Heterologous signals allow efficient targeting of a nuclear-encoded fusion protein to plastids and endoplasmic reticulum in diverse plant species

Approximately 30% of plant nuclear genes appear to encode proteins targeted to the plastids or endoplasmic reticulum (ER). The signals that direct proteins into these compartments are diverse in sequence, but, on the basis of a limited number of tests in heterologous systems, they appear to be functionally conserved across species. To further test the generality of this conclusion, we tested the ability of two plastid transit peptides and an ER signal peptide to target green fluorescent protein (GFP) in 12 crops, including three monocots (barley, sugarcane, wheat) and nine dicots ( Arabidopsis , broccoli, cabbage, carrot, cauliflower, lettuce, radish, tobacco, turnip). In all species, transient assays following microprojectile bombardment or vacuum infiltration using Agrobacterium showed that the plastid transit peptides from tomato DCL (defective chloroplast and leaves) and tobacco RbcS [ribulose bisphosphate carboxylase (Rubisco) small subunit] genes were effective in targeting GFP to the leaf plastids. GFP engineered as a fusion to the N-terminal ER signal peptide from Arabidopsis basic chitinase and a C-terminal HDEL signal for protein retention in the ER was accumulated in the ER of all species. The results in tobacco were confirmed in stably transformed cells. These signal sequences should be useful to direct proteins to the plastid stroma or ER lumen in diverse plant species of biotechnological interest for the accumulation of particular recombinant proteins or for the modification of particular metabolic streams.     

ER‐phagy: selective autophagy of the endoplasmic reticulum

Eukaryotic cells adequately control the mass and functions of organelles in various situations. Autophagy, an intracellular degradation system, largely contributes to this organelle control by degrading the excess or defective portions of organelles. The endoplasmic reticulum (ER) is an organelle with distinct structural domains associated with specific functions. The ER dynamically changes its mass, components, and shape in response to metabolic, developmental, or proteotoxic cues to maintain or regulate its functions. Therefore, elaborate mechanisms are required for proper degradation of the ER. Here, we review our current knowledge on diverse mechanisms underlying selective autophagy of the ER, which enable efficient degradation of specific ER subdomains according to different demands of cells.     

Dual recognition of the ribosome and the signal recognition particle by the SRP receptor during protein targeting to the endoplasmic reticulum

We have analyzed the interactions between the signal recognition particle (SRP), the SRP receptor (SR), and the ribosome using GTPase assays, biosensor experiments, and ribosome binding assays. Possible mechanisms that could contribute to an enhanced affinity between the SR and the SRP-ribosome nascent chain complex to promote protein translocation under physiological ionic strength conditions have been explored. Ribosomes or 60S large ribosomal subunits activate the GTPase cycle of SRP54 and SRalpha by providing a platform for assembly of the SRP-SR complex. Biosensor experiments revealed high-affinity, saturable binding of ribosomes or large ribosomal subunits to the SR. Remarkably, the SR has a 100-fold higher affinity for the ribosome than for SRP. Proteoliposomes that contain the SR bind nontranslating ribosomes with an affinity comparable to that shown by the Sec61 complex. An NH2-terminal 319-residue segment of SRalpha is necessary and sufficient for binding of SR to the ribosome. We propose that the ribosome-SR interaction accelerates targeting of the ribosome nascent chain complex to the RER, while the SRP-SR interaction is crucial for maintaining the fidelity of the targeting reaction.     

A close-up view of membrane contact sites between the endoplasmic reticulum and the endolysosomal system: From yeast to man

Abstract

Maintenance of organelle identity is crucial for the functionality of eukaryotic cells. Hence, transfer reactions between different compartments must be highly efficient and tightly regulated at the same time. Membrane contact sites (MCSs) represent an important route for inter-organelle transport and communication independent of vesicular trafficking. Due to extensive research, the mechanistic understanding of these sites increases constantly. However, how the formation and the versatile functions of MCSs are regulated is mainly unclear. Within this review, we focus on one well-known MCS, the nucleus–vacuole junction in yeast and discuss its analogy to endoplasmic reticulum-late endosome contacts in metazoan. Formation of the junction in yeast requires Vac8, a protein that is involved in various cellular processes at the yeast vacuole and a target of multiple posttranslational modifications. We discuss the possibility that dual functionality of proteins involved in contact formation is a common principle to coordinate inter-organelle transfer with organellar biogenesis.     

High capacity of the endoplasmic reticulum to prevent secretion and aggregation of amyloidogenic proteins

Protein aggregation is associated with neurodegeneration and various other pathologies. How specific cellular environments modulate the aggregation of disease proteins is not well understood. Here, we investigated how the endoplasmic reticulum (ER) quality control system handles β‐sheet proteins that were designed de novo to form amyloid‐like fibrils. While these proteins undergo toxic aggregation in the cytosol, we find that targeting them to the ER (ER‐β) strongly reduces their toxicity. ER‐β is retained within the ER in a soluble, polymeric state, despite reaching very high concentrations exceeding those of ER‐resident molecular chaperones. ER‐β is not removed by ER‐associated degradation (ERAD) but interferes with ERAD of other proteins. These findings demonstrate a remarkable capacity of the ER to prevent the formation of insoluble β‐aggregates and the secretion of potentially toxic protein species. Our results also suggest a generic mechanism by which proteins with exposed β‐sheet structure in the ER interfere with proteostasis.     

Mass transport of proform of a KDEL-tailed cysteine proteinase (SH-EP) to protein storage vacuoles by endoplasmic reticulum-derived vesicle is involved in protein mobilization in germinating seeds

A vacuolar cysteine proteinase, designated SH-EP, is expressed in the cotyledon of germinated Vigna mungo seeds and is responsible for the degradation of storage proteins. SH-EP is a characteristic vacuolar proteinase possessing a COOH-terminal endoplasmic reticulum (ER) retention sequence, KDEL. In this work, immunocytochemical analysis of the cotyledon cells of germinated V. mungo seeds was performed using seven kinds of antibodies to identify the intracellular transport pathway of SH-EP from ER to protein storage vacuoles. A proform of SH-EP synthesized in ER accumulated at the edge or middle region of ER where the transport vesicle was formed. The vesicle containing a large amount of proSH-EP, termed KV, budded off from ER, bypassed the Golgi complex, and was sorted to protein storage vacuoles. This massive transport of SH-EP via KV was thought to mediate dynamic protein mobilization in the cotyledon cells of germinated seeds. We discuss the possibilities that the KDEL sequence of KDEL-tailed vacuolar cysteine proteinases function as an accumulation signal at ER, and that the mass transport of the proteinases by ER-derived KV-like vesicle is involved in the protein mobilization of plants.     

Calcium binding proteins in the sarcoplasmic/endoplasmic reticulum of muscle and nonmuscle cells

In this paper we review some of the large quantities of information currently available concerning the identification, structure and function of Ca²⁺-binding proteins of endoplasmic and sarcoplasmic reticulum membranes. The review places particular emphasis on identification and discussion of Ca²⁺ storage proteins in these membranes. We believe that the evidence reviewed here supports the contention that the Ca²⁺-binding capacity of both calsequestrin and calreticulin favor their contribution as the major Ca²⁺-binding proteins of muscle and nonmuscle cells, respectively. Other Ca²⁺-binding proteins discovered in both endoplasmic reticulum and sarcoplasmic reticulum membranes probably contribute to the overall Ca²⁺ storage capacity of these membrane organelles, and they also play other important functional role such as posttranslational modification of newly synthesized proteins, a cytoskeletal (structural) function, or movement of Ca²⁺ within the lumen of the sarcoplasmic/endoplasmic reticulum towards the storage sites.Abbreviations SR Sarcoplasmic Reticulum - ER Endoplasmic Reticulum - InsP₃ Inositol 1,4,5-trisphosphate - SDS-PAGE Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis - PDI Protein Disulphide Isomerase - T₃BP Thyroid Hormone Binding Protein - Grp Glucose regulated proteins - HCP Histidine-rich Ca²⁺ binding Protein - LDL Low Density Lipoprotein     

Changes in levels of pancreatic endoplasmic reticulum proteins that function in translocation and maturation of secretory proteins in response to cholecystokinin

Two pathways operate to target newly-synthesised proteins to the endoplasmic reticulum. In one, the signal recognition particle attaches to the signal sequences of nascent chains on ribosomes and slows or stops translation until contact is made with the docking protein at the membrane. The second operates via molecular chaperons. The pathways converge at the level of a 43 kDa signal binding protein integrated into the membrane, where translocation through a proteinaceous pore is initiated. In the lumen, proteins fold and disulphide formation is catalysed by the enzyme protein disulphide isomerase. The heavy chain binding protein may attach to unassembled or unfolded proteins and prevent their exit from the ER to the Golgi. Cholecystokinin (CCK) treatment increases the biosynthesis and secretion of pancreatic proteins, increases the levels of PDI and the 43 kDa binding protein, and reduces levels of BiP. These proteins may be possible targets for genetic manipulation to improve processing of heterologous proteins from cultured mammalian cells.     

Production of a recombinant full-length collagen type I α-1 and of a 45-kDa collagen type I α-1 fragment in barley seeds

Recombinant DNA technology can be used to design and express collagen and gelatin-related proteins with predetermined composition and structure. Barley seed was chosen as a production host for a recombinant full-length collagen type I α1 (rCIa1) and a related 45-kDa rCIa1 fragment. The transgenic barley seeds were shown to accumulate both the rCIa1 and the 45-kDa rCIa1 fragment. Even when the amount of the rCIa1 was just above the detection threshold, this work using rCIa1 as a model demonstrated for the first time that barley seed can be used as a production system for collagen-related structural proteins. The 45-kDa rCI1a fragment expression, targeted to the endoplasmic reticulum, was controlled by three different promoters (a constitutive maize ubiquitin , seed endosperm-specific rice glutelin and germination-specific barley α - amylase fusion) to compare their effects on rCIa1 accumulation. Highest accumulation of the 45-kDa rCIa1 was obtained with the glutelin promoter (140 mg/kg seed), whereas the lowest accumulation was obtained with the α - amylase promoter. To induce homozygosity for stable 45-kDa rCIa1 production in the transgenic lines, doubled haploid (DH) progeny was generated through microspore culture. The 45-kDa rCIa1 expression levels achieved from the best DH lines were 13 mg/kg dry seeds under the ubiquitin promoter and 45 mg/kg dry seeds under the glutelin promoter. Mass spectroscopy and amino acid composition analysis of the purified 45-kDa rCIa1 fragment revealed that a small percent of prolines were hydroxylated with no additional detectable post-translational modifications.     

Localization of individual subunits of protein kinase CK2 to the endoplasmic reticulum and to the Golgi apparatus

The protein kinase CK2 is composed of two catalytic - or - and two regulatory -subunits. In mammalian cells there is ample evidence for the presence of individual CK2 subunits beside the holoenzyme. By immunofluorescence studies using peptide antibodies which allow us to detect the CK2-, - and -subunits we found all three subunits to be co-localized with a 58 KDa Golgi protein which is specific for the Golgi complex. Subfractionation studies using dog pancreas cells revealed the presence of all three subunits of CK2 at the smooth endoplasmic reticulum (sER)/Golgi fraction whereas the rough endoplasmic reticulum (rER) harboured only the catalytic - and -subunits. We found that the microsomal preparation from dog pancreas cells contained CK2 which phosphorylated a CK2 specific synthetic peptide and which was heparin sensitive. Furthermore, we could immunoprecipitate the CK2-subunit that exhibited a kinase activity which phosphorylated a CK2 specific substrate and which was heparin sensitive. Protease digestion experiments revealed that the CK2 subunits were located on the cytosolic side of the rER and the sER/Golgi complex. Thus, we could demonstrate an asymmetric distribution of the CK2 subunits at the rER and sER/Golgi complex. Since the CK2- and -subunits exhibit a substrate specificity which is different from the CK2 holoenzyme one might speculate that the asymmetric distribution of the CK2 holoenzyme and the CK2 catalytic subunits may have regulatory functions.     

Cytosolic aggregates in presence of non‐translocated proteins perturb endoplasmic reticulum structure and dynamics

Presence of cytosolic protein aggregates and membrane damage are two common attributes of neurodegenerative diseases. These aggregates delay degradation of non‐translocated protein precursors leading to their persistence and accumulation in the cytosol. Here, we find that cells with intracellular protein aggregates (of cytosolic prion protein or huntingtin) destabilize the endoplasmic reticulum (ER) morphology and dynamics when non‐translocated protein load is high. This affects trafficking of proteins out from the ER, relative distribution of the rough and smooth ER and three‐way junctions that are essential for the structural integrity of the membrane network. The changes in ER membranes may be due to high aggregation tendency of the ER structural proteins—reticulons, and altered distribution of those associated with the three‐way ER junctions—Lunapark. Reticulon4 is seen to be enriched in the aggregate fractions in presence of non‐translocated protein precursors. This could be mitigated by improving signal sequence efficiencies of the proteins targeted to the ER. These were observed using PrP variants and the seven‐pass transmembrane protein (CRFR1) with different signal sequences that led to diverse translocation efficiencies. This identifies a previously unappreciated consequence of cytosolic aggregates on non‐translocated precursor proteins—their persistent presence affects ER morphology and dynamics. This may be one of the ways in which cytosolic aggregates can affect endomembranes during neurodegenerative disease.     

70-kDa peroxisomal membrane protein related protein (P70R/ABCD4) localizes to endoplasmic reticulum not peroxisomes, and NH2-terminal hydrophobic property determines the subcellular localization of ABC subfamily D proteins

70-kDa peroxisomal membrane protein related protein (P70R/ABCD4) is a member of ATP-binding cassette (ABC) protein subfamily D. ABC subfamily D proteins are also known as peroxisomal ABC proteins. Therefore, P70R is thought to be a peroxisomal membrane protein. However, the subcellular localization of P70R is not extensively investigated. In this study, we transiently expressed P70R in fusion with HA (P70R-HA) in CHO cells and examined subcellular localization by immunofluorescence. Surprisingly, P70R-HA was localized to the endoplasmic reticulum (ER), not to peroxisomes. To examine the ER-targeting property of P70R, we expressed various NH₂-terminal deletion constructs of P70R. Among the NH₂-terminal deletion constructs, mutant proteins starting with hydrophobic transmembrane segment (TMS) were localized to ER, but the ones containing the NH₂-terminal hydrophilic cytosolic domain were not. ABC subfamily D proteins destined for peroxisomes have NH₂-terminal hydrophilic region adjacent to TMS1. However, only P70R lacks the region and is translated with NH₂-terminal hydrophobic TMS1. Furthermore, attachment of the NH₂-terminal hydrophilic domain to the NH₂-terminus of P70R excluded P70R from the ER-targeting pathway. These data suggest that P70R resides in the ER but not the peroxisomal membranes, and the hydrophobic property of NH₂-terminal region determines the subcellular localization of ABC subfamily D proteins.     

Alternatively spliced forms of the P180 ribosome receptor differ in their ability to induce the proliferation of rough endoplasmic reticulum

Expression of the canine 180-kDa ribosome receptor p180 in yeast induces the synthesis of RER, and increases the mRNAs of secretory pathway proteins, and protein secretion. To assess whether p180 is a master regulator of cell secretion in mammalian cells, we stably expressed red fluorescent forms of the human p180 variants p180DeltaR (no tandem repeats), p180R (26 repeats), and full-length p180FR (54 repeats) containing different lengths of the tandem repeat ribosome-binding domain in rat pancreatic RINm5F islet beta-cells. All three fluorescent p180 variants localized exclusively to the RER. Cells transfected with p180R were filled with ribosome-studded karmellae, whereas p180DeltaR and p180FR transfectants contained only increased amounts of mostly smooth ER. Unlike in yeast, over-expression of p180R failed to increase the secretory pathway proteins calnexin, SEC61beta, and calreticulin, or ribosome biogenesis. The data suggest that alternative splicing of the p180 tandem repeat domain is a means of regulating the ribosome-binding activity of p180, and potentially the secretory activity of the cell. However, p180 is not a master regulator of mammalian cell secretion as it does not concomitantly trigger the synthesis of protein machinery required to enhance protein synthesis and cell secretion.     

The steady-state distribution of glycosyltransferases between the Golgi apparatus and the endoplasmic reticulum is approximately 90:10

Several lines of evidence support a novel model for Golgi protein residency in which these proteins cycle between the Golgi apparatus and the endoplasmic reticulum (ER). However, to preserve the functional distinction between the two organelles, this pool of ER-resident Golgi enzymes must be small. We quantified the distribution for two Golgi glycosyltransferases in HeLa cells to test this prediction. We reasoned that best-practice, quantitative solutions would come from treating images as data arrays rather than pictures. Using deconvolution and computer calculated organellar boundaries, the Golgi fraction for both endogenous beta1,4-galactosyltransferase and UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferase 2 fused with green fluorescent protein (GFP) was 91% by fluorescence microscopy. Immunogold labeling followed by electron microscopy and model analysis yielded a similar value. Values reflect steady-state conditions, as inclusion of a protein synthesis inhibitor had no effect. These data strongly suggest that the fluorescence of a GFP chimera with an organellar protein can be a valid indicator of protein distribution and more generally that fluorescent microscopy can provide a valid, rapid approach for protein quantification. In conclusion, we find the ER pool of cycling Golgi glycosyltransferases is small and approximately 1/100 the concentration found in the Golgi apparatus.     

内质网应激偶联炎症反应与慢性病发病机制

内质网是合成细胞内分泌蛋白和膜蛋白并进行蛋白折叠的主要细胞器。新近研究证明,当内质网蛋白质合成与折叠的负担增加、非折叠或错误折叠蛋白质堆积,可激活内质网的几组特定信号转导通路,将这些应激信号传递到细胞浆和细胞核,引起未/错误折叠蛋白反应。这对维持细胞动态平衡和生物体的发育具有重要意义。更为重要的是,未/错误折叠蛋白反应能够与细胞内炎症反应信号转导通路偶联,是非感染性致病原引发炎症反应的主要原因。因此,内质网应激-未/错误折叠蛋白反应-炎症反应在特定的细胞发生偶联是许多炎症疾病的发病机制。本文综述该领域的研究进展,并介绍了内质网应激信号和炎症反应偶联参与一些慢性病发病的分子细胞机制。这些研究不仅加深人们对这些慢性病发病机制的了解,也有助于对调节内质网应激-炎症反应的药物的研发。     

Identification and functional study of the endoplasmic reticulum stress sensor IRE1 in Chlamydomonas reinhardtii

In many eukaryotes, endoplasmic reticulum (ER ) stress activates the unfolded protein response (UPR ) via the transmembrane endoribonuclease IRE 1 to maintain ER homeostasis. The ER stress response in microalgae has not been studied in detail. Here, we identified Chlamydomonas reinhardtii IRE 1 (CrIRE 1 ) and characterized two independent knock‐down alleles of this gene. CrIRE 1 is similar to IRE 1s identified in budding yeast, plants, and humans, in terms of conserved domains, but differs in having the tandem zinc‐finger domain at the C terminus. CrIRE 1 was highly induced under ER stress conditions, and the expression of a chimeric protein consisting of the luminal N‐terminal region of CrIRE 1 fused to the cytosolic C‐terminal region of yeast Ire1p rescued the yeast ?ire1 mutant. Both allelic ire1 knock‐down mutants ire1‐1 and ire1‐2 were much more sensitive than their parental strain CC ‐4533 to the ER stress inducers tunicamycin, dithiothreitol and brefeldin A. Treatment with a low concentration of tunicamycin resulted in growth arrest and cytolysis in ire1 mutants, but not in CC ‐4533 cells. Furthermore, in the mutants, ER stress marker gene expression was reduced, and reactive oxygen species (ROS ) marker gene expression was increased. The survival of ire1 mutants treated with tunicamycin improved in the presence of the ROS scavenger glutathione, suggesting that ire1 mutants failed to maintain ROS levels under ER stress. Together, these results indicate that CrIRE 1 functions as an important component of the ER stress response in Chlamydomonas, and suggest that the ER stress sensor IRE 1 is highly conserved during the evolutionary history.