Polypeptide translocation machinery of the yeast endoplasmic reticulum

Proteins enter the secretory pathway by two general routes. In one, the complete polypeptide is made in the cytoplasm and held in an incompletely folded state by chaperoning adenosine triphosphatases (ATPases) such as hsp70. InSaccharomyces cerevisiae, fully synthesized secretory precursors engage the endoplasmic reticulum (ER) membrane by interaction with a set of Sec proteins comprising the polypeptide translocation apparatus (Sec61p, Sec62p, Sec63p, Sec71p, Sec72p). Productive interaction requires displacement of hsp70 from the precursor, a reaction that is facilitated by Ydj1p, a homologue of theEscherichia coli DnaJ protein. Both DnaJ and Ydj1p regulate chaperone activity by stimulating the ATPase activity of their respective hsp70 partners (E. coli DnaK andS. cerevisiae Ssa1p, resepectively). In the ER lumen, another hsp70 chaperone, BiP, binds ATP and interacts with the ER membrane via its contact with a peptide loop of Sec63p. This loop represents yet another DnaJ homologue in that it contains a region of 70 residue similarity to the J box, the most conserved region of the DnaJ family of proteins. In the presence of ATP, under conditions in which BiP can bind to Sec63p, the secretory precursor passes from the cytosol into the lumen through a membrane channel formed by Sec61 p. A second route to the membrane pore that is used by many other secretory precursors, particularly in mammalian cells, requires that the polypeptide engage the ER membrane as the nascent chain emerges from the ribosome. Such cotranslational translocation bypasses the need for certain Sec proteins, instead utilizing an alternate set of cytosolic and membrane factors that allows the nascent chain to be inserted directly into the Sec61p channel.     

Mechanisms for exporting large-sized cargoes from the endoplasmic reticulum

Cargo proteins exported from the endoplasmic reticulum to the Golgi apparatus are typically transported in coat protein complex II (COPII)-coated vesicles of 60–90 nm diameter. Several cargo molecules including collagens and chylomicrons form structures that are too large to be accommodated by these vesicles, but their secretion still requires COPII proteins. Here, we first review recent progress on large cargo secretions derived especially from animal models and human diseases, which indicate the importance of COPII proteins. We then discuss the recent isolation of specialized factors that modulate the process of COPII-dependent cargo formation to facilitate the exit of large-sized cargoes from the endoplasmic reticulum. Based on these findings, we propose a model that describes the importance of the GTPase cycle for secretion of oversized cargoes. Next, we summarize reports that describe the structures of COPII proteins and how these results provide insight into the mechanism of assembly of the large cargo carriers. Finally, we discuss what issues remain to be solved in the future.     

Connection between a mitochondrion and endoplasmic reticulum in liver

Zusammenfassung Es wird der Zusammenhang von endoplasmatischem Retikulum und Mitochondrien in der Leber bestrahlter Rhesusaffen ermittelt.     

Similar paracrystals of endoplasmic reticulum in the photoemitters and the photoreceptors of scale-worms

Summary Paracrystals of tubular endoplasmic reticulum are the characteristic of the luminous cells in the elytra of polynoïd worms, and they are the sources of luminescence as well as of fluorescence. Structurally similar paracrystals are now found to be the main constituent of the lens in the eyes of all the Aphroditidae.     

Signaling pathways between the plasma membrane and endoplasmic reticulum calcium stores

This review discusses multiple ways in which the endoplasmic reticulum participates in and is influenced by signal transduction pathways. The endoplasmic reticulum provides a Ca2+ store that can be mobilized either by calcium-induced calcium release or by the diffusible messenger inositol 1,4,5-trisphosphate. Depletion of endoplasmic reticulum Ca2+ stores provides a signal that activates surface membrane Ca2+ channels, a process known as capacitative calcium entry. Depletion of endoplasmic reticulum stores can also signal long-term cellular responses such as gene expression and programmed cell death or apoptosis. In addition to serving as a source of cellular signals, the endoplasmic reticulum is also functionally and structurally modified by the Ca2+ and protein kinase C pathways. Elevated cytoplasmic Ca2+ causes a rearrangement and fragmentation of endoplasmic reticulum membranes. Protein kinase C activation reduces the storage capacity of the endoplasmic reticulum Ca2+ pool. In some cell types, protein kinase C inhibits capacitative calcium entry. Protein kinase C activation also protects the endoplasmic reticulum from the structural effects of high cytoplasmic Ca2+. The emerging view is one of a complex network of pathways through which the endoplasmic reticulum and the Ca2+ and protein kinase C signaling pathways interact at various levels regulating cellular structure and function.     

Role of the nuclear membrane in smooth endoplasmic reticulum formation in white rat pinealocytes

Résumé La glande pinéale du rat blanc est étudiée ici au microscope électronique. L'association observée entre les vésicules agranulaires et les «blebs» de la membrane nucléaire est un argument en faveur de la théorie selon laquelle l'origine du reticulum endoplasmique est à rechercher dans la membrane nucléaire.     

Annotating proteins from endoplasmic reticulum and Golgi apparatus in eukaryotic proteomes

The sub-cellular localization of a native protein constitutes one coarse-grained aspect of its function. Transport between compartments is often regulated through short sequence motifs. Here, we analyzed experimentally characterized endoplasmic reticulum (ER)/ Golgi retrieval motifs and investigated the accuracy of homology-transfer. Only the C-terminal ER retrieval motifs KDEL, HDEL and AIAKE were sufficiently specific. However, even unspecific motifs may help, provided we know the probability for localization given the motif. We provided such estimates. We also rigorously estimated the accuracy and coverage for inferring ER and Golgi localization through homology-transfer by sequence similarity. In entire proteomes, we could thereby annotate 3304 ER (3182 membrane) and 1853 Golgi (759 membrane) proteins. We identified another putative 5157 globular and 3941 membrane ER or Golgi proteins. Each experimental annotation yielded, on average, one to three high-accuracy and five to six low-accuracy homology-transfers in the six proteomes. These numbers will increase with each new experimental annotation.Received 6 January 2004; received after revision 10 March 2004; accepted 29 March 2004     

A concept of intracellular transmission of excitation by means of the endoplasmic reticulum

Zusammenfassung Die morphologische Beobachtung von getrennten cytoplasmatischen Phasen in geschlossenen Membranen führt zur Annahme von Membranpotentialen innerhalb der Zelle. Schreibt man den Membranen des endoplasmatischen Reticulums die Eigenschaften der Plasmamembran zu, so leiten sie die Erregung ins Zellinnere und dienen als Potentialspeicher.     

APP dimer formation is initiated in the endoplasmic reticulum and differs between APP isoforms

The amyloid precursor protein (APP) is part of a larger gene family, which has been found to form homo- or heterotypic complexes with its homologues, whereby the exact molecular mechanism and origin of dimer formation remains elusive. In order to assess the cellular location of dimerization, we have generated a cell culture model system in CHO-K1 cells, stably expressing human APP, harboring dilysine-based organelle sorting motifs [KKAA-endoplasmic reticulum (ER); KKFF-Golgi], accomplishing retention within early secretory compartments. We show that APP exists as disulfide-bonded dimers upon ER retention after it was isolated from cells, and analyzed by SDS-polyacrylamide gel electrophoresis under non-reducing conditions. In contrast, strong denaturing and reducing conditions, or deletion of the E1 domain, resulted in the disappearance of those dimers. Thus we provide first evidence that a fraction of APP can associate via intermolecular disulfide bonds, likely generated between cysteines located in the extracellular E1 domain. We particularly visualize APP dimerization itself and identified the ER as subcellular compartment of its origin using biochemical or split GFP approaches. Interestingly, we also found that minor amounts of SDS-resistant APP dimers were located to the cell surface, revealing that once generated in the oxidative environment of the ER, dimers remained stably associated during transport. In addition, we show that APP isoforms encompassing the Kunitz-type protease inhibitor (KPI) domain exhibit a strongly reduced ability to form cis-directed dimers in the ER, whereas trans-mediated cell aggregation of Drosophila Schneider S2-cells was isoform independent. Thus, suggesting that steric properties of KPI-APP might be the cause for weaker cis-interaction in the ER, compared to APP695. Finally, we provide evidence that APP/APLP1 heterointeractions are likewise initiated in the ER.     

Extensive endoplasmic reticulum: a distinguishing feature of auxin-producing plant cells

Résumé Le cytoplasme des cellules végétales capables de synthétiser l'auxine est caractérisé par l'abondance du réticulum endoplasmique. Les cellules dont la croissance est stimulée par de hautes concentrations de cette hormone contiennent beaucoup moins de réticulum endoplasmique. D'où l'hypothèse que le lieu de synthèse de l'auxine (dans les cellules capables de cette synthèse) est peut-être le réticulum endoplasmique.

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This work was begun at Manhattan College, Bronx, New York, supported by USPHS grant No. CA 06955, the Anna Fuller fund, and the Christine Sonntag Foundation. The work was continued at the Boyce Thompson Institute, Yonkers, New York, and completed at the Wistar Institute with support from USPHS grant No. CA 10815.

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I am grateful to Dr.G. Hagen for theNicotiana tissues, Dr.J. Lippincott for theP. savastanoi cultures and to Mrs.B. di Gregorio andE. Manasse for competent technical assistance.     

From the endoplasmic reticulum to the plasma membrane: mechanisms of CFTR folding and trafficking

CFTR biogenesis starts with its co-translational insertion into the membrane of endoplasmic reticulum and folding of the cytosolic domains, towards the acquisition of a fully folded compact native structure. Efficiency of this process is assessed by the ER quality control system that allows the exit of folded proteins but targets unfolded/misfolded CFTR to degradation. If allowed to leave the ER, CFTR is modified at the Golgi and reaches the post-Golgi compartments to be delivered to the plasma membrane where it functions as a cAMP- and phosphorylation-regulated chloride/bicarbonate channel. CFTR residence at the membrane is a balance of membrane delivery, endocytosis, and recycling. Several adaptors, motor, and scaffold proteins contribute to the regulation of CFTR stability and are involved in continuously assessing its structure through peripheral quality control systems. Regulation of CFTR biogenesis and traffic (and its dysregulation by mutations, such as the most common F508del) determine its overall activity and thus contribute to the fine modulation of chloride secretion and hydration of epithelial surfaces. This review covers old and recent knowledge on CFTR folding and trafficking from its synthesis to the regulation of its stability at the plasma membrane and highlights how several of these steps can be modulated to promote the rescue of mutant CFTR.     

BiP (GRP78), an essential hsp70 resident protein in the endoplasmic reticulum

BiP is a constitutively-expressed resident protein of the endoplasmic, reticulum (ER) of all eucaryotic cells, and belongs to the highly conserved hsp70 protein family. In the ER, BiP is involved in polypeptide translocation, protein folding and presumably protein degradation as well. These functions are essential to cell viability, as has been shown for yeast. In this review, I will summarize the structural features of hsp70 proteins and focus on those experiments which revealed the biological function of BiP.     

Effect of praseodymium on drug metabolism in rat liver smooth and rough endoplasmic reticulum

Summary A small i.v. dose (3 mg/kg) of a light lanthanon, praseodymium, impairs the drug metabolizing capacity of both the smooth and rough fractions of rat liver endoplasmic reticulum. This decrease in the activity of drug metabolizing enzymes and in the amount of cytochromes P-450 and b₅ is more pronounced in the rough endoplasmic reticulum fraction.