The role of the endoplasmic reticulum in the synthesis and transport of α-amylase in barley aleurone layers

The subcellular site of -amylase (EC 1.6.2.1) synthesis and transport was studied in barley aleurone layers incubated in the presence or absence of gibberellic acid (GA₃). Using [³⁵S]methionine as a marker, the site of amino-acid incorporation into organelles isolated from aleurone layers incubated with and without GA₃ was determined following purification by isopycnic sucrose-density-gradient centrifugation. Incorporation of radioactivity into trichloroacetic-acid-insoluble proteins was greatest in those fractions exhibiting activity of an endoplasmic reticulum (ER) marker enzyme. Further fractionation of densitygradient fractions by sodium-dodecyl-sulfate polyacrylamide-gel electrophoresis showed that a major portion of the radioactivity in the ER fractions was present in a protein co-migrating with marker -amylase. This protein was identified as authentic -amylase by immunoadsorbent chromatography and affinity chromatography. The newly synthesized -amylase associated with the ER was shown to be sequenstered within the lumen of the ER by experiments which showed that the enzyme was resistant to proteolytic degradation. The labelled -amylase sequestered in the ER can be chased from this organelle when tissue is incubated in unlabelled methionine following a 1-h pulse of labelled methionine. The isoenzymic forms of -amylase found in tissue homogenates and incubation media of aleurone layers incubated with and without GA₃ were characterized after chromatography on diethylaminoethyl cellulose. In homogenates of GA₃-treated aleurone layers, five peaks of -amylase activity were detected, while in homogenates of aleurone layers incubated with-out GA₃ only three peaks of activity were found. In incubation media, four isoenzymes were found after GA₃ treatment and two were found after incubation without GA₃. We conclude that at least five -amylase isoenzymes are synthesized by the ER of barley aleurone layers and that this membrane system is involved in the sequestration and transport of four of these isoenzymes.Abbreviations CHA cyclohepataamylose - DEAE-cellulose diethylaminoethyl-cellulose - ER endoplasmic reticulum - GA₃ gibberellic acid - SDS-PAGE sodium-dodecyl-sulfate polyacrylamide-gel electrophoresis     

Glycoprotein folding in the endoplasmic reticulum: a tale of three chaperones?

The endoplasmic reticulum (ER) is a major site of protein synthesis and its inside, or lumen, is a major site of protein folding. The lumen of the ER contains many folding factors and molecular chaperones, which facilitate protein folding by increasing both the rate and the efficiency of this process. Amongst the many ER folding factors, there are three components that specifically modulate the folding glycoproteins bearing N-linked carbohydrate side chains. These components are calnexin, calreticulin and ERp57, and this review focuses on the molecular basis for their capacity to influence glycoprotein folding.     

Influence of SelS gene silence on β-Mercaptoethanol-mediated endoplasmic reticulum stress and cell apoptosis in HepG2 cells

Selenoprotein S (SelS), a transmembrane selenoprotein, may be related to the response of endoplasmic reticulum (ER) stress. In this report, the influence of selenite supplementation and SelS gene silence on β-mercaptoethanol (β-ME)-mediated ER stress and cell apoptosis in HepG2 cells were examined. The results showed that SelS protein expression was markedly increased by 10 mM β-ME and 100 nM sodium selenite in HepG2 cells. GRP78 protein level was significantly increased after treatment with 10 mM β-ME in HepG2 cells, which suggested that β-ME was also an ER stress inducer. Meanwhile, β-ME (10 mM) was found to induce cell apoptosis, which was alleviated obviously when cells were pretreated with 100 nM selenite before exposure to β-ME. Moreover, the suppression of SelS gene by siRNA could aggravate HepG2 cell apoptosis induced by β-ME significantly. In conclusion, these results suggested that β-ME, also an ER stress agent, could induce cell apoptosis, and SelS may play an important role in protecting cells from apoptosis induced by ER stress in HepG2 cells.     

How are peroxisomes formed? The role of the endoplasmic reticulum and peroxins

Recent data from studies of peroxisome assembly and the subcellular sorting of peroxisomal matrix and membrane proteins have led to an expansion of the 'growth and division' and 'endoplasmic reticulum-vesiculation' models of peroxisome biogenesis into a more flexible, unified model. Within this context, we discuss the proposed role for the endoplasmic reticulum in the formation of preperoxisomes and the potential for 15 Arabidopsis peroxin homologs to function in the biogenesis of peroxisomes in plant cells.     

Oxidative folding in the endoplasmic reticulum: Towards a multiple oxidant hypothesis?

Oxidative protein folding in the luminal compartment of the endoplasmic reticulum is thought to be mediated by a proteinaceous electron relay system composed by PDI and ER oxidoreductin 1 (Ero1), transferring electrons from the cysteinyl residues of substrate proteins to oxygen. However, recent observations revealed that Ero1 isoforms are dispensable. Endoplasmic reticulum is known as a generator and accumulator of low molecular weight oxidants; some of them have already been shown to promote oxidative folding. On the basis of these observations a new theory of oxidative folding is proposed where the oxidative power is provided by the stochastic contribution of prooxidants.     

Escaping the endoplasmic reticulum: why does a molecular chaperone leave home for greener pastures?

Molecular chaperones reside in nearly every organelle within a eukaryotic cell, and in each of these compartments, they ensure that protein homeostasis (or proteostasis) is maintained. In this issue, Wiseman and colleagues find that an ER lumenal chaperone escapes this compartment when a specific stress pathway is activated. The chaperone, an Hsp40 homolog known as ERdj3, transits through the secretory pathway to the extracellular space. During this journey, ERdj3 can escort an aggregation‐prone protein or it can identify aggregation‐prone proteins extracellularly, thereby functioning outside of its normal environment.     

Participation of endoplasmic reticulum and mitochondrial calcium handling in apoptosis: more than just neighborhood?

Over the past few years, extensive progress has been made in elucidating the role of calcium in the signaling of apoptosis. This has led to the characterization of calcium's role in the induction of apoptosis and in the regulation of effector proteases. In this review, we attempt to summarize the current knowledge regarding a segment of these studies, the interaction between the endoplasmic reticulum (ER) and mitochondria. This interface has been shown to play a crucial role in transferring agonist induced Ca(2+) signals to mitochondria during physiological processes. Recent evidence, however, extended the role of this Ca(2+) transfer to apoptotic pathways, showing that modulation of mitochondrial Ca(2+) uptake from the ER side has a prominent role in modulating cellular fate.     

Nϵ-lysine acetylation in the lumen of the endoplasmic reticulum: A way to regulate autophagy and maintain protein homeostasis in the secretory pathway

The N?-lysine acetylation of cargo proteins in the lumen of the endoplasmic reticulum (ER) requires a membrane transporter (SLC33A1) and 2 acetyltransferases (NAT8B and NAT8). The ER acetylation machinery regulates the homeostatic balance between quality control/efficiency of the secretory pathway and autophagy-mediated disposal of toxic protein aggregates. We recently reported that the autophagy pathway that acts downstream of the ER acetylation machinery specifically targets protein aggregates that form within the secretory pathway. Genetic and biochemical manipulation of ER acetylation in a mouse model of Alzheimer disease is able to restore normal proteostasis and rescue the disease phenotype. Here we summarize these findings and offer an overview of the ER-acetylation machinery.     

What's in a name? The vervet predator calls and the limits of the Washburnian synthesis

After the Second World War, a renaissance in field primatology took place in the United States under the aegis of the 'new physical anthropology'. Its leader, Sherwood Washburn, envisioned a science uniting studies of hominid fossils with Darwinian population genetics, experimental functional anatomy, and field observation of non-human primates and human hunter-gatherers. Thanks to Washburn's stimulus, his colleague at Berkeley, the bird ethologist Peter Marler, took up the study of the natural communicative behaviour of apes and monkeys. When Marler's first primatological student, Thomas Struhsaker, reported in the mid-1960s that the vervet monkeys of Amboseli, Kenya, give acoustically distinct alarm calls to different predators, and respond to alarm calls as if to the sight of those predators, a debate broke out over whether the vervet calls thus function as names, translating as 'leopard', 'eagle' and 'python'. Washburn and his students argued that no matter what the behavioural evidence, vervet calls could not be predator names, since monkeys had been shown to lack the neuroanatomical basis of naming. This controversy thus reveals, first, the persistence of older patterns of disciplinary allegiance within the new, synthetic physical anthropology; and second, the impotence of adaptationist Darwinism--common to both sides of the debate--as a force for unity.     

Differential hepatic protein tyrosine nitration of mouse due to aging – Effect on mitochondrial energy metabolism,quality control machinery of the endoplasmic reticulum and metabolism of drugs

Aging is the inevitable fate of life which leads to the gradual loss of functions of different organs and organelles of all living organisms. The liver is no exception. Oxidative damage to proteins and other macromolecules is widely believed to be the primary cause of aging. One form of oxidative damage is tyrosine nitration of proteins, resulting in the potential loss of their functions. In this study, the effect of age on the nitration of tyrosine in mouse liver proteins was examined. Liver proteins from young (19–22 weeks) and old (24 months) C57/BL6 male mice were separated using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS–PAGE) and electroblotted onto nitrocellulose membranes. Proteins undergoing tyrosine nitration were identified using anti-nitrotyrosine antibody. Three different protein bands were found to contain significantly increased levels of nitrotyrosine in old mice (Wilconxon rank-sum test, p < 0.05). Electrospray ionization liquid chromatography tandem mass spectrometry (ESI-LC–MS/MS) was used to identify the proteins in these bands, which included aldehyde dehydrogenase 2, Aldehyde dehydrogenase family 1, subfamily A1, ATP synthase, H⁺ transporting, mitochondrial F1 complex, β subunit, selenium-binding protein 2, and protein disulfide-isomerase precursor. The possible impairment of their functions can lead to altered hepatic activity and have been discussed.     

Is cytochrome P-450 transported from the endoplasmic reticulum to the Golgi apparatus in rat hepatocytes?

The Golgi apparatus mediates intracellular transport of not only secretory and lysosomal proteins but also membrane proteins. As a typical marker membrane protein for endoplasmic reticulum (ER) of rat hepatocytes, we have selected phenobarbital (PB)-inducible cytochrome P- 450 (P-450[PB]) and investigated whether P-450(PB) is transported to the Golgi apparatus or not by combining biochemical and quantitative ferritin immunoelectron microscopic techniques. We found that P-450(PB) was not detectable on the membrane of Golgi cisternae either when P-450 was maximally induced by phenobarbital treatment or when P-450 content in the microsomes rapidly decreased after cessation of the treatment. The P-450 detected biochemically in the Golgi subcellular fraction can be explained by the contamination of the microsomal vesicles derived from fragmented ER membranes to the Golgi fraction. We conclude that when the transfer vesicles are formed by budding on the transitional elements of ER, P-450 is completely excluded from such regions and is not transported to the Golgi apparatus, and only the membrane proteins destined for the Golgi apparatus, plasma membranes, or lysosomes are selectively collected and transported.     

Association of spectrin-like proteins with the actin-organized aggregate of endoplasmic reticulum in the Spitzenkörper of gravitropically tip-growing plant cells

Spectrin-like epitopes were immunochemically detected and immunofluorescently localized in gravitropically tip-growing rhizoids and protonemata of characean algae. Antiserum against spectrin from chicken erythrocytes showed cross-reactivity with rhizoid proteins at molecular masses of about 170 and 195 kD. Confocal microscopy revealed a distinct spherical labeling of spectrin-like proteins in the apices of both cell types tightly associated with an apical actin array and a specific subdomain of endoplasmic reticulum (ER), the ER aggregate. The presence of spectrin-like epitopes, the ER aggregate, and the actin cytoskeleton are strictly correlated with active tip growth. Application of cytochalasin D and A23187 has shown that interfering with actin or with the calcium gradient, which cause the disintegration of the ER aggregate and abolish tip growth, inhibits labeling of spectrin-like proteins. At the beginning of the graviresponse in rhizoids the labeling of spectrin-like proteins remained in its symmetrical position at the cell tip, but was clearly displaced to the upper flank in gravistimulated protonemata. These findings support the hypothesis that a displacement of the Spitzenk?rper is required for the negative gravitropic response in protonemata, but not for the positive gravitropic response in rhizoids. It is evident that the actin/spectrin system plays a role in maintaining the organization of the ER aggregate and represents an essential part in the mechanism of gravitropic tip growth.     

Transfer RNATyr of melanoma tissues and cells: relevance to melanin synthesis?

The tRNA present in swine melanoma tumor tissue and normal gray skin tissue were compared by aminoacylation of the unfractionated tRNA preparations. Of the seventeen amino acids studied, seven showed differences in rate of acceptance to tRNAs from normal and tumor tissues; the tRNAs of two amino acids, tyrosine and glycine, showed dramatic three fold increases in melanoma tumor. As melanin biosynthesis proceeds from tyrosine oxidation the investigations focused on the increase in tyrosine tRNA. Kinetic analysis of tyrosine aminoacylation to normal and melanoma tRNAs revealed no differences. Analysis of the isoaccepting species of tRNATyr from normal skin and melanoma tumor tissues identified three isoacceptors; tRNATyr, represented the predominant species in normal gray skin, while tRNA2Tyr predominated in melanoma tumor tissue. The tyrosine acceptances by tRNAs from three human melanoma cell lines were analyzed and found to be variable, but isoaccepting species analysis of the tRNATyr of these three cell lines still showed a correlation between the preponderance of tRNA2Tyr and extent of tyrosine acceptance. Additionally the enzymatic activity for the oxidation of tyrosine was found to be related to tyrosine acceptance and tRNA2Tyr predominance..     

Diverse pathways for maturation of the Na,K-ATPase β1 and β2 subunits in the endoplasmic reticulum of Madin-Darby canine kidney cells

Proper folding of the Na,K-ATPase β subunits followed by assembly with the α subunits is necessary for their export from the endoplasmic reticulum (ER). Here we examine roles of the ER lectin chaperone, calnexin, and non-lectin chaperone, BiP, in folding and quality control of the β(1) and β(2) subunits in Madin-Darby canine kidney cells. Short term prevention of glycan-calnexin interactions by castanospermine slightly increases ER retention of β(1), suggesting minor involvement of calnexin in subunit folding. However, both prolonged incubation with castanospermine and removal of N-glycosylation sites do not affect the α(1)-assembly or trafficking of β(1) but increase the amount of the β(1)-bound BiP, showing that BiP can compensate for calnexin in assisting β(1) folding. In contrast to β(1), prevention of either N-glycosylation or glycan-calnexin interactions abolishes the α(1)-assembly and export of β(2) from the ER despite increased β(2)-BiP binding. Mutations in the α(1)-interacting regions of β(1) and β(2) subunits impair α(1) assembly but do not affect folding of the β subunits tested by their sensitivity to trypsin. At the same time, these mutations increase the amount of β-bound BiP but not of β-bound calnexin and increase ER retention of both β-isoforms. BiP, therefore, prevents the ER export of folded but α(1)-unassembled β subunits. These α(1)-unassembled β subunits are degraded faster than α(1)-bound β subunits, preventing ER overload. In conclusion, folding of the β(1) and β(2) subunits is assisted predominantly by BiP and calnexin, respectively. Folded β(1) and β(2) either assemble with α(1) or bind BiP. The α(1)-bound β subunits traffic to the Golgi, whereas BiP-bound β subunits are retained and degraded in the ER.     

All hands on deck—the role of chloroplasts, endoplasmic reticulum, and the nucleus in driving plant innate immunity

Plant innate immunity is mediated by cell membrane and intracellular immune receptors that function in distinct and overlapping cell-signaling pathways to activate defense responses. It is becoming increasingly evident that immune receptors rely on components from multiple organelles for the generation of appropriate defense responses. This review analyzes the defense-related functions of the chloroplast, nucleus, and endoplasmic reticulum (ER) during plant innate immunity. It details the role of the chloroplasts in synthesizing defense-specific second messengers and discusses the retrograde signal transduction pathways that exist between the chloroplast and nucleus. Because the activities of immune modulators are regulated, in part, by their subcellular localization, the review places special emphasis on the dynamics and nuclear–cytoplasmic transport of immune receptors and regulators and highlights the importance of this process in generating orderly events during an innate immune response. The review also covers the recently discovered contributions of the ER quality-control pathways in ensuring the signaling competency of cell surface immune receptors or immune regulators.     

Fluorescent phosphocholine—a specific marker for the endoplasmic reticulum and for lipid droplets in Chara internodal cells

The staining pattern of 1,2-bis(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-undecanoyl)-sn-glycero-3-phosphocholine (Bodipy PC) was investigated in internodal cells of the green alga Chara corallina. Ten minutes after dye addition, Bodipy-PC-derived fluorescence appeared in lipid droplets and after 1 h in the cortical endoplasmic reticulum (ER) and in the inner ER tubes. Staining of the ER required energy but was independent of an intact actin or microtubule cytoskeleton and independent of vesicular endocytosis. The size of the lipid droplets varied between 0.25 µm in elongating cells and 3.2 µm in senescent internodes. They moved together with or along the cortical ER cisternae in a cytoskeleton-independent manner or remained immobile up to several minutes. Detachment of lipid droplets from the cortical ER or fusion of lipid droplets was never observed. The results of this study suggest that Bodipy PC is a valuable, less toxic alternative to 3,3′-dihexyloxacarbocyanine iodide (DiOC6) staining of the ER in Chara. They confirm an earlier report about microtubule-dependent cortical ER morphology and dynamics in elongating internodes and offer new perspectives for the study of organelle interactions.     

Cdc37/Hsp90 protein-mediated regulation of IRE1α protein activity in endoplasmic reticulum stress response and insulin synthesis in INS-1 cells

IRE1α is an endoplasmic reticulum (ER) localized signaling molecule critical for unfolded protein response. During ER stress, IRE1α activation is induced by oligomerization and autophosphorylation in its cytosolic domain, a process triggered by dissociation of an ER luminal chaperone, binding immunoglobulin-protein (BiP), from IRE1α. In addition, inhibition of a cytosolic chaperone protein Hsp90 also induces IRE1α oligomerization and activation in the absence of an ER stressor. Here, we report that the Hsp90 cochaperone Cdc37 directly interacts with IRE1α through a highly conserved cytosolic motif of IRE1α. Cdc37 knockdown or disruption of Cdc37 interaction with IRE1α significantly increased basal IRE1α activity. In INS-1 cells, Hsp90 inhibition and disruption of IRE1α-Cdc37 interaction both induced an ER stress response and impaired insulin synthesis and secretion. These data suggest that Cdc37-mediated direct interaction between Hsp90/Cdc37 and an IRE1α cytosolic motif is important to maintain basal IRE1α activity and contributes to normal protein homeostasis and unfolded protein response under physiological stimulation.