Ser-Tyr-Gln-Tyr in SLN. The role of the C-terminal sequence of SLN tagged N-terminally with the FLAG epitope (NF-SLN) in endoplasmic reticulum (ER) retention was investigated by transfecting human embryonic kidney-293 cells with cDNAs encoding NF-SLN or a series of NF-SLN mutants in which C-terminal amino acids were deleted progressively. Immunofluorescence and immunoblotting of transfected cells by using anti-FLAG antibodies indicated that NF-SLN and PLN tagged at its N terminus with the FLAG epitope, even when overexpressed, were restricted to the ER. However, C-terminal truncation deletions of SLN, which lacked RSYQY, were not localized to ER and did not inhibit Ca(2+)-dependent Ca2+ uptake by SERCA. The shortest deletion constructs, NF-SLN 1-22 and NF-SLN 1-23, did not express stable protein products. However, all NF-SLN cDNA constructs, including NF-SLN 1-22 and NF-SLN 1-23, were expressed stably and localized to the ER when they were coexpressed with SERCA2a. These results show that NF-SLN subcellular distribution depends on SERCA coexpression and on its luminal, C-terminal RSYQY sequence. By using immunoprecipitation and MS, glucose-regulated protein 78/BiP and glucose-regulated protein 94 were identified as proteins that interact with NF-SLN through the RSYQY sequence. Thus, in the absence of SERCA, retention of NF-SLN in the ER is mediated through its association with other components through the C-terminal RSYQY sequence.

Regulating cell surface glycosylation by small molecule control of enzyme localization

Cell surface carbohydrates mediate interactions between the cell and its environment. Glycosyltransferases responsible for synthesis of cell surface oligosaccharides are therefore essential administrators of cellular communication. These enzymes often comprise large families. Redundancy of related family members and embryonic lethality both complicate genetic methods for deconvoluting functions of glycosyltransferases. We report a chemical method in which the activity of an individual glycosyltransferase is controlled by a small molecule. The approach exploits the requirement of Golgi localization, a common feature of glycosyltransferase superfamily members. In our approach, the glycosyltransferase is separated into two domains, one that determines localization and one responsible for catalysis. Control of enzyme activity is achieved using a small molecule to regulate association of the two domains. We used this method to regulate production of sialyl Lewis x by alpha1,3-fucosyltransferase VII in living cells.

Production and glycosylation of plant-made pharmaceuticals: the antibodies as a challenge

Antibodies have long been recognized for their diagnostic and therapeutic potential. The rapidly increasing number of monoclonal antibodies approved for immunotherapy has paved the way to an even greater demand for these molecules. In order to satisfy this growing demand and to increase the production capacity, alternative systems based on antibody production in transgenic organisms are being actively explored. In this paper, we focus on transgenic plants as a promising system for the scale-up and processing of plant-made pharmaceuticals. In particular, we point out the advantages and limitations induced by glycosylation of plant-made antibodies for human therapy.

Membrane-bound fatty acid desaturases are inserted co-translationally into the ER and contain different ER retrieval motifs at their carboxy termini

Fatty acid desaturases (FADs) play a prominent role in plant lipid metabolism and are located in various subcellular compartments, including the endoplasmic reticulum (ER). To investigate the biogenesis of ER-localized membrane-bound FADs, we characterized the mechanisms responsible for insertion of Arabidopsis FAD2 and Brassica FAD3 into ER membranes and determined the molecular signals that maintain their ER residency. Using in vitro transcription/translation reactions with ER-derived microsomes, we show that both FAD2 and FAD3 are efficiently integrated into membranes by a co-translational, translocon-mediated pathway. We also demonstrate that while the C-terminus of FAD3 (-KSKIN) contains a functional prototypic dilysine ER retrieval motif, FAD2 contains a novel C-terminal aromatic amino acid-containing sequence (-YNNKL) that is both necessary and sufficient for maintaining localization in the ER. Co-expression of a membrane-bound reporter protein containing the FAD2 C-terminus with a dominant-negative mutant of ADP-ribosylation factor (Arf)1 abolished transient localization of the reporter protein in the Golgi, indicating that the FAD2 peptide signal acts as an ER retrieval motif. Mutational analysis of the FAD2 ER retrieval signal revealed a sequence-specific motif consisting of Phi-X-X-K/R/D/E-Phi-COOH, where -Phi- are large hydrophobic amino acid residues. Interestingly, this aromatic motif was present in a variety of other known and putative ER membrane proteins, including cytochrome P450 and the peroxisomal biogenesis factor Pex10p. Taken together, these data describe the insertion and retrieval mechanisms of FADs and define a new ER localization signal in plants that is responsible for the retrieval of escaped membrane proteins back to the ER.

Phylogenetic analyses and expression studies reveal two distinct groups of calreticulin isoforms in higher plants

Calreticulin (CRT) is a multifunctional protein mainly localized to the endoplasmic reticulum in eukaryotic cells. Here, we present the first analysis, to our knowledge, of evolutionary diversity and expression profiling among different plant CRT isoforms. Phylogenetic studies and expression analysis show that higher plants contain two distinct groups of CRTs: a CRT1/CRT2 group and a CRT3 group. To corroborate the existence of these isoform groups, we cloned a putative CRT3 ortholog from Brassica rapa. The CRT3 gene appears to be most closely related to the ancestral CRT gene in higher plants. Distinct tissue-dependent expression patterns and stress-related regulation were observed for the isoform groups. Furthermore, analysis of posttranslational modifications revealed differences in the glycosylation status among members within the CRT1/CRT2 isoform group. Based on evolutionary relationship, a new nomenclature for plant CRTs is suggested. The presence of two distinct CRT isoform groups, with distinct expression patterns and posttranslational modifications, supports functional specificity among plant CRTs and could account for the multiple functional roles assigned to CRTs.

Human T-cell lymphotropic virus type 1 p12I enhances interleukin-2 production during T-cell activation

Human T-cell lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATLL) and a variety of lymphoproliferative disorders. The early virus-cell interactions that determine a productive infection remain unclear. However, it is well recognized that T-cell activation is required for effective retroviral integration into the host cell genome and subsequent viral replication. The HTLV-1 pX open reading frame I encoding protein, p12(I), is critical for the virus to establish persistent infection in vivo and for infection in quiescent primary lymphocytes in vitro. p12(I) localizes in the endoplasmic reticulum (ER) and cis-Golgi apparatus, increases intracellular calcium and activates nuclear factor of activated T cells (NFAT)-mediated transcription. To clarify the function of p12(I), we tested the production of IL-2 from Jurkat T cells and peripheral blood mononuclear cells (PBMC) expressing p12(I). Lentiviral vector expressed p12(I) in Jurkat T cells enhanced interleukin-2 (IL-2) production in a calcium pathway-dependent manner during T-cell receptor (TCR) stimulation. Expression of p12(I) also induced higher NFAT-mediated reporter gene activities during TCR stimulation in Jurkat T cells. In contrast, p12 expression in PBMC elicited increased IL-2 production in the presence of phorbal ester stimulation, but not during TCR stimulation. Finally, the requirement of ER localization for p12(I)-mediated NFAT activation was demonstrated and two positive regions and two negative regions in p12(I) were identified for the activation of this transcription factor by using p12(I) truncation mutants. These results are the first to indicate that HTLV-1, an etiologic agent associated with lymphoproliferative diseases, uses a conserved accessory protein to induce T-cell activation, an antecedent to efficient viral infection.

Structure-function analysis of herpes simplex virus type 1 gD and gH-gL: clues from gDgH chimeras

In alphaherpesviruses, glycoprotein B (gB), gD, gH, and gL are essential for virus entry. A replication-competent gL-null pseudorabies virus (PrV) (B. G. Klupp and T. C. Mettenleiter, J. Virol. 73:3014-3022, 1999) was shown to express a gDgH hybrid protein that could replace gD, gH, and gL in cell-cell fusion and null virus complementation assays. To study this phenomenon in herpes simplex virus type 1 (HSV-1), we constructed four gDgH chimeras, joining the first 308 gD amino acids to various gH N-terminal truncations. The chimeras were named for the first amino acid of gH at which each was truncated: 22, 259, 388, and 432. All chimeras were immunoprecipitated with both gD and gH antibodies to conformational epitopes. Normally, transport of gH to the cell surface requires gH-gL complex formation. Chimera 22 contains full-length gH fused to gD308. Unlike PrV gDgH, chimera 22 required gL for transport to the surface of transfected Vero cells. Interestingly, although chimera 259 failed to reach the cell surface, chimeras 388 and 432 exhibited gL-independent transport. To examine gD and gH domain function, each chimera was tested in cell-cell fusion and null virus complementation assays. Unlike PrV gDgH, none of the HSV-1 chimeras substituted for gL for fusion. Only chimera 22 was able to replace gH for fusion and could also replace either gH or gD in the complementation assay. Surprisingly, this chimera performed very poorly as a substitute for gD in the fusion assay despite its ability to complement gD-null virus and bind HSV entry receptors (HveA and nectin-1). Chimeras 388 and 432, which contain the same portion of gD as that in chimera 22, substituted for gD for fusion at 25 to 50% of wild-type levels. However, these chimeras functioned poorly in gD-null virus complementation assays. The results highlight the fact that these two functional assays are measuring two related but distinct processes.

Protein transport into secondary plastids and the evolution of primary and secondary plastids

Chloroplasts are key organelles in algae and plants due to their photosynthetic abilities. They are thought to have evolved from prokaryotic cyanobacteria taken up by a eukaryotic host cell in a process termed primary endocytobiosis. In addition, a variety of organisms have evolved by subsequent secondary endocytobioses, in which a heterotrophic host cell engulfed a eukaryotic alga. Both processes dramatically enhanced the complexity of the resulting cells. Since the first version of the endosymbiotic theory was proposed more than 100 years ago, morphological, physiological, biochemical, and molecular data have been collected substantiating the emerging picture about the origin and the relationship of individual organisms with different primary or secondary chloroplast types. Depending on their origin, plastids in different lineages may have two, three, or four envelope membranes. The evolutionary success of endocytobioses depends, among other factors, on the specific exchange of molecules between the host and endosymbiont. This raises questions concerning how targeting of nucleus-encoded proteins into the different plastid types occurs and how these processes may have developed. Most studies of protein translocation into plastids have been performed on primary plastids, but in recent years more complex protein-translocation systems of secondary plastids have been investigated. Analyses of transport systems in different algal lineages with secondary plastids reveal that during evolution existing translocation machineries were recycled or recombined rather than being developed de novo. This review deals with current knowledge about the evolution and function of primary and secondary plastids and the respective protein-targeting systems.

ER retention may play a role in sorting of the nuclear pore membrane protein POM121

Integral membrane proteins of the nuclear envelope (NE) are synthesized on the rough endoplasmic reticulum (ER) and following free diffusion in the continuous ER/NE membrane system are targeted to their proper destinations due to interactions of specific domains with other components of the NE. By studying the intracellular distribution and dynamics of a deletion mutant of an integral membrane protein of the nuclear pores, POM121, which lacks the pore-targeting domain, we investigated if ER retention plays a role in sorting of integral membrane proteins to the nuclear envelope. A nascent membrane protein lacking sorting determinants is believed to diffuse laterally in the continuous ER/NE lipid bilayer and expected to follow vesicular traffic to the plasma membrane. The GFP-tagged deletion mutant, POM121(1-129)-GFP, specifically distributed within the ER membrane, but was completely absent from the Golgi compartment and the plasma membrane. Experiments using fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) demonstrated that despite having very high mobility within the whole ER network (D = 0.41 +/- 0.11 micro m(2)/s) POM121(1-129)-GFP was unable to exit the ER. It was also not detected in post-ER compartments of cells incubated at 15 degrees C. Taken together, these experiments show that amino acids 1-129 of POM121 are able to retain GFP in the ER membrane and suggest that this retention occurs by a direct mechanism rather than by a retrieval mechanism. Our data suggest that ER retention might be important for sorting of POM121 to the nuclear pores.

Cloning, expression, and functional characterization of a Ca(2+)-dependent endoplasmic reticulum nucleoside diphosphatase

We have isolated and characterized the cDNA encoding a Ca(2+)-dependent nucleoside diphosphatase (EC ) related to two secreted ATP- and ADP-hydrolyzing apyrases of the bloodsucking insects, Cimex lectularius and Phlebotomus papatasi. The rat brain-derived cDNA has an open reading frame of 1209 bp encoding a protein of 403 amino acids and a calculated molecular mass of 45.7 kDa. The mRNA was expressed in all tissues investigated, revealing two major transcripts with varying preponderance. The immunohistochemical analysis of the Myc-His-tagged enzyme expressed in Chinese hamster ovary cells revealed its association with the endoplasmic reticulum and also with pre-Golgi intermediates. Ca(2+)-dependent nucleoside diphosphatase is a membrane protein with its catalytic site facing the organelle lumen. It hydrolyzes nucleoside 5'-diphosphates in the order UDP >GDP = IDP >>>CDP but not ADP. Nucleoside 5'-triphosphates were hydrolyzed to a minor extent, and no hydrolysis of nucleoside 5'-monophosphates was observed. The enzyme was strongly activated by Ca(2+), insensitive to Mg(2+), and had a K(m) for UDP of 216 microm. Ca(2+)-dependent nucleoside diphosphatase may support glycosylation reactions related to quality control in the endoplasmic reticulum.

A nuclear localization signal and a membrane association domain contribute to the cellular localization of the Tobacco mosaic virus 126-kDa replicase protein

A transient expression system using onion epidermal cells was used to investigate domains of the Tobacco mosaic virus (TMV) 126-kDa replicase protein involved in cellular localization. Initially, a nuclear localization signal (NLS), identified within the amino-terminus of the 126-kDa protein, was investigated for its functionality using fusion constructs containing the green fluorescent protein (GFP). Fusion of the amino-terminal 70 amino acids of the 126-kDa protein, containing the NLS, to a beta-glucuronidase-GFP open reading frame (ORF), directed the accumulation of fluorescence to the nucleus. In contrast, similar constructs lacking the NLS or containing a mutated NLS sequence failed to accumulate within the nucleus. Additional investigations using GFP fusion constructs containing the first 178 or 388 amino acids of the 126-kDa protein also displayed nuclear localization. However, fusion constructs encoding the first 781 amino acids or the entire 126-kDa ORF did not accumulate within the nucleus but instead associated with the endoplasmic reticulum (ER), forming spot-like inclusions. Thus, a dominant ER association domain exists between amino acids 388 and 781 of the 126-kDa protein. Interestingly, a full-length 126-kDa GFP fusion construct encoding a nonfunctional NLS mutation also localized to the ER but did not form inclusions. Furthermore, a TMV mutant containing the same nonfunctional NLS mutation failed to replicate in protoplasts. Together these findings suggest that both the NLS and the ER retention domain contribute to the functional localization of the 126-kDa protein.

Analysis of the C-terminal membrane anchor domains of hepatitis C virus glycoproteins E1 and E2: toward a topological model

The hepatitis C virus (HCV) glycoproteins E1 and E2 should be anchored in the viral membrane by their C-terminal domains. During synthesis, they are translocated to the endoplasmic reticulum (ER) lumen where they remain. The 31 C-terminal residues of the E1 protein and the 29 C-terminal residues of the E2 protein are implicated in the ER retention. Moreover, the E1 and E2 C termini are implicated in E1-E2 heterodimerization. We studied the E1 and E2 C-terminal sequences of 25 HCV strains in silico using molecular modeling techniques. We conclude that both C-terminal domains should adopt a similar and peculiar configuration: one amphipathic alpha-helix followed by a pair of transmembrane beta-strands. Several three-dimensional (3-D) models were generated. After energy minimization, their ability to interact with membranes was studied using the molecular hydrophobicity potentials calculation and the IMPALA procedure. The latter simulates interactions with a membrane by a Monte Carlo minimization of energy. These methods suggest that the beta-hairpins could anchor the glycoproteins in the ER membrane at least transiently. Anchoring could be stabilized by the adsorption of the nearby amphipathic alpha-helices at the membrane surface. The 3-D models correlate with experimental results which indicate that the E1-E2 transmembrane domains are involved in the heterodimerization and have ER retention properties.

Molecular characterization of a Bombyx mori protein disulfide isomerase (bPDI)

We have isolated a complementary deoxyribonucleic acid clone that encodes the protein disulfide isomerase of Bombyx mori (bPDI). This protein has a putative open reading frame of 494 amino acids and a predicted size of 55.6 kDa. In addition, 2 thioredoxin active sites, each with a CGHC sequence, and an endoplasmic reticulum (ER) retention signal site with a KDEL motif were found at the C-terminal. Both sites are typically found in members of the PDI family of proteins. The expression of bPDI messenger ribonucleic acid (mRNA) was markedly increased during ER stress induced by stimulation with calcium ionophore A23187, tunicamycin, and dithiothreitol, all of which are known to cause an accumulation of unfolded proteins in the ER. We also examined the tissue distribution of bPDI mRNA and found pronounced expression in the fat body of insects. Hormonal regulation studies showed that juvenile hormone, insulin, and a combination of juvenile hormone and transferrin (although not transferrin alone) affected bPDI mRNA expression. A challenge with exogenous bacteria also affected expression, and the effect peaked 16 hours after infection. These results suggest that bPDI is a member of the ER-stress protein group, that it may play an important role in exogenous bacterial infection of the fat body, and that its expression is hormone regulated.

Endoplasmic reticulum and cis-Golgi localization of human T-lymphotropic virus type 1 p12(I): association with calreticulin and calnexin

Human T-lymphotropic virus type 1 (HTLV-1) is a complex retrovirus encoding regulatory and accessory genes in four open reading frames (ORF I to IV) of the pX region. We have demonstrated an important role of pX ORF I expression, which encodes p12(I), in establishment of HTLV-1 infection in a rabbit model and for optimal viral infectivity in quiescent primary lymphocytes. These data indicated that p12(I) may enhance lymphocyte activation and thereby promote virus infection. To further define the role of p12(I) in cell activation, we characterized the subcellular localization of p12(I) in transfected 293T cells and HeLa-Tat cells by multiple methods, including immunofluorescence confocal microscopy, electron microscopy, and subcellular fractionation. Herein, we demonstrate that p12(I) accumulates in the endoplasmic reticulum (ER) and cis-Golgi apparatus. The location of p12(I) was unchanged following treatments with both cycloheximide (blocking de novo protein synthesis) and brefeldin A (disrupting ER-to-Golgi protein transport), indicating that the protein is retained in the ER and cis-Golgi. Moreover, using coimmunoprecipitation assays, we identify the direct binding of p12(I) with both calreticulin and calnexin, resident ER proteins which regulate calcium storage. Our results indicate that p12(I) directly binds key regulatory proteins involved in calcium-mediated cell signaling and suggest a role of p12(I) in the establishment of HTLV-1 infection by activation of host cells.

The immunologically protective P-4 antigen of Leishmania amastigotes. A developmentally regulated single strand-specific nuclease associated with the endoplasmic reticulum

The purified membrane-associated Leishmania pifanoi amastigote protein P-4 has been shown to induce protective immunity against infection and to elicit preferentially a T helper 1-like response in peripheral blood mononuclear cells of patients with American cutaneous leishmaniasis. As this molecule is potentially important for future vaccine studies, the L. pifanoi gene encoding the P-4 membrane protein was cloned and sequenced. Southern blot analyses indicate the presence of six tandemly arrayed copies of the P-4 gene in L. pifanoi; homologues of the P-4 gene are found in all other species of the genus Leishmania examined. DNA-derived protein sequence data indicated an identity to the P1 zinc-dependent nuclease of Penicillium citrinum (20.8%) and the C-terminal domain of the 3' nucleotidase of Leishmania donovani (33.7%). Consistent with these sequence analyses, purified L. pifanoi P-4 protein possesses single strand nuclease (DNA and RNA) and phosphomonoesterase activity, with a preference for UMP > TMP > AMP >> CMP. Double-labeling immunofluorescence microscopic analyses employing anti-binding protein antibodies revealed that the P-4 protein is localized in the endoplasmic reticulum of the amastigote. Northern blot analyses indicated that the gene is selectively expressed in the intracellular amastigote stage (mammalian host) but not in the promastigote stage (insect) of the parasite. Based upon its subcellular localization and single-stranded specific nuclease activity, possible roles of the P-4 nuclease in the amastigote in RNA stability (gene expression) or DNA repair are discussed.

Subcellular sorting of small membrane-associated triple gene block proteins: TGBp3-assisted targeting of TGBp2

We studied subcellular distribution of green fluorescent protein (GFP)-tagged movement proteins encoded by the second and the third genes of poa semilatent hordeivirus (PSLV) triple gene block (TGB), 15K TGBp2 and 18K TGBp3. GFP-15K transiently expressed in Nicotiana benthamiana leaf epidermal cells was associated with the endomembrane system elements. GFP-18K appeared in the membrane bodies at cell periphery. Mutation analysis demonstrated that subcellular targeting of GFP-15K depended on the protein transmembrane segment(s), whereas the TGBp3 central hydrophilic region was responsible for targeting of GFP-18K. Coexpression of GFP-15K with the intact 18K protein induced drastic changes in the TGBp2 localization: GFP-15K appeared in the cell peripheral bodies similar to those in the cells expressing GFP-18K alone. Coexpression experiments with mutant forms of both proteins argue against involvement of direct interaction between small TGB proteins in the TGBp3-assisted targeting of TGBp2 to the cell peripheral compartments. This conclusion was further confirmed by similar effects on the PSLV 15K TGBp2 localization induced by TGBp3 proteins of PSLV and potato virus X, which have no detectable sequence similarity to each other.