The dynamic nature of the Golgi complex

PD-L1 translocation to the plasma membrane enables tumor immune evasion through MIB2 ubiquitination

Programmed death-ligand 1 (PD-L1), a critical immune checkpoint ligand, is a transmembrane protein synthesized in the endoplasmic reticulum of tumor cells and transported to the plasma membrane to interact with programmed death 1 (PD-1) expressed on T cell surface. This interaction delivers coinhibitory signals to T cells, thereby suppressing their function and allowing evasion of antitumor immunity. Most companion or complementary diagnostic devices for assessing PD-L1 expression levels in tumor cells used in the clinic or in clinical trials require membranous staining. However, the mechanism driving PD-L1 translocation to the plasma membrane after de novo synthesis is poorly understood. Herein, we showed that mind bomb homolog 2 (MIB2) is required for PD-L1 transportation from the trans-Golgi network (TGN) to the plasma membrane of cancer cells. MIB2 deficiency led to fewer PD-L1 proteins on the tumor cell surface and promoted antitumor immunity in mice. Mechanistically, MIB2 catalyzed nonproteolytic K63-linked ubiquitination of PD-L1, facilitating PD-L1 trafficking through Ras-associated binding 8-mediated (RAB8-mediated) exocytosis from the TGN to the plasma membrane, where it bound PD-1 extrinsically to prevent tumor cell killing by T cells. Our findings demonstrate that nonproteolytic ubiquitination of PD-L1 by MIB2 is required for its transportation to the plasma membrane and tumor cell immune evasion.

PI(4,5)P2: signaling the plasma membrane

In the almost 70 years since the first hints of its existence, the phosphoinositide, phosphatidyl-D-myo-inositol 4,5-bisphosphate has been found to be central in the biological regulation of plasma membrane (PM) function. Here, we provide an overview of the signaling, transport and structural roles the lipid plays at the cell surface in animal cells. These include being substrate for second messenger generation, direct modulation of receptors, control of membrane traffic, regulation of ion channels and transporters, and modulation of the cytoskeleton and cell polarity. We conclude by re-evaluating PI(4,5)P2's designation as a signaling molecule, instead proposing a cofactor role, enabling PM-selective function for many proteins.

The Golgi Apparatus and its Next-Door Neighbors

The Golgi apparatus represents a central compartment of membrane traffic. Its apparent architecture, however, differs considerably among species, from unstacked and scattered cisternae in the budding yeast Saccharomyces cerevisiae to beautiful ministacks in plants and further to gigantic ribbon structures typically seen in mammals. Considering the well-conserved functions of the Golgi, its fundamental structure must have been optimized despite seemingly different architectures. In addition to the core layers of cisternae, the Golgi is usually accompanied by next-door compartments on its cis and trans sides. The trans-Golgi network (TGN) can be now considered as a compartment independent from the Golgi stack. On the cis side, the intermediate compartment between the ER and the Golgi (ERGIC) has been known in mammalian cells, and its functional equivalent is now suggested for yeast and plant cells. High-resolution live imaging is extremely powerful for elucidating the dynamics of these compartments and has revealed amazing similarities in their behaviors, indicating common mechanisms conserved along the long course of evolution. From these new findings, I would like to propose reconsideration of compartments and suggest a new concept to describe their roles comprehensively around the Golgi and in the post-Golgi trafficking.

Comparison of the Cisterna Maturation-Progression Model with the Kiss-and-Run Model of Intra-Golgi Transport: Role of Cisternal Pores and Cargo Domains

The Golgi complex is the central station of the secretory pathway. Knowledge about the mechanisms of intra-Golgi transport is inconsistent. Here, we compared the explanatory power of the cisterna maturation-progression model and the kiss-and-run model. During intra-Golgi transport, conventional cargoes undergo concentration and form cisternal distensions or distinct membrane domains that contain only one membrane cargo. These domains and distension are separated from the rest of the Golgi cisternae by rows of pores. After the arrival of any membrane cargo or a large cargo aggregate at the Golgi complex, the cis-Golgi SNAREs become enriched within the membrane of cargo-containing domains and then replaced by the trans-Golgi SNAREs. During the passage of these domains, the number of cisternal pores decreases. Restoration of the cisternal pores is COPI-dependent. Our observations are more in line with the kiss-and-run model.

Ribosome-associated vesicles: A dynamic subcompartment of the endoplasmic reticulum in secretory cells

The endoplasmic reticulum (ER) is a highly dynamic network of membranes. Here, we combine live-cell microscopy with in situ cryo-electron tomography to directly visualize ER dynamics in several secretory cell types including pancreatic β-cells and neurons under near-native conditions. Using these imaging approaches, we identify a novel, mobile form of ER, ribosome-associated vesicles (RAVs), found primarily in the cell periphery, which is conserved across different cell types and species. We show that RAVs exist as distinct, highly dynamic structures separate from the intact ER reticular architecture that interact with mitochondria via direct intermembrane contacts. These findings describe a new ER subcompartment within cells.

Defects in COG-Mediated Golgi Trafficking Alter Endo-Lysosomal System in Human Cells

The conserved oligomeric complex (COG) is a multi-subunit vesicle tethering complex that functions in retrograde trafficking at the Golgi. We have previously demonstrated that the formation of enlarged endo-lysosomal structures (EELSs) is one of the major glycosylation-independent phenotypes of cells depleted for individual COG complex subunits. Here, we characterize the EELSs in HEK293T cells using microscopy and biochemical approaches. Our analysis revealed that the EELSs are highly acidic and that vATPase-dependent acidification is essential for the maintenance of this enlarged compartment. The EELSs are accessible to both trans-Golgi enzymes and endocytic cargo. Moreover, the EELSs specifically accumulate endolysosomal proteins Lamp2, CD63, Rab7, Rab9, Rab39, Vamp7, and STX8 on their surface. The EELSs are distinct from lysosomes and do not accumulate active Cathepsin B. Retention using selective hooks (RUSH) experiments revealed that biosynthetic cargo mCherry-Lamp1 reaches the EELSs much faster as compared to both receptor-mediated and soluble endocytic cargo, indicating TGN origin of the EELSs. In support to this hypothesis, EELSs are enriched with TGN specific lipid PI4P. Additionally, analysis of COG4/VPS54 double KO cells revealed that the activity of the GARP tethering complex is necessary for EELSs’ accumulation, indicating that protein mistargeting and the imbalance of Golgi-endosome membrane flow leads to the formation of EELSs in COG-deficient cells. The EELSs are likely to serve as a degradative storage hybrid organelle for mistargeted Golgi enzymes and underglycosylated glycoconjugates. To our knowledge this is the first report of the formation of an enlarged hybrid endosomal compartment in a response to malfunction of the intra-Golgi trafficking machinery.

The Interplay of Viral and Host Factors in Chikungunya Virus Infection: Targets for Antiviral Strategies

Chikungunya virus (CHIKV) has re-emerged as one of the many medically important arboviruses that have spread rampantly across the world in the past decade. Infected patients come down with acute fever and rashes, and a portion of them suffer from both acute and chronic arthralgia. Currently, there are no targeted therapeutics against this debilitating virus. One approach to develop potential therapeutics is by understanding the viral-host interactions. However, to date, there has been limited research undertaken in this area. In this review, we attempt to briefly describe and update the functions of the different CHIKV proteins and their respective interacting host partners. In addition, we also survey the literature for other reported host factors and pathways involved during CHIKV infection. There is a pressing need for an in-depth understanding of the interaction between the host environment and CHIKV in order to generate potential therapeutics.

Land-locked mammalian Golgi reveals cargo transport between stable cisternae

The Golgi is composed of a stack of cis, medial, trans cisternae that are biochemically distinct. The stable compartments model postulates that permanent cisternae communicate through bi-directional vesicles, while the cisternal maturation model postulates that transient cisternae biochemically mature to ensure anterograde transport. Testing either model has been constrained by the diffraction limit of light microscopy, as the cisternae are only 10-20 nm thick and closely stacked in mammalian cells. We previously described the unstacking of Golgi by the ectopic adhesion of Golgi cisternae to mitochondria. Here, we show that cargo processing and transport continue-even when individual Golgi cisternae are separated and "land-locked" between mitochondria. With the increased spatial separation of cisternae, we show using three-dimensional live imaging that cis-Golgi and trans-Golgi remain stable in their composition and size. Hence, we provide new evidence in support of the stable compartments model in mammalian cells.The different composition of Golgi cisternae gave rise to two different models for intra-Golgi traffic: one where stable cisternae communicate via vesicles and another one where cisternae biochemically mature to ensure anterograde transport. Here, the authors provide evidence in support of the stable compartments model.

Sphingomyelin metabolism controls the shape and function of the Golgi cisternae

The flat Golgi cisterna is a highly conserved feature of eukaryotic cells, but how is this morphology achieved and is it related to its function in cargo sorting and export? A physical model of cisterna morphology led us to propose that sphingomyelin (SM) metabolism at the trans-Golgi membranes in mammalian cells essentially controls the structural features of a Golgi cisterna by regulating its association to curvature-generating proteins. An experimental test of this hypothesis revealed that affecting SM homeostasis converted flat cisternae into highly curled membranes with a concomitant dissociation of membrane curvature-generating proteins. These data lend support to our hypothesis that SM metabolism controls the structural organization of a Golgi cisterna. Together with our previously presented role of SM in controlling the location of proteins involved in glycosylation and vesicle formation, our data reveal the significance of SM metabolism in the structural organization and function of Golgi cisternae.

DOI:http://dx.doi.org/10.7554/eLife.24603.001

The Golgi complex is a hub inside cells that transports many proteins to various parts of the cell. It also receives freshly made proteins and modifies them to help them mature into their final active forms. The complex is made up of a stack of disc-like membrane structures called cisternae.

Are the shapes of the cisternae important for the Golgi complex to work properly? Membranes are made of mixtures of molecules known as lipids and proteins. Previous experiments show that when the mixture of lipids in the Golgi membranes changes in a specific manner, the cisternae curl into an onion-like shape and the Golgi cannot process or send out proteins anymore.

Campelo et al. used mathematics and experimental approaches to investigate what causes the Golgi to change shape when the lipid mixture of the cisternae changes. A mathematical description of the shape of the Golgi predicted that some proteins keep the cisternae flat by holding the membrane rim that connects the two faces of a cisterna. To test this prediction, Campelo et al. performed experiments in human cells, which showed that when the mixture of lipids in the Golgi membranes changes, certain proteins jump from the rim, causing the cisternae to curl. These same proteins are also needed to transport cargo proteins out of the Golgi, meaning that there is a connection between the shape of the Golgi and the tasks it carries out.

The shape of the Golgi complex is altered in Alzheimer’s disease and many other neurodegenerative diseases. The next challenges are to understand how these shape changes happen, how this affects cells, and if it could be possible to develop drugs that prevent these changes from occurring in patients.

DOI:http://dx.doi.org/10.7554/eLife.24603.002

Quantifying Golgi structure using EM: combining volume-SEM and stereology for higher throughput

Investigating organelles such as the Golgi complex depends increasingly on high-throughput quantitative morphological analyses from multiple experimental or genetic conditions. Light microscopy (LM) has been an effective tool for screening but fails to reveal fine details of Golgi structures such as vesicles, tubules and cisternae. Electron microscopy (EM) has sufficient resolution but traditional transmission EM (TEM) methods are slow and inefficient. Newer volume scanning EM (volume-SEM) methods now have the potential to speed up 3D analysis by automated sectioning and imaging. However, they produce large arrays of sections and/or images, which require labour-intensive 3D reconstruction for quantitation on limited cell numbers. Here, we show that the information storage, digital waste and workload involved in using volume-SEM can be reduced substantially using sampling-based stereology. Using the Golgi as an example, we describe how Golgi populations can be sensed quantitatively using single random slices and how accurate quantitative structural data on Golgi organelles of individual cells can be obtained using only 5–10 sections/images taken from a volume-SEM series (thereby sensing population parameters and cell–cell variability). The approach will be useful in techniques such as correlative LM and EM (CLEM) where small samples of cells are treated and where there may be variable responses. For Golgi study, we outline a series of stereological estimators that are suited to these analyses and suggest workflows, which have the potential to enhance the speed and relevance of data acquisition in volume-SEM.

Growth of the Mammalian Golgi Apparatus during Interphase

During the cell cycle, genetic materials and organelles are duplicated to ensure that there is sufficient cellular content for daughter cells. While Golgi growth in interphase has been observed in lower eukaryotes, the elaborate ribbon structure of the mammalian Golgi apparatus has made it challenging to monitor. Here we demonstrate the growth of the mammalian Golgi apparatus in its protein content and volume during interphase. Through ultrastructural analyses, physical growth of the Golgi apparatus was revealed to occur by cisternal elongation of the individual Golgi stacks. By examining the timing and regulation of Golgi growth, we established that Golgi growth starts after passage through the cell growth checkpoint at late G1 phase and continues in a manner highly correlated with cell size growth. Finally, by identifying S6 kinase 1 as a major player in Golgi growth, we revealed the coordination between cell size and Golgi growth via activation of the protein synthesis machinery in early interphase.

Control of protein trafficking by reversible masking of transport signals

A system for controlled trafficking of proteins is based on modifying the streptavidin-binding peptide with trafficking signals and appending it to reporter proteins. Coexpression with streptavidin results in signal masking, which is reversed upon biotin addition.

Systems that allow the control of protein traffic between subcellular compartments have been valuable in elucidating trafficking mechanisms. Most current approaches rely on ligand or light-controlled dimerization, which results in either retardation or enhancement of the transport of a reporter. We developed an alternative approach for trafficking regulation that we term “controlled unmasking of targeting elements” (CUTE). Regulated trafficking is achieved by reversible masking of the signal that directs the reporter to its target organelle, relying on the streptavidin–biotin system. The targeting signal is generated within or immediately after a 38–amino acid streptavidin-binding peptide (SBP) that is appended to the reporter. The binding of coexpressed streptavidin to SBP causes signal masking, whereas addition of biotin causes complex dissociation and triggers protein transport to the target organelle. We demonstrate the application of this approach to the control of nuclear and peroxisomal protein import and the generation of biotin-dependent trafficking through the endocytic and COPI systems. By simultaneous masking of COPI and endocytic signals, we were able to generate a synthetic pathway for efficient transport of a reporter from the plasma membrane to the endoplasmic reticulum.

siRNA Screen Identifies Trafficking Host Factors that Modulate Alphavirus Infection

Little is known about the repertoire of cellular factors involved in the replication of pathogenic alphaviruses. To uncover molecular regulators of alphavirus infection, and to identify candidate drug targets, we performed a high-content imaging-based siRNA screen. We revealed an actin-remodeling pathway involving Rac1, PIP5K1- α, and Arp3, as essential for infection by pathogenic alphaviruses. Infection causes cellular actin rearrangements into large bundles of actin filaments termed actin foci. Actin foci are generated late in infection concomitantly with alphavirus envelope (E2) expression and are dependent on the activities of Rac1 and Arp3. E2 associates with actin in alphavirus-infected cells and co-localizes with Rac1-PIP5K1-α along actin filaments in the context of actin foci. Finally, Rac1, Arp3, and actin polymerization inhibitors interfere with E2 trafficking from the trans-Golgi network to the cell surface, suggesting a plausible model in which transport of E2 to the cell surface is mediated via Rac1- and Arp3-dependent actin remodeling.

Fluorescent Protein-Tagged Sindbis Virus E2 Glycoprotein Allows Single Particle Analysis of Virus Budding from Live Cells

Sindbis virus (SINV) is an enveloped, mosquito-borne alphavirus. Here we generated and characterized a fluorescent protein-tagged (FP-tagged) SINV and found that the presence of the FP-tag (mCherry) affected glycoprotein transport to the plasma membrane whereas the specific infectivity of the virus was not affected. We examined the virions by transmission electron cryo-microscopy and determined the arrangement of the FP-tag on the surface of the virion. The fluorescent proteins are arranged icosahedrally on the virus surface in a stable manner that did not adversely affect receptor binding or fusion functions of E2 and E1, respectively. The delay in surface expression of the viral glycoproteins, as demonstrated by flow cytometry analysis, contributed to a 10-fold reduction in mCherry-E2 virus titer. There is a 1:1 ratio of mCherry to E2 incorporated into the virion, which leads to a strong fluorescence signal and thus facilitates single-particle tracking experiments. We used the FP-tagged virus for high-resolution live-cell imaging to study the spatial and temporal aspects of alphavirus assembly and budding from mammalian cells. These processes were further analyzed by thin section microscopy. The results demonstrate that SINV buds from the plasma membrane of infected cells and is dispersed into the surrounding media or spread to neighboring cells facilitated by its close association with filopodial extensions.

Golgi fragmentation in amyotrophic lateral sclerosis, an overview of possible triggers and consequences

Amyotrophic Lateral Sclerosis (ALS) is an invariably fatal neurodegenerative disorder, which specifically targets motor neurons in the brain, brain stem and spinal cord. Whilst the etiology of ALS remains unknown, fragmentation of the Golgi apparatus is detected in ALS patient motor neurons and in animal/cellular disease models. The Golgi is a highly dynamic organelle that acts as a dispatching station for the vesicular transport of secretory/transmembrane proteins. It also mediates autophagy and maintains endoplasmic reticulum (ER) and axonal homeostasis. Both the trigger for Golgi fragmentation and the functional consequences of a fragmented Golgi apparatus in ALS remain unclear. However, recent evidence has highlighted defects in vesicular trafficking as a pathogenic mechanism in ALS. This review summarizes the evidence describing Golgi fragmentation in ALS, with possible links to other disease processes including cellular trafficking, ER stress, defective autophagy, and axonal degeneration.

Dual pulse-chase microscopy reveals early divergence in the biosynthetic trafficking of the Na,K-ATPase and E-cadherin

The trafficking of newly synthesized Na,K-ATPase and E-cadherin is observed in polarized epithelial cells. E-cadherin’s exit from the Golgi complex is not susceptible to 19°C temperature block. Furthermore, these proteins exit the Golgi and are delivered to the basolateral cell surface in separate vascular carriers.

Recent evidence indicates that newly synthesized membrane proteins that share the same distributions in the plasma membranes of polarized epithelial cells can pursue a variety of distinct trafficking routes as they travel from the Golgi complex to their common destination at the cell surface. In most polarized epithelial cells, both the Na,K-ATPase and E-cadherin are localized to the basolateral domains of the plasma membrane. To examine the itineraries pursued by newly synthesized Na,K-ATPase and E-cadherin in polarized MDCK epithelial cells, we used the SNAP and CLIP labeling systems to fluorescently tag temporally defined cohorts of these proteins and observe their behaviors simultaneously as they traverse the secretory pathway. These experiments reveal that E-cadherin is delivered to the cell surface substantially faster than is the Na,K-ATPase. Furthermore, the surface delivery of newly synthesized E-cadherin to the plasma membrane was not prevented by the 19°C temperature block that inhibits the trafficking of most proteins, including the Na,K-ATPase, out of the trans-Golgi network. Consistent with these distinct behaviors, populations of newly synthesized E-cadherin and Na,K-ATPase become separated from one another within the trans-Golgi network, suggesting that they are sorted into different carrier vesicles that mediate their post-Golgi trafficking.

COPI-mediated retrieval of SCAP is crucial for regulating lipogenesis under basal and sterol-deficient conditions

Retrograde trafficking from the Golgi complex to endoplasmic reticulum (ER) through COPI-coated vesicles has been implicated in lipid homeostasis. Here, we find that a block in COPI-dependent retrograde trafficking promotes processing and nuclear translocation of sterol regulatory element binding proteins (SREBPs), and upregulates the expression of downstream genes that are involved in lipid biosynthesis. This elevation in SREBP processing and activation is not caused by mislocalization of S1P or S2P (also known as MBTPS1 and MBTPS2, respectively), two Golgi-resident endoproteases that are involved in SREBP processing, but instead by increased Golgi residence of SREBPs, leading to their increased susceptibility to processing by the endoproteases. Analyses using a processing-defective SREBP mutant suggest that a fraction of SREBP molecules undergo basal cycling between the ER and Golgi in complex with SREBP cleavage-activating protein (SCAP). Furthermore, we show that SCAP alone is retrieved from the Golgi and moves to the ER after processing of SREBP under sterol-deficient conditions. Thus, our observations indicate that COPI-mediated retrograde trafficking is crucial for preventing unnecessary SREBP activation, by retrieving the small amounts of SCAP-SREBP complex that escape from the sterol-regulated ER retention machinery, as well as for the reuse of SCAP.

Role of tetanus neurotoxin insensitive vesicle-associated membrane protein in membrane domains transport and homeostasis

Biological membranes in eukaryotes contain a large variety of proteins and lipids often distributed in domains in plasma membrane and endomembranes. Molecular mechanisms responsible for the transport and the organization of these membrane domains along the secretory pathway still remain elusive. Here we show that vesicular SNARE TI-VAMP/VAMP7 plays a major role in membrane domains composition and transport. We found that the transport of exogenous and endogenous GPI-anchored proteins was altered in fibroblasts isolated from VAMP7-knockout mice. Furthermore, disassembly and reformation of the Golgi apparatus induced by Brefeldin A treatment and washout were impaired in VAMP7-depleted cells, suggesting that loss of VAMP7 expression alters biochemical properties and dynamics of the Golgi apparatus. In addition, lipid profiles from these knockout cells indicated a defect in glycosphingolipids homeostasis. We conclude that VAMP7 is required for effective transport of GPI–anchored proteins to cell surface and that VAMP7-dependent transport contributes to both sphingolipids and Golgi homeostasis.

Communication between the N and C termini is required for copper-stimulated Ser/Thr phosphorylation of Cu(I)-ATPase (ATP7B)

The Wilson disease protein ATP7B exhibits copper-dependent trafficking. In high copper, ATP7B exits the trans-Golgi network and moves to the apical domain of hepatocytes where it facilitates elimination of excess copper through the bile. Copper levels also affect ATP7B phosphorylation. ATP7B is basally phosphorylated in low copper and becomes more phosphorylated ("hyperphosphorylated") in elevated copper. The functional significance of hyperphosphorylation remains unclear. We showed that hyperphosphorylation occurs even when ATP7B is restricted to the trans-Golgi network. We performed comprehensive phosphoproteomics of ATP7B in low versus high copper, which revealed that 24 Ser/Thr residues in ATP7B could be phosphorylated, and only four of these were copper-responsive. Most of the phosphorylated sites were found in the N- and C-terminal cytoplasmic domains. Using truncation and mutagenesis, we showed that inactivation or elimination of all six N-terminal metal binding domains did not block copper-dependent, reversible, apical trafficking but did block hyperphosphorylation in hepatic cells. We showed that nine of 15 Ser/Thr residues in the C-terminal domain were phosphorylated. Inactivation of 13 C-terminal phosphorylation sites reduced basal phosphorylation and eliminated hyperphosphorylation, suggesting that copper binding at the N terminus propagates to the ATP7B C-terminal region. C-terminal mutants with either inactivating or phosphomimetic substitutions showed little effect upon copper-stimulated trafficking, indicating that trafficking does not depend on phosphorylation at these sites. Thus, our studies revealed that copper-dependent conformational changes in the N-terminal region lead to hyperphosphorylation at C-terminal sites, which seem not to affect trafficking and may instead fine-tune copper sequestration.

How Rab proteins determine Golgi structure

Rab proteins, small GTPases, are key regulators of mammalian Golgi apparatus organization. Based on the effect of Rab activation state, Rab proteins fall into two functional classes. In Class1, inactivation induces Golgi ribbon fragmentation and/or redistribution of Golgi enzymes to the Endoplasmic Reticulum, while overexpression of wild type or activation has little, if any, effect on Golgi ribbon organization. In Class 2, the reverse is true. We give emphasis to Rab6, the most abundant Golgi-associated Rab protein. Rab6 depletion in HeLa cells causes an increase in Golgi cisternal number, longer, more continuous cisternae, and a pronounced accumulation of vesicles; the effect of Rab6 on Golgi ribbon organization is probably through regulation of vesicle transport. In effector studies, motor proteins and their regulators are found to be key Rab6 effectors. A related Rab, Rab41, affects Golgi ribbon organization in a contrasting manner. The balance between minus- and plus-end directed motor recruitment may well be the major Rab-dependent factor in Golgi ribbon organization.

Positive feedback between Golgi membranes, microtubules and ER exit sites directs de novo biogenesis of the Golgi

The Golgi complex is the central organelle of the secretory pathway. It undergoes dynamic changes during the cell cycle, but how it acquires and maintains its complex structure is unclear. To address this question, we have used laser nanosurgery to deplete BSC1 cells of the Golgi complex and have monitored its biogenesis by quantitative time-lapse microscopy and correlative electron microscopy. After Golgi depletion, endoplasmic reticulum (ER) export is inhibited and the number of ER exit sites (ERES) is reduced and does not increase for several hours. Occasional fusion of small post-ER carriers to form the first larger structures triggers a rapid and drastic growth of Golgi precursors, due to the capacity of these structures to attract more carriers by microtubule nucleation and to stimulate ERES biogenesis. Increasing the chances of post-ER carrier fusion close to ERES by depolymerizing microtubules results in the acceleration of Golgi and ERES biogenesis. Taken together, on the basis of our results, we propose a self-organizing principle of the early secretory pathway that integrates Golgi biogenesis, ERES biogenesis and the organization of the microtubule network by positive-feedback loops.

The Golgi ribbon structure facilitates anterograde transport of large cargoes

Breaking down the ribbon of mammalian cells strongly inhibits intra-Golgi transport of large cargoes without altering the rate of transport of smaller cargoes. These results imply that the ribbon structure is an essential requirement for transport of large cargoes in mammalian cells.

In mammalian cells, individual Golgi stacks fuse laterally to form the characteristic perinuclear ribbon structure. Yet the purpose of this remarkable structure has been an enigma. We report that breaking down the ribbon of mammalian cells strongly inhibits intra-Golgi transport of large cargoes without altering the rate of transport of smaller cargoes. In addition, insect cells that naturally harbor dispersed Golgi stacks have limited capacity to transport artificial oversized cargoes. These results imply that the ribbon structure is an essential requirement for transport of large cargoes in mammalian cells, and we suggest that this is because it enables the dilated rims of cisternae (containing the aggregates) to move across the stack as they transfer among adjacent stacks within the ribbon structure.

Reduction in Golgi apparatus dimension in the absence of a residential protein, N-acetylglucosaminyltransferase V

Various proteins are involved in the generation and maintenance of the membrane complex known as the Golgi apparatus. We have used mutant Chinese hamster ovary (CHO) cell lines Lec4 and Lec4A lacking N-acetylglucosaminyltransferase V (GlcNAcT-V, MGAT5) activity and protein in the Golgi apparatus to study the effects of the absence of a single glycosyltransferase on the Golgi apparatus dimension. Quantification of immunofluorescence in serial confocal sections for Golgi α-mannosidase II and electron microscopic morphometry revealed a reduction in Golgi volume density up to 49 % in CHO Lec4 and CHO Lec4A cells compared to parental CHO cells. This reduction in Golgi volume density could be reversed by stable transfection of Lec4 cells with a cDNA encoding Mgat5. Inhibition of the synthesis of β1,6-branched N-glycans by swainsonine had no effect on Golgi volume density. In addition, no effect on Golgi volume density was observed in CHO Lec1 cells that contain enzymatically active GlcNAcT-V, but cannot synthesize β1,6-branched glycans due to an inactive GlcNAcT-I in their Golgi apparatus. These results indicate that it may be the absence of the GlcNAcT-V protein that is the determining factor in reducing Golgi volume density. No dimensional differences existed in cross-sectioned cisternal stacks between Lec4 and control CHO cells, but significantly reduced Golgi stack hits were observed in cross-sectioned Lec4 cells. Therefore, the Golgi apparatus dimensional change in Lec4 and Lec4A cells may be due to a compaction of the organelle.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Docosahexaenoic acid impairs the maturation of very low density lipoproteins in rat hepatic cells

One mechanism of the lipid-lowering effects of the fish oil n-3 fatty acids [e.g., docosahexaenoic acid (DHA)] in cell and animal models is induced hepatic apolipoprotein B100 (apoB) presecretory degradation. This degradation occurs post-endoplasmic reticulum, but whether DHA induces it before or after intracellular VLDL formation remains unanswered. We found in McA-RH7777 rat hepatic cells that DHA and oleic acid (OA) treatments allowed formation of pre-VLDL particles and their transport to the Golgi, but, in contrast to OA, with DHA pre-VLDL particles failed to quantitatively assemble into fully lipidated (mature) VLDL. This failure required lipid peroxidation and was accompanied by the formation of apoB aggregates (known to be degraded by autophagy). Preventing the exit of proteins from the Golgi blocked the aggregation of apoB but did not restore VLDL maturation, indicating that failure to fully lipidate apoB preceded its aggregation. ApoB autophagic degradation did not appear to require an intermediate step of cytosolic aggresome formation. Taken with other examples in the literature, the results of this study suggest that pre-VLDL particles that are competent to escape endoplasmic reticulum quality control mechanisms but fail to mature in the Golgi remain subject to quality control surveillance late in the secretory pathway.

Proteostasis in complex dendrites

Like all cells, neurons are made of proteins that have characteristic synthesis and degradation profiles. Unlike other cells, however, neurons have a unique multipolar architecture that makes ∼10,000 synaptic contacts with other neurons. Both the stability and modifiability of the neuronal proteome are crucial for its information-processing, storage and plastic properties. The cell biological mechanisms that synthesize, modify, deliver and degrade dendritic and synaptic proteins are not well understood but appear to reflect unique solutions adapted to the particular morphology of neurons.

Intra-Golgi Transport: Roles for Vesicles, Tubules, and Cisternae

The Golgi complex is considered the central station of the secretory pathway where cargo proteins and lipids are properly modified, classified, packed into specific carriers and delivered to their final destinations. Early electron microscope studies showed the extraordinary structural complexity of this organelle. However, despite the large volume of incoming and outgoing traffic, it is able to maintain its architecture, although it is also flexible enough to adapt to the functional status of the cell. Many components of the molecular machinery involved in membrane traffic and other Golgi functions have been identified. However, some basic aspects of Golgi functioning remain unsolved. For instance, how cargo moves through the stack remains controversial and two classical models have been proposed: vesicular transport and cisternal maturation. Since neither of these models explains all the experimental data, a combination of these models as well as new models have been proposed. In this context, the specific role of the cisternae, vesicles and tubules needs to be clarified. In this review, we summarize our current knowledge of the Golgi organization and function, focusing on the mechanisms of intra-Golgi transport.

Changes in Rab3D expression and distribution in the acini of Sjögren's syndrome patients are associated with loss of cell polarity and secretory dysfunction

OBJECTIVE

Oral and ocular dryness are frequent and serious symptoms of Sjögren's syndrome (SS) that reflect problems in secretion due to glandular dysfunction. Exocytosis, an important process in the secretory pathway, requires the participation of Rab family GTPases. This study was undertaken to analyze the expression and localization of Rab3D and Rab8A and to examine their correlation with acinar cell polarity and glandular secretory function.

METHODS

Nineteen patients with SS and 17 controls were evaluated. Levels of Rab3D and Rab8A messenger RNA (mRNA) and protein were determined by real-time polymerase chain reaction and Western blotting. Subcellular localization of proteins was determined by indirect immunofluorescence analysis.

RESULTS

In patients with SS, total Rab3D protein levels decreased significantly, while mRNA levels remained unchanged. For Rab8A, no changes in either mRNA or protein levels were detected. In serous acini of labial salivary glands from patients with SS, the following 4 patterns of Rab3D staining were distinguishable: severely decreased, distribution throughout the cytoplasm, distribution throughout the cytoplasm combined with loss of nuclear polarity, and normal apical localization. Basal localization of Rab8A was not modified. Rab3D changes were accompanied by apicobasolateral redistribution of ezrin, loss of nuclear polarity, thicker Golgi stacks, and mucin 7 accumulation in the cytoplasm. Finally, low Rab3D protein levels correlated with alterations in scintigraphy measurements.

CONCLUSION

Our findings indicate that Rab3D regulates the exocytosis of many components critical for the maintenance of oral physiology. Hence, the changes observed in Rab3D expression and distribution are likely to contribute to the decrease in or loss of saliva components (i.e., mucins), which may explain the variety of oral and ocular symptoms associated with SS.

Probing the early temporal and spatial interaction of the Sindbis virus capsid and E2 proteins with reverse genetics

A 7-Å cryoelectron microscopy-based reconstruction of Sindbis virus (SINV) was recently generated. Fitting the crystal structure of the SINV capsid protein (Cp) into the density map revealed that the F2-G2 loop of the Cp was shifted away from cytoplasmic domain of E2 (cdE2) in the 7-Å reconstruction relative to its position in the Cp crystal structure. Furthermore, the reconstruction demonstrated that residue E395 in region I of the cytoplasmic domain of the E2 envelope protein (cdE2-RI) and K252 of Cp, part of the Cp F2-G2 loop, formed a putative salt bridge in the virion. We generated amino acid substitutions at residues K250 and K252 of the SINV Cp and explored the resulting phenotypes. In the context of cells infected with wild-type or mutant virus, reversing the charge of these two residues resulted in the appearance of Cp aggregates around cytopathic vacuole type I (CPV-I) structures, the absence of nucleocapsid (NC) formation, and a lack of virus particle release in the infected mammalian cell. However, expressing the same Cp mutants in the cell without the envelope proteins or expressing and purifying the mutants from an Escherichia coli expression system and assembling in vitro yielded NC assembly in all cases. In addition, second-site mutations within cdE2 restored NC assembly but not release of infectious particles. Our data suggest an early temporal and spatial interaction between cdE2-RI and the Cp F2-G2 loop that, when ablated, leads to the absence of NC assembly. This interaction also appears to be important for budding of virus particles.

Crystal structure of the conserved domain of the DC lysosomal associated membrane protein: implications for the lysosomal glycocalyx

Background

The family of lysosome-associated membrane proteins (LAMP) comprises the multifunctional, ubiquitous LAMP-1 and LAMP-2, and the cell type-specific proteins DC-LAMP (LAMP-3), BAD-LAMP (UNC-46, C20orf103) and macrosialin (CD68). LAMPs have been implicated in a multitude of cellular processes, including phagocytosis, autophagy, lipid transport and aging. LAMP-2 isoform A acts as a receptor in chaperone-mediated autophagy. LAMP-2 deficiency causes the fatal Danon disease. The abundant proteins LAMP-1 and LAMP-2 are major constituents of the glycoconjugate coat present on the inside of the lysosomal membrane, the 'lysosomal glycocalyx'. The LAMP family is characterized by a conserved domain of 150 to 200 amino acids with two disulfide bonds.

Results

The crystal structure of the conserved domain of human DC-LAMP was solved. It is the first high-resolution structure of a heavily glycosylated lysosomal membrane protein. The structure represents a novel β-prism fold formed by two β-sheets bent by β-bulges and connected by a disulfide bond. Flexible loops and a hydrophobic pocket represent possible sites of molecular interaction. Computational models of the glycosylated luminal regions of LAMP-1 and LAMP-2 indicate that the proteins adopt a compact conformation in close proximity to the lysosomal membrane. The models correspond to the thickness of the lysosomal glycoprotein coat of only 5 to 12 nm, according to electron microscopy.

Conclusion

The conserved luminal domain of lysosome-associated membrane proteins forms a previously unknown β-prism fold. Insights into the structure of the lysosomal glycoprotein coat were obtained by computational models of the LAMP-1 and LAMP-2 luminal regions.

Trafficking of the NMDAR2B receptor subunit distal cytoplasmic tail from endoplasmic reticulum to the synapse

NMDA receptor NR2A/B subunits have PDZ-binding domains on their extreme C-termini that are known to interact with the PSD-95 family and other PDZ proteins. We explore the interactions between PSD-95 family proteins and the NR2A/B cytoplasmic tails, and the consequences of these interactions, from the endoplasmic reticulum (ER) through delivery to the synapse in primary rat hippocampal and cortical cultured neurons. We find that the NR2A/B cytoplasmic tails cluster very early in the secretory pathway and interact serially with SAP102 beginning at the intermediate compartment, and then PSD-95. We further establish that colocalization of the distal C-terminus of NR2B and PSD-95 begins at the trans-Golgi Network (TGN). Formation of NR2B/PSD-95/SAP102 complexes is dependent on the PDZ binding domain of NR2B subunits, but association with SAP102 and PSD-95 plays no distinguishable role in cluster pre-formation or initial targeting to the vicinity of the synapse. Instead the PDZ binding domain plays a role in restricting cell-surface clusters to postsynaptic targets.

Electron tomography reveals Rab6 is essential to the trafficking of trans-Golgi clathrin and COPI-coated vesicles and the maintenance of Golgi cisternal number

We have shown previously that Rab6, a small, trans-Golgi-localized GTPase, acts upstream of the conserved oligomeric Golgi complex (COG) and ZW10/RINT1 retrograde tether complexes to maintain Golgi homeostasis. In this article, we present evidence from the unbiased and high-resolution approach of electron microscopy and electron tomography that Rab6 is essential to the trans-Golgi trafficking of two morphological classes of coated vesicles; the larger corresponds to clathrin-coated vesicles and the smaller to coat protein I (COPI)-coated vesicles. On the basis of the site of coated vesicle accumulation, cisternal dilation and the normal kinetics of cargo transport from the endoplasmic reticulum (ER) to Golgi followed by delayed Golgi to cell surface transport, we suggest that Golgi function in cargo transport is preferentially inhibited at the trans-Golgi/trans-Golgi network (TGN). The >50% increase in Golgi cisternae number in Rab6-depleted HeLa cells that we observed may well be coupled to the trans-Golgi accumulation of COPI-coated vesicles; depletion of the individual Rab6 effector, myosin IIA, produced an accumulation of uncoated vesicles with if anything a decrease in cisternal number. These results are the first evidence for a Rab6-dependent protein machine affecting Golgi-proximal, coated vesicle accumulation and probably transport at the trans-Golgi and the first example of concomitant cisternal proliferation and increased Golgi stack organization under inhibited transport conditions.

Imaging of membrane systems and membrane traffic in living cells

Eukaryotic cells are composed of an intricate system of internal membranes that are organized into different compartments--including the endoplasmic reticulum (ER), the nuclear envelope, the Golgi complex (GC), lysosomes, endosomes, caveolae, mitochondria, and peroxisomes--that perform specialized tasks within the cell. The localization and dynamics of intracellular compartments are now being studied in living cells because of the availability of green fluorescent protein (GFP)-fusion proteins and recent advances in fluorescent microscope imaging systems. Results using these techniques are revealing how intracellular compartments maintain their steady-state organization and distributions, how they undergo growth and division, and how they transfer protein and lipid components between themselves through the formation and trafficking of membrane transport intermediates. This article describes methods using GFP-fusion proteins to visualize the behavior of organelles and to track membrane-bound transport intermediates moving between them. Practical issues related to the construction and expression of GFP-fusion proteins are discussed first. These are essential for optimizing the brightness and expression levels of GFP-fusion proteins so that intracellular membrane-bound structures containing these fusion proteins can be readily visualized. Next, techniques for performing time-lapse imaging using a confocal laser-scanning microscope (CLSM) are detailed, including the use of photobleaching to highlight organelles and transport intermediates. Methods for the acquisition and analysis of data are then discussed. Finally, commonly used and exciting new approaches for perturbing membrane traffic are outlined.

Architecture of the mammalian Golgi

Since its first visualization in 1898, the Golgi has been a topic of intense morphological research. A typical mammalian Golgi consists of a pile of stapled cisternae, the Golgi stack, which is a key station for modification of newly synthesized proteins and lipids. Distinct stacks are interconnected by tubules to form the Golgi ribbon. At the entrance site of the Golgi, the cis-Golgi, vesicular tubular clusters (VTCs) form the intermediate between the endoplasmic reticulum and the Golgi stack. At the exit site of the Golgi, the trans-Golgi, the trans-Golgi network (TGN) is the major site of sorting proteins to distinct cellular locations. Golgi functioning can only be understood in light of its complex architecture, as was revealed by a range of distinct electron microscopy (EM) approaches. In this article, a general concept of mammalian Golgi architecture, including VTCs and the TGN, is described.

AP-1 and clathrin are essential for secretory granule biogenesis in Drosophila

Regulated secretion of hormones, digestive enzymes, and other biologically active molecules requires the formation of secretory granules. Clathrin and the clathrin adaptor protein complex 1 (AP-1) are necessary for maturation of exocrine, endocrine, and neuroendocrine secretory granules. However, the initial steps of secretory granule biogenesis are only minimally understood. Powerful genetic approaches available in the fruit fly Drosophila melanogaster were used to investigate the molecular pathway for biogenesis of the mucin-containing "glue granules" that form within epithelial cells of the third-instar larval salivary gland. Clathrin and AP-1 colocalize at the trans-Golgi network (TGN) and clathrin recruitment requires AP-1. Furthermore, clathrin and AP-1 colocalize with secretory cargo at the TGN and on immature granules. Finally, loss of clathrin or AP-1 leads to a profound block in secretory granule formation. These findings establish a novel role for AP-1- and clathrin-dependent trafficking in the biogenesis of mucin-containing secretory granules.

Rescue of progranulin deficiency associated with frontotemporal lobar degeneration by alkalizing reagents and inhibition of vacuolar ATPase

Numerous loss-of-function mutations in the progranulin (GRN) gene cause frontotemporal lobar degeneration with ubiquitin and TAR-DNA binding protein 43-positive inclusions by reduced production and secretion of GRN. Consistent with the observation that GRN has neurotrophic properties, pharmacological stimulation of GRN production is a promising approach to rescue GRN haploinsufficiency and prevent disease progression. We therefore searched for compounds capable of selectively increasing GRN levels. Here, we demonstrate that four independent and highly selective inhibitors of vacuolar ATPase (bafilomycin A1, concanamycin A, archazolid B, and apicularen A) significantly elevate intracellular and secreted GRN. Furthermore, clinically used alkalizing drugs, including chloroquine, bepridil, and amiodarone, similarly stimulate GRN production. Elevation of GRN levels occurs via a translational mechanism independent of lysosomal degradation, autophagy, or endocytosis. Importantly, alkalizing reagents rescue GRN deficiency in organotypic cortical slice cultures from a mouse model for GRN deficiency and in primary cells derived from human patients with GRN loss-of-function mutations. Thus, alkalizing reagents, specifically those already used in humans for other applications, and vacuolar ATPase inhibitors may be therapeutically used to prevent GRN-dependent neurodegeneration.

Structural evidence of glycoprotein assembly in cellular membrane compartments prior to Alphavirus budding

Membrane glycoproteins of alphavirus play a critical role in the assembly and budding of progeny virions. However, knowledge regarding transport of viral glycoproteins to the plasma membrane is obscure. In this study, we investigated the role of cytopathic vacuole type II (CPV-II) through in situ electron tomography of alphavirus-infected cells. The results revealed that CPV-II contains viral glycoproteins arranged in helical tubular arrays resembling the basic organization of glycoprotein trimers on the envelope of the mature virions. The location of CPV-II adjacent to the site of viral budding suggests a model for the transport of structural components to the site of budding. Thus, the structural characteristics of CPV-II can be used in evaluating the design of a packaging cell line for replicon production.

Arf3 is activated uniquely at the trans-Golgi network by brefeldin A-inhibited guanine nucleotide exchange factors

Arf3 associates with the TGN in a manner that is both temperature-sensitive and uniquely dependent on BIGs. TGN localization and release at 20°C are readily separated and depend on pairs of residues absolutely conserved and unique to Arf3 present at opposite ends of the protein. These results suggest that Arf3 plays a unique function at the TGN.

It is widely assumed that class I and II Arfs function interchangeably throughout the Golgi complex. However, we report here that in vivo, Arf3 displays several unexpected properties. Unlike other Golgi-localized Arfs, Arf3 associates selectively with membranes of the trans-Golgi network (TGN) in a manner that is both temperature-sensitive and uniquely dependent on guanine nucleotide exchange factors of the BIGs family. For example, BIGs knockdown redistributed Arf3 but not Arf1 from Golgi membranes. Furthermore, shifting temperature to 20°C, a temperature known to block cargo in the TGN, selectively redistributed Arf3 from Golgi membranes. Arf3 redistribution occurred slowly, suggesting it resulted from a change in membrane composition. Arf3 knockdown and overexpression experiments suggest that redistribution is not responsible for the 20°C block. To investigate in more detail the mechanism for Arf3 recruitment and temperature-dependent release, we characterized several mutant forms of Arf3. This analysis demonstrated that those properties are readily separated and depend on pairs of residues present at opposite ends of the protein. Furthermore, phylogenetic analysis established that all four critical residues were absolutely conserved and unique to Arf3. These results suggest that Arf3 plays a unique function at the TGN that likely involves recruitment by a specific receptor.

Membrane traffic within the Golgi apparatus

Newly synthesized secretory cargo molecules pass through the Golgi apparatus while resident Golgi proteins remain in the organelle. However, the pathways of membrane traffic within the Golgi are still uncertain. Most of the available data can be accommodated by the cisternal maturation model, which postulates that Golgi cisternae form de novo, carry secretory cargoes forward and ultimately disappear. The entry face of the Golgi receives material that has been exported from transitional endoplasmic reticulum sites, and the exit face of the Golgi is intimately connected with endocytic compartments. These conserved features are enhanced by cell-type-specific elaborations such as tubular connections between mammalian Golgi cisternae. Key mechanistic questions remain about the formation and maturation of Golgi cisternae, the recycling of resident Golgi proteins, the origins of Golgi compartmental identity, the establishment of Golgi architecture, and the roles of Golgi structural elements in membrane traffic.

A mathematical model to derive N-glycan structures and cellular enzyme activities from mass spectrometric data

Effective representation and characterization of biosynthetic pathways of glycosylation can be facilitated by mathematical modeling. This paper describes the expansion of a previously developed detailed model for N-linked glycosylation with the further application of the model to analyze MALDI-TOF mass spectra of human N-glycans in terms of underlying cellular enzyme activities. The glycosylation reaction network is automatically generated by the model, based on the reaction specificities of the glycosylation enzymes. The use of a molecular mass cutoff and a network pruning method typically limits the model size to about 10,000 glycan structures. This allows prediction of the complete glycan profile and its abundances for any set of assumed enzyme concentrations and reaction rate parameters. A synthetic mass spectrum from model-calculated glycan profiles is obtained and enzyme concentrations are adjusted to bring the theoretically calculated mass spectrum into agreement with experiment. The result of this process is a complete characterization of a measured glycan mass spectrum containing hundreds of masses in terms of the activities of 19 enzymes. In addition, a complete annotation of the mass spectrum in terms of glycan structure is produced, including the proportions of isomers within each peak. The method was applied to mass spectrometric data of normal human monocytes and monocytic leukemia (THP1) cells to derive glycosyltransferase activity changes underlying the differences in glycan structure between the normal and diseased cells. Model predictions could lead to a better understanding of the changes associated with disease states, identification of disease-associated biomarkers, and bioengineered glycan modifications.

The length of cargo-protein transmembrane segments drives secretory transport by facilitating cargo concentration in export domains

The cellular destination of secretory proteins is determined by interactions of their targeting motifs with coat-protein complexes. The transmembrane domain (TMD) of secretory proteins also plays a central role in their transport and targeting. However, a comprehensive model that considers both TMD- and targeting-sequence-mediated transport has never been advanced. We focused on the secretory transport of two fluorescently tagged membrane proteins: vesicular stomatitis virus G tsO45 (VSVG), which is a cargo protein that is a thermoreversible mutant, and the Golgi-resident protein GalT-CFP. A quantitative approach was applied to analyze, in living cells, secretory transport dynamics, as well as cargo concentration of YFP-tagged VSVG mutants with one, three, five, seven, eight or nine amino acids deleted from their TMD, as well as two or four amino acids added to their TMD. Changes in TMD length affected secretory transport dynamics and the extent of cargo concentration in the ER exit sites, demonstrating that the capacity of the transport machinery to concentrate cargo depends on the length of the TMD of the cargo protein.

Differential trafficking of Src, Lyn, Yes and Fyn is specified by the state of palmitoylation in the SH4 domain

Src-family tyrosine kinases (SFKs), which participate in a variety of signal transduction events, are known to localize to the cytoplasmic face of the plasma membrane through lipid modification. Recently, we showed that Lyn, an SFK member, is exocytosed to the plasma membrane via the Golgi region along the secretory pathway. We show here that SFK trafficking is specified by the palmitoylation state. Yes is also a monopalmitoylated SFK and is biosynthetically transported from the Golgi pool of caveolin to the plasma membrane. This pathway can be inhibited in the trans-Golgi network (TGN)-to-cell surface delivery by temperature block at 19 degrees C or dominant-negative Rab11 GTPase. A large fraction of Fyn, a dually palmitoylated SFK, is directly targeted to the plasma membrane irrespective of temperature block of TGN exit. Fyn(C6S), which lacks the second palmitoylation site, is able to traffic in the same way as Lyn and Yes. Moreover, construction of Yes(S6C) and chimeric Lyn or Yes with the Fyn N-terminus further substantiates the importance of the dual palmitoylation site for plasma membrane targeting. Taken together with our recent finding that Src, a nonpalmitoylated SFK, is rapidly exchanged between the plasma membrane and late endosomes/lysosomes, these results suggest that SFK trafficking is specified by the palmitoylation state in the SH4 domain.

Presecretory oxidation, aggregation, and autophagic destruction of apoprotein-B: a pathway for late-stage quality control

Hepatic secretion of apolipoprotein-B (apoB), the major protein of atherogenic lipoproteins, is regulated through posttranslational degradation. We reported a degradation pathway, post-ER pre secretory proteolysis (PERPP), that is increased by reactive oxygen species (ROS) generated within hepatocytes from dietary polyunsaturated fatty acids (PUFA). We now report the molecular processes by which PUFA-derived ROS regulate PERPP of apoB. ApoB exits the ER; undergoes limited oxidant-dependent aggregation; and then, upon exit from the Golgi, becomes extensively oxidized and converted into large aggregates. The aggregates slowly degrade by an autophagic process. None of the oxidized, aggregated material leaves cells, thereby preventing export of apoB-lipoproteins containing potentially toxic lipid peroxides. In summary, apoB secretory control via PERPP/autophagosomes is likely a key component of normal and pathologic regulation of plasma apoB levels, as well as a means for remarkably late-stage quality control of a secreted protein.