Rough Endoplasmic Reticulum Membranes Research Articles

Cargo selection by specific kinesin light chain 1 isoforms.

Kinesin-1 drives the movement of diverse cargoes, and it has been proposed that specific kinesin light chain (KLC) isoforms target kinesin-1 to these different structures. Here, we test this hypothesis using two in vitro motility assays, which reconstitute the movement of rough endoplasmic reticulum (RER) and vesicles present in a Golgi membrane fraction. We generated GST-tagged fusion proteins of KLC1B and KLC1D that included the tetratricopeptide repeat domain and the variable C-terminus. We find that preincubation of RER with KLC1B inhibits RER motility, whereas KLC1D does not. In contrast, Golgi fraction vesicle movement is inhibited by KLC1D but not KLC1B reagents. Both RER and vesicle movement is inhibited by preincubation with the GST-tagged C-terminal domain of ubiquitous kinesin heavy chain (uKHC), which binds to the N-terminal domain of uKHC and alters its interaction with microtubules. We propose that although the TRR domains are required for cargo binding, it is the variable C-terminal region of KLCs that are vital for targeting kinesin-1 to different cellular structures.

Evidence for a large conductance voltage gated cationic channel in rough endoplasmic reticulum of rat hepatocytes

In this work, we report the single channel characterization of a voltage gated cationic channel from rough endoplasmic reticulum (RER) membranes of rat hepatocytes incorporated into a planar lipid bilayer. The channel was found to be cation selective with a main conductance of 598 ± 20 pS in 200 mM KCl cis/50 mM KCl trans. The channel open probability appeared voltage dependent with a voltage for half activation ( V 1/2) of 38 mV and an effective gating charge z of −6.66. Adding either 4-AP (5 mM) or ATP (2.5 mM) to the side corresponding to the cell internal medium caused a strong inhibition of the channel activity. This channel is likely to be involved in maintaining proper cation homeostasis in the RER of hepatocytes.

Postmortem stability of pituitary hormones in the human adenohypophysis

The hypophysis is embedded in the fossa at the base of skull, having important functions in the hormonal system. The present study investigated its postmortem morphological changes and the stability of adenohypopyseal hormones. The pituitaries were collected at autopsy 6 h to 20 days postmortem and were studied by histology, immunohistochemistry and electron microscopy. To avoid the influence of prolonged brain hypoxia or swelling, subjects who survived not longer than 12 h were examined. Histological changes were seen in the nucleus 6 h after death, followed by cytoplasmic changes, and the cell shapes were hardly identifiable 7 days postmortem. Electron microscopy revealed evident ultra-structural changes 6 h postmortem, involving rough endoplasmic reticulum, Golgi complexes, mitochondria, nuclei and cell membranes. However, secretory granules remained well preserved 7 days postmortem. Immunostaining showed positivities for growth hormone, prolactin, adenocorticotropic hormone, luteinizing hormone and thyroid-stimulating hormone up to 15 days after death. These findings suggest the usefulness of immunohistochemical investigation of the adenohypophysis for estimating the time of death and endocrinologic evaluation in decomposed cadavers.

Herpes Simplex Virus 1 Envelopment Follows Two Diverse Pathways

Herpesvirus envelopment is assumed to follow an uneconomical pathway including primary envelopment at the inner nuclear membrane, de-envelopment at the outer nuclear membrane, and reenvelopment at the trans-Golgi network. In contrast to the hypothesis of de-envelopment by fusion of the primary envelope with the outer nuclear membrane, virions were demonstrated to be transported from the perinuclear space to rough endoplasmic reticulum (RER) cisternae. Here we show by high-resolution microscopy that herpes simplex virus 1 envelopment follows two diverse pathways. First, nuclear envelopment includes budding of capsids at the inner nuclear membrane into the perinuclear space whereby tegument and a thick electron dense envelope are acquired. The substance responsible for the dense envelope is speculated to enable intraluminal transportation of virions via RER into Golgi cisternae. Within Golgi cisternae, virions are packaged into transport vacuoles containing one or several virions. Second, for cytoplasmic envelopment, capsids gain direct access from the nucleus to the cytoplasm via impaired nuclear pores. Cytoplasmic capsids could bud at the outer nuclear membrane, at membranes of RER, Golgi cisternae, and large vacuoles, and at banana-shaped membranous entities that were found to continue into Golgi membranes. Envelopes originating by budding at the outer nuclear membrane and RER membrane also acquire a dense substance. Budding at Golgi stacks, designated wrapping, results in single virions within small vacuoles that contain electron-dense substances between envelope and vacuolar membranes.

Endocytosis Function of a Ligand-Gated Ion Channel Homolog in Caenorhabditis elegans

Ligand-gated ion channels are transmembrane proteins that respond to a variety of transmitters, including acetylcholine, gamma-aminobutyric acid (GABA), glycine, and glutamate [1 and 2]. These proteins play key roles in neurotransmission and are typically found in the nervous system and at neuromuscular junctions [3]. Recently, acetylcholine receptor family members also have been found in nonneuronal cells, including macrophages [4], keratinocytes [5], bronchial epithelial cells [5], and endothelial cells of arteries [6]. The function of these channels in nonneuronal cells in mammals remains to be elucidated, though it has been shown that the acetylcholine receptor alpha7 subunit is required for acetylcholine-mediated inhibition of tumor necrosis factor release by activated macrophages [4]. We show that cup-4, a gene required for efficient endocytosis of fluids by C. elegans coelomocytes, encodes a protein that is homologous to ligand-gated ion channels, with the highest degree of similarity to nicotinic acetylcholine receptors. Worms lacking CUP-4 have reduced phosphatidylinositol 4,5-bisphosphate levels at the plasma membrane, suggesting that CUP-4 regulates endocytosis through modulation of phospholipase C activity.

Interaction of Microtubule-associated Protein-2 and p63: A NEW LINK BETWEEN MICROTUBULES AND ROUGH ENDOPLASMIC RETICULUM MEMBRANES IN NEURONS

Neurons are polarized cells presenting two distinct compartments, dendrites and an axon. Dendrites can be distinguished from the axon by the presence of rough endoplasmic reticulum (RER). The mechanism by which the structure and distribution of the RER is maintained in these cells is poorly understood. In the present study, we investigated the role of the dendritic microtubule-associated protein-2 (MAP2) in the RER membrane positioning by comparing their distribution in brain subcellular fractions and in primary hippocampal cells and by examining the MAP2-microtubule interaction with RER membranes in vitro. Subcellular fractionation of rat brain revealed a high MAP2 content in a subfraction enriched with the endoplasmic reticulum markers ribophorin and p63. Electron microscope morphometry confirmed the enrichment of this subfraction with RER membranes. In cultured hippocampal neurons, MAP2 and p63 were found to concomitantly compartmentalize to the dendritic processes during neuronal differentiation. Protein blot overlays using purified MAP2c protein revealed its interaction with p63, and immunoprecipitation experiments performed in HeLa cells showed that this interaction involves the projection domain of MAP2. In an in vitro reconstitution assay, MAP2-containing microtubules were observed to bind to RER membranes in contrast to microtubules containing tau, the axonal MAP. This binding of MAP2c microtubules was reduced when an anti-p63 antibody was added to the assay. The present results suggest that MAP2 is involved in the association of RER membranes with microtubules and thereby could participate in the differential distribution of RER membranes within a neuron.

Ultrastructural characterization of the hemocytes of Culex quinquefasciatus (DIPTERA: Culicidae)

Six hemocytes cell types from Culex quinquefasciatus were identified by light and transmission electron microscopy: They are prohemocytes (9.3%), spherulocytes (1.6%), adipohemocytes (0.8%), oenocytoids (4.6%), plasmatocytes (43.4%) and granulocytes (40.3%). The prohemocytes were the smallest hemocytes encountered in the hemolymph, displaying a large and centrally located nucleus, almost filling the whole cell. The spherulocytes, which were small hemocytes, presented small and numerous spherules with a lamellar pattern and an electron-dense core. Rare adipohemocytes were observed in the C. quinquefasciatus hemolymph, presenting large nucleus with an evident nucleolus, cytoplasm containing rough endoplasmic reticulum (RER), mitochondriae and lipid inclusions. C. quinquefasciatus oenocytoids showed homogeneous cytoplasm with several granules, completely or partially filled with amorphous material. These cells showed abundant smooth endoplasmic reticulum (SER) and dense mitochondriae. By light microscopy analysis we identified two morphological types of plasmatocytes, granular and agranular. However, ultrastructural investigation revealed that the granular cells contained lipid inclusion between RER membranes, instead of membrane-bounded granules. The granulocytes presented a fusiform or circular profile and displayed a unique and very complex process of granules formation, including organization of polysomes inside vesicles that protrude from the Golgi system, synthesis of a proteinaceous material, condensation of the granule matrix and recycling of endoplasmic membranes. Intense endocytic pathways were also observed in the granulocytes.

Cyclooxygenase‐2 expression and relationship to tumour progression in human renal cell carcinoma

Cyclooxygenase (COX), which catalyses the synthesis of prostaglandins from arachidonic acid, has two isoforms; COX-1 and COX-2. There is ample evidence to suggest an important role for COX-2 in cancer. The aim of this study was to evaluate the clinical significance of COX-2 expression and its localization in the development and progression of human renal cell carcinoma (RCC). The expression and localization of COX-2 were evaluated in human RCC tissues from 75 patients by immunohistochemistry. Immunoreactive COX-2 protein was observed in all cases of RCC, and the levels of COX-2 expression were correlated with tumour grade and pathological stage. Expression of COX-2 was higher in the granular cell subtype than in the clear cell subtype of RCC. Immunoelectron microscopy revealed that COX-2 was expressed in the nuclear membrane, rough endoplasmic reticulum, Golgi complex and mitochondrial membrane of RCC cells. COX-2 overexpression within these intracellular organelles in RCC may be associated with renal cell carcinogenesis and COX-2 may be a useful biomarker in RCC.

Differences in membrane association and sub-cellular distribution between NS2-3 and NS3 of bovine viral diarrhoea virus

The sub-cellular location and mechanism of membrane association of NS3 and NS2-3 polypeptides of bovine viral diarrhoea virus (BVDV) have been examined. Both NS3 and NS2-3 proteins were detected in post-nuclear membrane fractions but not in cytosolic fractions of BVDV infected cells; a proportion of NS3, but not NS2-3, could be dissociated from the membranes with 800 mM KCl or at pH 11. Following extraction with 1% Triton X-114, NS3 was predominantly present in the aqueous phase, but NS2-3 was only recovered in the detergent phase. Confocal microscopy showed that in BVDV infected cells, NS3 and/or NS2-3 co-localise with the endoplasmic reticulum (ER) protein, ERP60, but not Golgi or lysosomal proteins. Sub-cellular fractionation analysis demonstrated that NS2-3 was almost exclusively associated with the rough ER membrane but a significant proportion of NS3 was present in the smooth ER membrane fractions in addition to the rough ER membrane. These differences in the distribution of NS2-3 and NS3 on ER membranes in cells infected with cytopathogenic (CP) strains of BVDV were also observed using confocal microscopy and antibodies that are specific to either NS2 or NS3. This distinct distribution of NS3 and NS2-3 on the ER membrane has revealed a further difference between CP and non-cytopathogenic (NCP) strains of BVDV.

Gangliosides are receptors for murine polyoma virus and SV40.

Polyoma virus (Py) and simian virus 40 (SV40) travel from the plasma membrane to the endoplasmic reticulum (ER) from where they enter the cytosol and then the nucleus to initiate infection. Here we demonstrate that specific gangliosides can serve as plasma membrane receptors for these viruses, GD1a and GT1b for Py and GM1 for SV40. Binding and flotation assays were used to show that addition of these gangliosides to phospholipid vesicles allowed specific binding of the respective viruses. The crystal structure of polyoma VP1 with a sialic acid-containing oligosaccharide was used to derive a model of how the two terminal sugars (sialic acid-alpha2,3-galactose) in one branch of GD1a and GT1b are recognized by the virus. A rat cell line deficient in ganglioside synthesis is poorly infectible by polyoma and SV40, but addition of the appropriate gangliosides greatly facilitates virus uptake, transport to the ER and infection. Lipid binding sites for polyoma are shown to be present in rough ER membranes, suggesting that the virus travel with the ganglioside(s) from the plasma membranes to the ER.

Expression of Inducible Nitric-oxide Synthase and Intracellular Protein Tyrosine Nitration in Vascular Smooth Muscle Cells: ROLE OF REACTIVE OXYGEN SPECIES

A significant increase in the induction of inducible nitric-oxide synthase (iNOS) protein expression and in the levels of nitrite plus nitrate was observed in rat aortic smooth muscle cells (RASMCs) stably transfected with catalase (RASMC-2C2) as compared with empty vector-transfected RASMC-V4 cells after exposure to cytokines and lipopolysaccharide. The increased expression of iNOS protein in the RASMC-2C2 cells was associated with a significant activation of nuclear transcription factor kappaB, one of the transcriptional regulators of iNOS expression. The induction of iNOS was also accompanied by increased protein tyrosine nitration in both cell types as revealed by immunocytochemical staining and high pressure liquid chromatography with on-line electrospray ionization tandem mass spectrometry. Nitrotyrosine formation was inhibited by 1400W, an iNOS inhibitor, by 4-(2-aminoethyl) benzenesulfonyl fluoride, an inhibitor of NADPH oxidase, and by the superoxide dismutase mimetic M40403, but not by the peroxidase inhibitor 4-aminobenzoic hydrazide. Electron microscopy using affinity-purified anti-nitrotyrosine antibodies revealed labeling at the cytosolic side of the rough endoplasmic reticulum membranes, in the nucleus, occasionally in mitochondria, and consistently within the fibrillar layer underneath the plasma membrane. Collectively, the data in this model system indicate that hydrogen peroxide, by inhibiting the activation of nuclear transcription factor kappaB, prevents iNOS expression, whereas superoxide contributes in a precise pattern of intracellular protein tyrosine nitration.

Targeting of rough endoplasmic reticulum membrane proteins and ribosomes in invertebrate neurons.

The endoplasmic reticulum (ER) is divided into rough and smooth domains (RER and SER). The two domains share most proteins, but RER is enriched in some membrane proteins by an unknown mechanism. We studied RER protein targeting by expressing fluorescent protein fusions to ER membrane proteins in Caenorhabditis elegans. In several cell types RER and general ER proteins colocalized, but in neurons RER proteins were concentrated in the cell body, whereas general ER proteins were also found in neurites. Surprisingly RER membrane proteins diffused rapidly within the cell body, indicating they are not localized by immobilization. Ribosomes were also concentrated in the cell body, suggesting they may be in part responsible for targeting RER membrane proteins.

Zein protein interactions, rather than the asymmetric distribution of zein mRNAs on endoplasmic reticulum membranes, influence protein body formation in maize endosperm.

Prolamin-containing protein bodies in maize endosperm are composed of four different polypeptides, the alpha-, beta-, gamma-, and delta-zeins. The spatial organization of zeins within the protein body, as well as interactions between them, suggests that the localized synthesis of gamma-zeins could initiate and target protein body formation at specific regions of the rough endoplasmic reticulum. To investigate this possibility, we analyzed the distribution of mRNAs encoding the 22-kD alpha-zein and the 27-kD gamma-zein proteins on cisternal and protein body rough endoplasmic reticulum membranes. In situ hybridization revealed similar frequencies of the mRNAs in both regions of the endoplasmic reticulum, indicating that the transcripts are distributed more or less randomly. This finding implies that zein protein interactions determine protein body assembly. To address this question, we expressed cDNAs encoding alpha-, beta-, gamma-, and delta-zeins in the yeast two-hybrid system. We found strong interactions among the 50-, 27-, and 16-kD gamma-zeins and the 15-kD beta-zein, consistent with their colocalization in developing protein bodies. Interactions between the 19- and 22-kD alpha-zeins were relatively weak, although each of them interacted strongly with the 10-kD delta-zein. Strong interactions were detected between the alpha- and delta-zeins and the 16-kD gamma-zein and the 15-kD beta-zein; however, the 50- and 27-kD gamma-zeins did not interact with the alpha- and delta-zein proteins. We identified domains within the 22-kD alpha-zein that bound preferentially the alpha- and delta-zeins and the beta- and gamma-zeins. Affinities between zeins generally were consistent with results from immunolocalization experiments, suggesting an important role for the 16-kD gamma-zein and the 15-kD beta-zein in the binding and assembly of alpha-zeins within the protein body.

The Cost of Exposing a Hydrophobic Loop and Implications for the Functional Role of 4.5 S RNA in the Escherichia coli Signal Recognition Particle

The signal recognition particle (SRP) is an RNA-protein complex that directs ribosomes to the rough endoplasmic reticulum membrane by binding to targeting signals found on the nascent chain of proteins destined for export to the endoplasmic reticulum. We found evidence from studies with fragments of the protein component of the Escherichia coli SRP that a long hydrophobic loop (the so-called "finger loop") is detrimental to the stability of its signal peptide-binding domain, the M domain. This hydrophobic loop is highly conserved and thus may have a critical role in the function of the SRP. Given our previously reported evidence that 4.5 S RNA stabilizes the tertiary fold of the M domain (Zheng, N., and Gierasch, L. M. (1997) Mol. Cell 1, 79-87), we now propose that the functional requirement for 4.5 S RNA resides in its ability to counteract the destabilizing influence of the finger loop.

Expression of apoptosis-related proteins in involuting mammary gland of sow

The expression of apoptosis-related proteins: TGF-β1 (local inductor), TGF-β-receptor, Bax (promoter), Bcl-2 (inhibitor) and CPP-32 (executor of apoptosis); the subcellular distribution of Bax; as well as the number and morphology of apoptotic cells in low-, moderate-, and high-involuted mammary glands of sow (four to six days after weaning) were investigated. The immunohistochemical study demonstrated a statistically significant increase in the integrated optical density (IOD) of lobuloalveolar mammary tissue labelling with anti-Bax antibody from low- through moderate-, to high-involuted glands. The immunoelectron microscopy revealed that Bax was localised in the cytosol, on the membranes of mitochondrium and rough endoplasmic reticulum, in nuclear envelope pores, and over heterochromatin of mammary epithelial cells. The increase in Bax/Bcl-2 ratio (2.3, 2.6 and 5.6 for low-, moderate-, and high-involuted glands, respectively) indicated the increasing susceptibility of mammary epithelial cells to apoptosis in the course of involution. The highest Bax/Bcl-2 ratio in high-involuted glands coincided with the highest expression of CPP-32 (caspase 3), TGF-β1 and TGF-β1 receptor. The number of apoptotic cells (simultaneous TUNEL and Hoechst 33342 staining) was 2.7, 3.4 and 3.8% for low-, moderate-, and high-involuted glands, respectively. The ultrastructural evaluation showed characteristic morphological features of apoptosis such as: margination and condensation of chromatin; pyknosis and fragmentation of the nucleus; and formation of apoptotic bodies. Phagocytosis of apoptotic cells by macrophages was also documented. The results of the present study suggest the involvement of Bax/Bcl-2 check-point in the regulation, CPP-32 in the execution, but TGF-β1 in the induction of apoptosis of mammary epithelial cells in the involuting mammary gland of sow.

In Vitro Binding of Ribosomes to the β Subunit of the Sec61p Protein Translocation Complex

The Sec61p complex forms the core element of the protein translocation complex (translocon) in the rough endoplasmic reticulum (rough ER) membrane. Translating or nontranslating ribosomes bind with high affinity to ER membranes that have been stripped of ribosomes or to liposomes containing purified Sec61p. Here we present evidence that the beta subunit of the complex (Sec61beta) makes contact with nontranslating ribosomes. A fusion protein containing the Sec61beta cytoplasmic domain (Sec61beta(c)) prevents the binding of ribosomes to stripped ER-derived membranes and also binds to ribosomes directly with an affinity close to the affinity of ribosomes for stripped ER-derived membranes. The ribosome binding activity of Sec61beta(c), like that of native ER membranes, is sensitive to high salt concentrations and is not based on an unspecific charge-dependent interaction of the relatively basic Sec61beta(c) domain with ribosomal RNA. Like stripped ER membranes, the Sec61beta(c) sequence binds to large ribosomal subunits in preference over small subunits. Previous studies have shown that Sec61beta is inessential for ribosome binding and protein translocation, but translocation is impaired by the absence of Sec61beta, and it has been proposed that Sec61beta assists in the insertion of nascent proteins into the translocation pore. Our results suggest a physical interaction of the ribosome itself with Sec61beta; this may normally occur alongside interactions between the ribosome and other elements of Sec61p, or it may represent one stage in a temporal sequence of binding.

Paraspermatogenesis in Littoraria (Palustorina) articulata, with reference to other Littorinidae (Littorinoidea, Caenogastropoda)

Abstract. The ultrastructure of paraspermatogenesis is examined in the littorinid subfamily Littorininae, with special emphasis on Littoraria (Palustorina) articulata (PHILIPPI 1846). In particular the study focuses on the fate of the nucleus and origin of the rod bodies during parasperm development. Parasperm of the Littorininae are rounded or oblong cells, which undergo an abortive meiosis and eliminate part of the nucleus but often retain a nuclear remnant. The cytoplasm is filled with numerous spherical vesicles in all Littorininae, but in Littoraria (and in certain species of Nodilittorina, Tectarius and Cenchritis) dense ‘rod‐bodies’ also occur. Littoraria (Palustorina) are unique in possessing a flagellum‐like structure termed the ‘pseudotrich”, which lacks an axoneme but contains microtubules during its development. Paraspermatogonia differ from euspermatogonia in the structure of the nucleus and in the extensive rough endoplasmic reticulum (RER) and swollen cytoplasm. Two types of secretions develop in Littoraria: (1) numerous, spherical granules (composed of putative glycoprotein, also seen in other Littorininae) and (2) rhomboid granules (composition uncertain but reacting positively to RNA stains; these granules arising within RER cisternae close to the nucleus). As the rhomboid granules fuse to form the larger, rod‐bodies (polygonal in cross section), the RER membrane enclosing the rod‐bodies becomes confluent with the outer nuclear membrane, thereby forming a common compartment. Results of this study clearly show that the rod‐bodies are secretions of the RER cisternae and not, as claimed in some light microscopic accounts, the product of fusion of eusperm nuclei which have entered the parasperm cytoplasm (either by active eusperm penetration or by phagocytosis). Developmental characteristics of littorinid parasperm show differences between species and may, in some cases, provide characters diagnostic of subgenera.

Ultrastructural and immunohistochemical analysis of cholangiocarcinoma in immunized Syrian golden hamsters infected with Opisthorchis viverrini and administered with dimethylnitrosamine

Utilizing the experimental model in Syrian golden hamsters, we explored the role of immunization in carcinogenesis. The animals, which were infected with liver flukes ( Opisthorchis viverrini) , and administered a subcarcinogenic dose of dimethylnitrosamine, developed cancer. Pre-immunizing with a crude somatic antigen did not reduce cancer development, but accelerated carcinogenesis. Histopathological analysis of the cancer tissues was done once at week 30 and again at week 39 using H and E staining, immunostaining for the p53 tumor suppressor phosphoprotein, and electron microscopy. Thirty weeks after immunization, the immunized hamsters developed tubular adenocarcinoma at a higher rate (71.43%) than the non-immunized group (20.00%). This rate (20.00%) increased to 63.64% by week 39. The small foci cancer in the non-immunized group decreased in frequency from 80.00% (at week 30) to 36.36% (by week 39), suggesting the small foci cancer progressed to tubular adenocarcinoma during the 9-week interval. Most of the observed tubular adenocarcinoma was well differentiated. Nearly all hamsters that tested positive for cancer also tested positive for p53 immunostaining in the epithelia of the small bile ducts. The positive reaction for p53-immunostaining was localized in the rough endoplasmic reticulum, Golgi apparatus and perinuclear membranes. The electron micrographs of these positive p53-immunostained cells showed characteristics of early cancer. The detection of p53 in early cancer development makes it a candidate as a tumor marker.

Developing prolamine protein bodies are associated with the cortical cytoskeleton in rice endosperm cells.

The mRNAs that encode the prolamine storage proteins in rice (Oryza sativa L.) endosperm cells are enriched on the surface of the prolamine protein bodies (PBs), a subcellular structure consisting of a prolamine intracisternal granule surrounded by rough endoplasmic reticulum membrane. Previous biochemical studies (D.G. Muench et al., 1998, Plant Physiol. 116: 559-569) have shown that prolamine mRNAs may be anchored to the PB surface via the cytoskeleton. To better understand the mechanism and role of mRNA localization in rice endosperm cells, we studied the subcellular development of prolamine PBs and their relationship with the cytoskeleton in rice endosperm cells. Confocal microscopy of endosperm cells showed that, unlike the glutelin PBs, the developing prolamine PBs are not randomly distributed within the cell, but instead are often enriched in the cortical region of the cell only a few micrometers beneath the plasma membrane. In addition, the peripheral prolamine PBs are closely associated with the cortical microtubule and actin filament networks. The cortical enrichment of rice prolamine protein bodies represents a unique example of endoplasmic reticulum subdomain localization in plant cells. The interaction of this endoplasmic reticulum subdomain with the cytoskeleton provides new insights on the possible mechanism and role of mRNA localization in plants.

Transport of free and N-linked oligomannoside species across the rough endoplasmic reticulum membranes.

The N-glycosylation process occurs in the rough endoplasmic reticulum. It requires the transport of glycosyl donors into the lumen and the exit of the glycosylated products toward the secretory pathway. Besides this main flow, the formation of free oligomannosides, glycopeptides, and misfolded glycoproteins which do not enter the secretory pathway and are cleared out of the endoplasmic reticulum by specific transports has been demonstrated. This review focuses on the export mechanisms of these three side products of the N-glycosylation process and discusses their physiological significance.