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Concept: Secretory pathway

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The Golgi apparatus has attracted intense attentions due to its fascinating morphology and vital role as the pivot of cellular secretory pathway since its discovery. However, its complex structure at the molecular level remains elusive due to limited approaches. In this study, the structure of Golgi apparatus, including the Golgi stack, cisternal structure, relevant tubules and vesicles, were directly visualized by high-resolution atomic force microscope. We imaged both sides of Golgi apparatus membranes and revealed that the outer leaflet of Golgi membranes is relatively smooth while the inner membrane leaflet is rough and covered by dense proteins. With the treatment of methyl-β-cyclodextrin and Triton X-100, we confirmed the existence of lipid rafts in Golgi apparatus membrane, which are mostly in the size of 20 nm -200 nm and appear irregular in shape. Our results may be of significance to reveal the structure-function relationship of the Golgi complex and pave the way for visualizing the endomembrane system in mammalian cells at the molecular level.

Concepts: Cell, Cell membrane, Golgi apparatus, Organelle, Endoplasmic reticulum, Protein targeting, Endomembrane system, Secretory pathway

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Protein traffic is of critical importance for normal cellular physiology. In eukaryotes, spherical transport vesicles move proteins and lipids from one internal membrane-bound compartment to another within the secretory pathway. The process of directing each individual protein to a specific destination (known as protein sorting) is a crucial event that is intrinsically linked to vesicle biogenesis. In this review, we summarize the principles of cargo sorting by the vesicle traffic machinery and consider the diverse mechanisms by which cargo proteins are selected and captured into different transport vesicles. We focus on the first two compartments of the secretory pathway: the endoplasmic reticulum and Golgi. We provide an overview of the complexity and diversity of cargo adaptor function and regulation, focusing on recent mechanistic discoveries that have revealed insight into protein sorting in cells.

Concepts: Protein, Cell, Golgi apparatus, Organelle, Endoplasmic reticulum, Lysosome, Protein targeting, Secretory pathway

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The Golgi complex has a central role in the intracellular sorting of secretory proteins. Anterograde transport through the Golgi has been explained by the movement of Golgi cisternae, known as cisternal maturation. Because this explanation is now appreciated to be incomplete, interest has developed in understanding tubules that connect the Golgi cisternae. Here we show that the coat protein I (COPI) complex sorts anterograde cargoes into these tubules in human cells. Moreover, the small GTPase CDC42 regulates bidirectional Golgi transport by targeting the dual functions of COPI in cargo sorting and carrier formation. CDC42 also directly imparts membrane curvature to promote COPI tubule formation. Our findings further reveal that COPI tubular transport complements cisternal maturation in explaining how anterograde Golgi transport is achieved, and that bidirectional COPI transport is modulated by environmental cues through CDC42.

Concepts: Cell, Cell membrane, Golgi apparatus, Organelle, Secretion, Endoplasmic reticulum, Protein targeting, Secretory pathway

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The signal recognition particle (SRP) is central to membrane protein targeting; SRP RNA is essential for SRP assembly, elongation arrest, and activation of SRP guanosine triphosphatases. In eukaryotes, SRP function relies on the SRP68-SRP72 heterodimer. We present the crystal structures of the RNA-binding domain of SRP68 (SRP68-RBD) alone and in complex with SRP RNA and SRP19. SRP68-RBD is a tetratricopeptide-like module that binds to a RNA three-way junction, bends the RNA, and inserts an α-helical arginine-rich motif (ARM) into the major groove. The ARM opens the conserved 5f RNA loop, which in ribosome-bound SRP establishes a contact to ribosomal RNA. Our data provide the structural basis for eukaryote-specific, SRP68-driven RNA remodeling required for protein translocation.

Concepts: Protein, Cell nucleus, RNA, Ribosome, Endoplasmic reticulum, Chaperone, Protein targeting, Secretory pathway

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We have designed a membrane ‘staple’, which consists of membrane-anchored repeats of the trans-aggregating FM domain that face the lumen of the secretory pathway. In the presence of the disaggregating drug these proteins transit the secretory pathway. When the drug is removed these proteins form electron-dense plaques which we term staples. Unexpectedly, when initially positioned within the cis-Golgi, staples remained at the cis face of the Golgi even after many hours. By contrast, soluble FM-aggregates transited the Golgi. Staples and soluble aggregates placed in cis-Golgi cisternae therefore have different fates. Whereas the membrane staples are located in the flattened, stacked central regions of the cisternae, the soluble aggregates are in the dilated rims. This suggests that while the cisternae are static on the time scale of protein traffic, the dilated rims are mobile and progress in the cis → trans direction via a mechanism that we term ‘Rim Progression’. DOI:http://dx.doi.org/10.7554/eLife.00558.001.

Concepts: Cell, Cell membrane, Golgi apparatus, Organelle, Endoplasmic reticulum, Lumen, Protein targeting, Secretory pathway

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The endoplasmic reticulum (ER) consists of the nuclear envelope and a reticulated interconnected network of tubules and sheets. ER sheets are studded with ribosomes and provide the entryway for proteins into the secretory pathway. ER tubules move dynamically on microtubules and form membrane contact sites with other organelles, where membranes are tethered, but not fused. This Snapshot reviews key biological processes that take place at ER contact sites with the Golgi, endosomes, and mitochondria.

Concepts: Cell, Eukaryote, Golgi apparatus, Organelle, Ribosome, Endoplasmic reticulum, Organelles, Secretory pathway

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Many secretory cells increase the synthesis and secretion of cargo proteins in response to specific stimuli. How cells couple increased cargo load with a coordinate rise in secretory capacity to ensure efficient transport is not well understood. We used thyroid cells stimulated with thyrotropin (TSH) to demonstrate a coordinate increase in the production of thyroid-specific cargo proteins and ER-Golgi transport factors, and a parallel expansion of the Golgi complex. TSH also increased expression of the CREB3L1 transcription factor, which alone caused amplified transport factor levels and Golgi enlargement. Furthermore, CREB3L1 potentiated the TSH-induced increase in Golgi volume. A dominant negative CREB3L1 construct hampered the ability of TSH to induce Golgi expansion, implying that this transcription factor contributes to Golgi expansion. Our findings support a model in which CREB3L1 acts as a downstream effector of TSH to regulate the expression of cargo proteins, and simultaneously increase the synthesis of transport factors and the expansion of the Golgi to synchronize the rise in cargo load with the amplified capacity of the secretory pathway.

Concepts: DNA, Gene, Cell, Golgi apparatus, Secretion, Endoplasmic reticulum, Protein targeting, Secretory pathway

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More than one-third of newly synthesized proteins are targeted to the early secretory pathway, which is comprised of the endoplasmic reticulum (ER), Golgi apparatus, and other intermediate compartments. The early secretory pathway plays a key role in controlling the folding, assembly, maturation, modification, trafficking, and degradation of such proteins. A considerable proportion of the secretome requires zinc as an essential factor for its structural and catalytic functions, and recent findings reveal that zinc plays a pivotal role in the function of the early secretory pathway. Hence, a disruption of zinc homeostasis and metabolism involving the early secretory pathway will lead to pathway dysregulation, resulting in various defects, including an exacerbation of homeostatic ER stress. The accumulated evidence indicates that specific members of the family of Zn transporters (ZNTs) and Zrt- and Irt-like proteins (ZIPs), which operate in the early secretory pathway, play indispensable roles in maintaining zinc homeostasis by regulating the influx and efflux of zinc. In this review, the biological functions of these transporters are discussed, focusing on recent aspects of their roles. In particular, we discuss in depth how specific ZNT transporters are employed in the activation of zinc-requiring ectoenzymes. The means by which early secretory pathway functions are controlled by zinc, mediated by specific ZNT and ZIP transporters, are also subjects of this review.

Concepts: Cell, Metabolism, Golgi apparatus, Secretion, Endoplasmic reticulum, Lysosome, Protein targeting, Secretory pathway

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Cells address challenges to protein folding in the secretory pathway by engaging endoplasmic reticulum (ER)-localized protective mechanisms that are collectively termed the unfolded protein response (UPR). By the action of the transmembrane signal transducers IRE1, PERK, and ATF6, the UPR induces networks of genes whose products alleviate the burden of protein misfolding. The UPR also plays instructive roles in cell differentiation and development, aids in the response to pathogens, and coordinates the output of professional secretory cells. These functions add to and move beyond the UPR’s classical role in addressing proteotoxic stress. Thus, the UPR is not just a reaction to protein misfolding, but also a fundamental driving force in physiology and pathology. Recent efforts have yielded a suite of chemical genetic methods and small molecule modulators that now provide researchers with both stress-dependent and -independent control of UPR activity. Such tools provide new opportunities to perturb the UPR and thereby study mechanisms for maintaining proteostasis in the secretory pathway. Numerous observations now hint at the therapeutic potential of UPR modulation for diseases related to the misfolding and aggregation of ER client proteins. Growing evidence also indicates the promise of targeting ER proteostasis nodes downstream of the UPR. Here, we review selected advances in these areas, providing a resource to inform ongoing studies of secretory proteostasis and function as they relate to the UPR.

Concepts: DNA, Proteins, Protein, Gene, Cell, Endoplasmic reticulum, Protein folding, Secretory pathway

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The Golgi complex consists of serially stacked membrane cisternae which can be further categorized into sub-Golgi regions, including the cis-Golgi, medial-Golgi, trans-Golgi and trans-Golgi network. Cellular functions of the Golgi are determined by the characteristic distribution of its resident proteins. The spatial resolution of conventional light microscopy is too low to resolve sub-Golgi structure or cisternae. Thus, the immuno-gold electron microscopy is a method of choice to localize a protein at the sub-Golgi level. However, the technique and instrument are beyond the capability of most cell biology labs. We describe here our recently developed super-resolution method called Golgi protein localization by imaging centers of mass (GLIM) to systematically and quantitatively localize a Golgi protein. GLIM is based on standard fluorescence labeling protocols and conventional wide-field or confocal microscopes. It involves the calibration of chromatic-shift aberration of the microscopic system, the image acquisition and the post-acquisition analysis. The sub-Golgi localization of a test protein is quantitatively expressed as the localization quotient. There are four main advantages of GLIM; it is rapid, based on conventional methods and tools, the localization result is quantitative, and it affords ~ 30 nm practical resolution along the Golgi axis. Here we describe the detailed protocol of GLIM to localize a test Golgi protein.

Concepts: Protein, Cell, Cell membrane, Golgi apparatus, Secretion, Endoplasmic reticulum, Protein targeting, Secretory pathway