Anterograde and retrograde traffic between the rough endoplasmic reticulum and the Golgi complex

LncRNA-encoded peptides: unveiling their significance in cardiovascular physiology and pathology-current research insights

Long non-coding RNAs (lncRNAs), which are RNA transcripts exceeding 200 nucleotides were believed to lack any protein-coding capacity. But advancements in -omics technology have revealed that some lncRNAs have small open reading frames (sORFs) that can be translated by ribosomes to encode peptides, some of which have important biological functions. These encoded peptides subserve important biological functions by interacting with their targets to modulate transcriptional or signalling axes, thereby enhancing or suppressing cardiovascular disease (CVD) occurrence and progression. In this review, we summarize what is known about the research strategy of lncRNA-encoded peptides, mainly comprising predictive websites/tools and experimental methods that have been widely used for prediction, identification, and validation. More importantly, we have compiled a list of lncRNA- encoded peptides, with a focus on those that play significant roles in cardiovascular physiology and pathology, including ENSRNOT (RNO)-sORF6/RNO-sORF7/RNO-sORF8, dwarf open reading frame (DOWRF), myoregulin (NLN), etc. Additionally, we have outlined the functions and mechanisms of these peptides in cardiovascular physiology and pathology, such as cardiomyocyte hypertrophy, myocardial contraction, myocardial infarction, and vascular remodelling. Finally, an overview of the existing challenges and potential future developments in the realm of lncRNA-encoded peptides was provided, with consideration given to prospective avenues for further research. Given that many lncRNA-encoded peptides have not been functionally annotated yet, their application in CVD diagnosis and treatment still requires further research.

The development of an endoplasmic reticulum-targeting fluorescent probe for the imaging of 1,4-dithiothreitol (DTT) in living cells

1,4-Dithiothreitol (DTT) is a robust reducing agent that contributes significantly to the folding process of proteins and maintaining endoplasmic reticulum (ER) homeostasis. Abnormally high levels of DTT can lead to severe endoplasmic reticulum stress (ERS), which induces cell death. In addition, DTT can also hinder cell growth and enhance reactive oxygen species (ROS) production in the ER. Herein, an effective turn-on ER-targeting fluorescent probe, ER-DTT, was designed to image DTT for the first time. The probe ER-DTT was based upon naphthalimide as a fluorophore, p-toluenesulfonamide as an exceptional unit for ER-targeting, and sulfoxide as a response site for imaging DTT based on an intramolecular charge transfer (ICT) mechanism. Optical-response experiments showed that the probe ER-DTT had good selectivity and sensitivity for DTT. Furthermore, confocal microscopy indicated that ER-DTT was suitable for selectively targeting ER in living cells and could be implemented to recognize cellular DTT.

Tunnels for Protein Export from the Endoplasmic Reticulum

The functions of coat protein complex II (COPII) coats in cargo packaging and the creation of vesicles at the endoplasmic reticulum are conserved in eukaryotic protein secretion. Standard COPII vesicles, however, cannot handle the secretion of metazoan-specific cargoes such as procollagens, apolipoproteins, and mucins. Metazoans have thus evolved modules centered on proteins like TANGO1 (transport and Golgi organization 1) to engage COPII coats and early secretory pathway membranes to engineer a novel mode of cargo export at the endoplasmic reticulum.

Vesicular and uncoated Rab1-dependent cargo carriers facilitate ER to Golgi transport

Secretory cargo is recognized, concentrated and trafficked from endoplasmic reticulum (ER) exit sites (ERES) to the Golgi. Cargo export from the ER begins when a series of highly conserved COPII coat proteins accumulate at the ER and regulate the formation of cargo-loaded COPII vesicles. In animal cells, capturing live de novo cargo trafficking past this point is challenging; it has been difficult to discriminate whether cargo is trafficked to the Golgi in a COPII-coated vesicle. Here, we describe a recently developed live-cell cargo export system that can be synchronously released from ERES to illustrate de novo trafficking in animal cells. We found that components of the COPII coat remain associated with the ERES while cargo is extruded into COPII-uncoated, non-ER associated, Rab1 (herein referring to Rab1a or Rab1b)-dependent carriers. Our data suggest that, in animal cells, COPII coat components remain stably associated with the ER at exit sites to generate a specialized compartment, but once cargo is sorted and organized, Rab1 labels these export carriers and facilitates efficient forward trafficking.This article has an associated First Person interview with the first author of the paper.

Probing the Heterogeneity of Protein Kinase Activation in Cells by Super-resolution Microscopy

Heterogeneity of mitogen-activated protein kinase (MAPK) activation in genetically identical cells, which occurs in response to epidermal growth factor receptor (EGFR) signaling, remains poorly understood. MAPK cascades integrate signals emanating from different EGFR spatial locations, including the plasma membrane and endocytic compartment. We previously hypothesized that in EGF-stimulated cells the MAPK phosphorylation (pMAPK) level and activity are largely determined by the spatial organization of the EGFR clusters within the cell. For experimental testing of this hypothesis, we used super-resolution microscopy to define EGFR clusters by receptor numbers (N) and average intracluster distances (d). From these data, we predicted the extent of pMAPK with 85% accuracy on a cell-to-cell basis with control data returning 54% accuracy (P < 0.001). For comparison, the prediction accuracy was only 61% (P = 0.382) when the diffraction-limited averaged fluorescence intensity/cluster was used. Large clusters (N ≥ 3) with d > 50 nm were most predictive for pMAPK level in cells. Electron microscopy revealed that these large clusters were primarily localized to the limiting membrane of multivesicular bodies (MVB). Many tighter packed dimers/multimers (d < 50 nm) were found on intraluminal vesicles within MVBs, where they were unlikely to activate MAPK because of the physical separation. Our results suggest that cell-to-cell differences in N and d contain crucial information to predict EGFR-activated cellular pMAPK levels and explain pMAPK heterogeneity in isogenic cells.

Rapid and facile quantitation of polyplex endocytic trafficking

Design of safe and effective synthetic nucleic acid delivery vectors such as polycation/DNA or polycation/siRNA complexes (polyplexes) will be facilitated by quantitative understanding of the mechanisms by which such materials escort cargo from the cell surface to the nucleus. In particular, the mechanisms of cellular internalization by various endocytosis pathways and subsequent endocytic vesicle trafficking have been shown to strongly affect nucleic acid delivery efficiency. Fluorescence microscopy and subcellular fractionation methods are commonly employed to follow intracellular trafficking of biomolecules and nanoparticulate delivery systems such as polyplexes. However, it is difficult to obtain quantitative data from microscopy and subcellular fractionation is experimentally difficult and low throughput. We have developed a method for quantifying the transport of polyplexes through important endocytic vesicles. The method is based on polymerization of 3,3'-diaminobenzidine by endocytosed horseradish peroxidase, causing an increase in the vesicle density, resistance to being solubilized by detergent and quenching of fluorophores within the vesicles, which makes them easy to separate and quantify. Using this method in HeLa cells, we have observed polyethylenimine/siRNA polyplexes initially appearing in early endosomes and rapidly moving to other compartments within 30min post-transfection. At the same time, we observed the kinetics of accumulation of the polyplexes in lysosomes at a similar rate. The results from the new method are consistent with similar measurements by confocal fluorescence microscopy and subcellular fractionation of endocytic vesicles on a Percoll gradient. The relative ease of this new method will aid investigation of gene delivery mechanisms by providing the means to rapidly quantify endocytic trafficking of polyplexes and other vectors.

Mitotic Golgi disassembly is required for bipolar spindle formation and mitotic progression

During mitosis, the mammalian Golgi vesiculates and, upon partitioning, reassembles in each daughter cell; however, it is not clear whether the disassembly process per se is important for partitioning or is merely an outcome of mitotic entry. Here, we show that Golgi vesiculation is required for progression to metaphase. To prevent Golgi disassembly, we expressed HRP linked to a Golgi-resident protein and acutely triggered the polymerization of 3,3'-diaminobenzidine (DAB) in the Golgi lumen. The DAB polymer does not affect interphase cell viability, but inhibits Golgi fragmentation by nocodazole and brefeldin A and also halts cells in early mitosis. The arrest is Golgi specific and does not occur when DAB is polymerized in the endosomes. Cells with a DAB polymer in the Golgi enter mitosis normally but arrest with an intact Golgi clustered at a monopolar spindle and an active spindle assembly checkpoint (SAC). Mitotic progression is restored upon centrosome depletion by the Polo-like kinase 4 inhibitor, centrinone, indicating that the link between the Golgi and the centrosomes must be dissolved to reach metaphase. These results demonstrate that Golgi disassembly is required for mitotic progression because failure to vesiculate the Golgi activates the canonical SAC. This requirement suggests that cells actively monitor Golgi integrity in mitosis.

Methods for monitoring lysosomal morphology

Lysosomes are abundant organelles best known for their crucial role in macromolecule turnover. Lysosome dysfunction features in several diseases exemplified by the lysosomal storage disorders and is often associated with marked changes in lysosome structure. Lysosomal morphology may therefore serve as a sensitive readout of endocytic well-being. Here we describe methods for monitoring lysosome morphology in fixed and live cells using fluorescent probes and electron microscopy.

A genetically specified connectomics approach applied to long-range feeding regulatory circuits

Synaptic connectivity and molecular composition provide a blueprint for information processing in neural circuits. Detailed structural analysis of neural circuits requires nanometer resolution, which can be obtained with serial-section electron microscopy. However, this technique remains challenging for reconstructing molecularly defined synapses. We used a genetically encoded synaptic marker for electron microscopy (GESEM) based on intra-vesicular generation of electron-dense labeling in axonal boutons. This approach allowed the identification of synapses from Cre recombinase-expressing or GAL4-expressing neurons in the mouse and fly with excellent preservation of ultrastructure. We applied this tool to visualize long-range connectivity of AGRP and POMC neurons in the mouse, two molecularly defined hypothalamic populations that are important for feeding behavior. Combining selective ultrastructural reconstruction of neuropil with functional and viral circuit mapping, we characterized some basic features of circuit organization for axon projections of these cell types. Our findings demonstrate that GESEM labeling enables long-range connectomics with molecularly defined cell types.

The TBC/RabGAP Armus coordinates Rac1 and Rab7 functions during autophagy

Autophagy is an evolutionarily conserved process that enables catabolic and degradative pathways. These pathways commonly depend on vesicular transport controlled by Rabs, small GTPases inactivated by TBC/RabGAPs. The Rac1 effector TBC/RabGAP Armus (TBC1D2A) is known to inhibit Rab7, a key regulator of lysosomal function. However, the precise coordination of signaling and intracellular trafficking that regulates autophagy is poorly understood. We find that overexpression of Armus induces the accumulation of enlarged autophagosomes, while Armus depletion significantly delays autophagic flux. Upon starvation-induced autophagy, Rab7 is transiently activated. This spatiotemporal regulation of Rab7 guanosine triphosphate/guanosine diphosphate cycling occurs by Armus recruitment to autophagosomes via interaction with LC3, a core autophagy regulator. Interestingly, autophagy potently inactivates Rac1. Active Rac1 competes with LC3 for interaction with Armus and thus prevents its appropriate recruitment to autophagosomes. The precise coordination between Rac1 and Rab7 activities during starvation suggests that Armus integrates autophagy with signaling and endocytic trafficking.

Silencing of mammalian Sar1 isoforms reveals COPII-independent protein sorting and transport

The Sar1 GTPase coordinates the assembly of coat protein complex-II (COPII) at specific sites of the endoplasmic reticulum (ER). COPII is required for ER-to-Golgi transport, as it provides a structural and functional framework to ship out protein cargoes produced in the ER. To investigate the requirement of COPII-mediated transport in mammalian cells, we used small interfering RNA (siRNA)-mediated depletion of Sar1A and Sar1B. We report that depletion of these two mammalian forms of Sar1 disrupts COPII assembly and the cells fail to organize transitional elements that coordinate classical ER-to-Golgi protein transfer. Under these conditions, minimal Golgi stacks are seen in proximity to juxtanuclear ER membranes that contain elements of the intermediate compartment, and from which these stacks coordinate biosynthetic transport of protein cargo, such as the vesicular stomatitis virus G protein and albumin. Here, transport of procollagen-I is inhibited. These data provide proof-of-principle for the contribution of alternative mechanisms that support biosynthetic trafficking in mammalian cells, providing evidence of a functional boundary associated with a bypass of COPII.

Distinct and opposing roles for Rab27a/Mlph/MyoVa and Rab27b/Munc13-4 in mast cell secretion

Mediator release from mast cells is a critical step in allergic and inflammatory disease. However, the processes regulating the latter stages of granule release are yet to be fully understood. Rab27 small GTPases regulate release of secretory lysosomes in a variety of cells, including mast cell granules. In the present study, using murine bone marrow-derived mast cells (BMMC) from Rab27-deficient mutant mice, we found that, in contrast to Rab27b, Rab27a primarily plays an inhibitory role in regulating degranulation. Immunofluorescence analysis revealed that resting Rab27a-deficient (ashen) BMMCs display abnormal cortical F-actin distribution. Actin disassembly prior to IgE cross-linking increased wild-type BMMC secretion to ashen levels, suggesting that changes in the integrity of cortical F-actin underlie the ashen phenotype. Comparison of the secretory impairment of Rab27b knockout and Rab27a/b double knockout BMMCs highlighted a secondary positive role for Rab27a in enhancing degranulation. Rab27 is known to interact with actin via its effectors melanophilin (Mlph) and myosin Va (MyoVa) in other cell types. To better understand the differing roles of Rab27 proteins, we analysed the secretory phenotype of BMMCs derived from mice lacking Rab27 effector proteins. These experiments revealed that the phenotype of BMMCs deficient in Mlph (leaden) and BMMCs deficient in MyoVa (dilute) resembles the hyper-secretion of ashen BMMCs, while Munc13-4-deficient (jinx) BMMCs phenocopy the Rab27b knockout and double Rab27a/b knockout secretory impairment. We conclude that Rab27a and Rab27b regulate distinct steps in the BMMC degranulation pathway, with Rab27a/Mlph/MyoVa regulating cortical actin stability upstream of Rab27a/b/Munc13-4-dependent granule exocytosis.

The kiss-and-run model of intra-Golgi transport

The Golgi apparatus (GA) is the main station along the secretory pathway. Mechanisms of intra-Golgi transport remain unresolved. Three models compete with each other for the right to be defined as the paradigm. The vesicular model cannot explain the following: (1) lipid droplets and aggregates of procollagen that are larger than coatomer I (COPI)-dependent vesicles are transported across the GA; and (2) most anterograde cargoes are depleted in COPI vesicles. The compartment progression/maturation model has the following problems: (1) most Golgi-resident proteins are depleted in COPI vesicles; (2) there are no COPI vesicles for the recycling of the resident proteins in the trans-most-Golgi cisterna; and (3) different proteins have different rates of intra-Golgi transport. The diffusion model based on permanent inter-cisternal connections cannot explain the existence of lipid, ionic and protein gradients across the Golgi stacks. In contrast, the kiss-and-run model has the potential to explain most of the experimental observations. The kiss-and-run model can be symmetric when fusion and then fission occurs in the same place, and asymmetric when fusion takes place in one location, whereas fission takes place in another. The asymmetric kiss-and-run model resembles the carrier maturation mechanism, and it can be used to explain the transport of large cargo aggregates.

Interaction between FIP5 and SNX18 regulates epithelial lumen formation

The Rab11 GTPase-binding protein FIP5 collaborates with the sorting nexin 18 to transport proteins to the apical surface and to tubulate membranes during epithelial apical lumen formation.

During the morphogenesis of the epithelial lumen, apical proteins are thought to be transported via endocytic compartments to the site of the forming lumen, although the machinery mediating this transport remains to be elucidated. Rab11 GTPase and its binding protein, FIP5, are important regulators of polarized endocytic transport. In this study, we identify sorting nexin 18 as a novel FIP5-interacting protein and characterize the role of FIP5 and SNX18 in epithelial lumen morphogenesis. We show that FIP5 mediates the transport of apical proteins from apical endosomes to the apical plasma membrane and, along with SNX18, is required for the early stages of apical lumen formation. Furthermore, both proteins bind lipids, and FIP5 promotes the capacity of SNX18 to tubulate membranes, which implies a role for FIP5 and SNX18 in endocytic carrier formation and/or scission. In summary, the present findings support the hypothesis that this FIP5-SNX18 complex plays a pivotal role in the polarized transport of apical proteins during apical lumen initiation in epithelial cells.

Endophilin drives the fast mode of vesicle retrieval in a ribbon synapse

Compensatory endocytosis of exocytosed membrane and recycling of synaptic vesicle components is essential for sustained synaptic transmission at nerve terminals. At the ribbon-type synapse of retinal bipolar cells, manipulations expected to inhibit the interactions of the clathrin adaptor protein complex (AP2) affect only the slow phase of endocytosis (τ = 10-15 s), leading to the conclusion that fast endocytosis (τ = 1-2 s) occurs by a mechanism that differs from the classical pathway of clathrin-coated vesicle retrieval from the plasma membrane. Here we investigate the role of endophilin in endocytosis at this ribbon synapse. Endophilin A1 is a synaptically enriched N-BAR domain-containing protein, suggested to function in clathrin-mediated endocytosis. Internal dialysis of the synaptic terminal with dominant-negative endophilin A1 lacking its linker and Src homology 3 (SH3) domain inhibited the fast mode of endocytosis, while slow endocytosis continued. Dialysis of a peptide that binds endophilin SH3 domain also decreased fast retrieval. Electron microscopy indicated that fast endocytosis occurred by retrieval of small vesicles in most instances. These results indicate that endophilin is involved in fast retrieval of synaptic vesicles occurring by a mechanism that can be distinguished from the classical pathway involving clathrin-AP2 interactions.

Neuronal protein trafficking: emerging consequences of endoplasmic reticulum dynamics

The highly polarized morphology and complex geometry of neurons is determined to a great extent by the structural and functional organization of the secretory pathway. It is intuitive to propose that the spatial arrangement of secretory organelles and their dynamic behavior impinge on protein trafficking and neuronal function, but these phenomena and their consequences are not well delineated. Here we analyze the architecture and motility of the archetypal endoplasmic reticulum (ER), and their relationship to the microtubule cytoskeleton and post-translational modifications of tubulin. We also review the dynamics of the ER in axons, dendrites and spines, and discuss the role of ER dynamics on protein mobility and trafficking in neurons.

Organisation of human ER-exit sites: requirements for the localisation of Sec16 to transitional ER

The COPII complex mediates the selective incorporation of secretory cargo and relevant machinery into budding vesicles at specialised sites on the endoplasmic reticulum membrane called transitional ER (tER). Here, we show using confocal microscopy, immunogold labelling of ultrathin cryosections and electron tomography that in human cells at steady state, Sec16 localises to cup-like structures of tER that are spatially distinct from the localisation of other COPII coat components. We show that Sec16 defines the tER, whereas Sec23-Sec24 and Sec13-Sec31 define later structures that precede but are distinct from the intermediate compartment. Steady-state localisation of Sec16 is independent of the localisation of downstream COPII components Sec23-Sec24 and Sec13-Sec31. Sec16 cycles on and off the membrane at a slower rate than other COPII components with a greater immobile fraction. We define the region of Sec16A that dictates its robust localisation of tER membranes and find that this requires both a highly charged region as well as a central domain that shows high sequence identity between species. The central conserved domain of Sec16 binds to Sec13 linking tER membrane localisation with COPII vesicle formation. These data are consistent with a model where Sec16 acts as a platform for COPII assembly at ERES.

Assembly, organization, and function of the COPII coat

A full mechanistic understanding of how secretory cargo proteins are exported from the endoplasmic reticulum for passage through the early secretory pathway is essential for us to comprehend how cells are organized, maintain compartment identity, as well as how they selectively secrete proteins and other macromolecules to the extracellular space. This process depends on the function of a multi-subunit complex, the COPII coat. Here we describe progress towards a full mechanistic understanding of COPII coat function, including the latest findings in this area. Much of our understanding of how COPII functions and is regulated comes from studies of yeast genetics, biochemical reconstitution and single cell microscopy. New developments arising from clinical cases and model organism biology and genetics enable us to gain far greater insight in to the role of membrane traffic in the context of a whole organism as well as during embryogenesis and development. A significant outcome of such a full understanding is to reveal how the machinery and processes of membrane trafficking through the early secretory pathway fail in disease states.

Rab11-FIP3 is critical for the structural integrity of the endosomal recycling compartment

Rab11-FIP3 is an endosomal recycling compartment (ERC) protein that is implicated in the process of membrane delivery from the ERC to sites of membrane insertion during cell division. Here we report that Rab11-FIP3 is critical for the structural integrity of the ERC during interphase. We demonstrate that knockdown of Rab11-FIP3 and expression of a mutant of Rab11-FIP3 that is Rab11-binding deficient cause loss of all ERC-marker protein staining from the pericentrosomal region of A431 cells. Furthermore, we find that fluorophore-labelled transferrin cannot access the pericentrosomal region of cells in which Rab11-FIP3 function has been perturbed. We find that this Rab11-FIP3 function appears to be specific because expression of the equivalent Rab11-binding deficient mutant of Rab-coupling protein does not perturb ERC morphology. In addition, we find that other organelles such as sorting and late endosomes are unaffected by loss of Rab11-FIP3 function. Finally, we demonstrate the presence of an extensive coiled-coil region between residues 463 and 692 of Rab11-FIP3, which exists as a dimer in solution and is critical to support its function on the ERC. Together, these data indicate that Rab11-FIP3 is necessary for the structural integrity of the pericentrosomal ERC.

Neurexin Ibeta and neuroligin are localized on opposite membranes in mature central synapses

Synaptogenesis requires formation of trans-synaptic complexes between neuronal cell-adhesion receptors. Heterophilic receptor pairs, such as neurexin Ibeta and neuroligin, can mediate distinct intracellular signals and form different cytoplasmic scaffolds in the pre- and post-synaptic neuron, and may be particularly important for synaptogenesis. However, the functions of neurexin and neuroligin depend on their distribution in the synapse. Neuroligin has been experimentally assigned to the post-synaptic membrane, while the localization of neurexin remains unclear. To study the subcellular distribution of neurexin Ibeta and neuroligin in mature cerebrocortical synapses, we have developed a novel method for the physical separation of junctional membranes and their direct analysis by western blotting. Using urea and dithiothreitol, we disrupted trans-synaptic protein links, without dissolving the lipid phase, and fractionated the pre- and post-synaptic membranes. The purity of these fractions was validated by electron microscopy and western blotting using multiple synaptic markers. A quantitative analysis has confirmed that neuroligin is localized strictly in the post-synaptic membrane. We have also demonstrated that neurexin Ibeta is largely (96%) pre-synaptic. Thus, neurexin Ibeta and neuroligin normally form trans-synaptic complexes and can transduce bidirectional signals.

Analysis of de novo Golgi complex formation after enzyme-based inactivation

The Golgi complex is characterized by its unique morphology of closely apposed flattened cisternae that persists despite the large quantity of lipids and proteins that transit bidirectionally. Whether such a structure is maintained through endoplasmic reticulum (ER)-based recycling and auto-organization or whether it depends on a permanent Golgi structure is strongly debated. To further study Golgi maintenance in interphase cells, we developed a method allowing for a drug-free inactivation of Golgi dynamics and function in living cells. After Golgi inactivation, a new Golgi-like structure, containing only certain Golgi markers and newly synthesized cargoes, was produced. However, this structure did not acquire a normal Golgi architecture and was unable to ensure a normal trafficking activity. This suggests an integrative model for Golgi maintenance in interphase where the ER is able to autonomously produce Golgi-like structures that need pre-existing Golgi complexes to be organized as morphologically normal and active Golgi elements.

Neurotrophins Redirect p75NTRfrom a Clathrin‐Independent to a Clathrin‐Dependent Endocytic Pathway Coupled to Axonal Transport

The p75 neurotrophin receptor (p75(NTR)) plays multiple roles in neuronal physiology through interactions with many ligands and coreceptors. However, its intracellular neuronal trafficking prior to and after neurotrophin activation is still poorly characterized. We have previously shown that in response to nerve growth factor (NGF), p75(NTR) is retrogradely transported along the axons of motor neurons (MNs) in carriers shared with NGF, brain-derived neurotrophic factor and the tyrosine kinase receptor TrkB. Here, we report that NGF does not enhance the internalization or degradation of p75(NTR), which undergoes a rapid dynamin-dependent and clathrin-independent recycling process in MNs. Instead, incubation of cells with NGF leads to the redirection of a pool of plasma membrane p75(NTR) into clathrin-coated pits. The subsequent internalization of p75(NTR) via clathrin-mediated endocytosis, as well as the activity of Rab5, are essential for the sorting of the p75(NTR)-containing endosomes to the axonal retrograde transport pathway and for the delivery of p75(NTR) to the soma. Our findings suggest that the spatial regulation of p75(NTR) signalling is controlled by these ligand-driven routes of endocytosis.

Membranous structures transfer cell surface proteins across NK cell immune synapses

Intercellular transfer of cell surface proteins is widespread and facilitates several recently discovered means for immune cell communication. Here, we examined the molecular mechanism for intercellular exchange of the natural killer (NK) cell receptor KIR2DL1 and HLA-C, prototypical proteins that swap between NK cells and target cells. Transfer was contact dependent and enhanced for cells expressing cognate receptor/ligand pairs but did not depend on KIR2DL1 signaling. To a lesser extent, proteins transferred independent from specific recognition. Intracellular domains of transferred proteins were not exposed to the extracellular environment and transferred proteins were removed by brief exposure to low pH. By fluorescence microscopy, transferred proteins localized to discrete regions on the recipient cell surface. Higher resolution scanning electron micrographs revealed that transferred proteins were located within specific membranous structures. Transmission electron microscopy of the immune synapse revealed that membrane protrusions from one cell interacted with the apposing cell surface within the synaptic cleft. These data, coupled with previous observations, lead us to propose that intercellular protein transfer is mediated by membrane protrusions within and surrounding the immunological synapse.

Tetanus toxin is internalized by a sequential clathrin-dependent mechanism initiated within lipid microdomains and independent of epsin1

Ligand-receptor complexes are internalized by a variety of endocytic mechanisms. Some are initiated within clathrin-coated membranes, whereas others involve lipid microdomains of the plasma membrane. In neurons, where alternative targeting to short- or long-range trafficking routes underpins the differential processing of synaptic vesicle components and neurotrophin receptors, the mechanism giving access to the axonal retrograde pathway remains unknown. To investigate this sorting process, we examined the internalization of a tetanus neurotoxin fragment (TeNT HC), which shares axonal carriers with neurotrophins and their receptors. Previous studies have shown that the TeNT HC receptor, which comprises polysialogangliosides, resides in lipid microdomains. We demonstrate that TeNT HC internalization also relies on a specialized clathrin-mediated pathway, which is independent of synaptic vesicle recycling. Moreover, unlike transferrin uptake, this AP-2-dependent process is independent of epsin1. These findings identify a pathway for TeNT, beginning with the binding to a lipid raft component (GD1b) and followed by dissociation from GD1b as the toxin internalizes via a clathrin-mediated mechanism using a specific subset of adaptor proteins.

P-Selectin and CD63 Use Different Mechanisms for Delivery to Weibel-Palade Bodies

The biogenesis of endothelial-specific Weibel-Palade bodies (WPB) is poorly understood, despite their key role in both haemostasis and inflammation. Biogenesis of specialized organelles of haemopoietic cells is often adaptor protein complex 3-dependent (AP-3-dependent), and AP-3 has previously been shown to play a role in the trafficking of both WPB membrane proteins, P-selectin and CD63. However, WPB are thought to form at the trans Golgi network (TGN), which is inconsistent with a role for AP-3, which operates in post-Golgi trafficking. We have therefore investigated in detail the mechanisms of delivery of these two membrane proteins to WPB. We find that P-selectin is recruited to forming WPB in the trans-Golgi by AP-3-independent mechanisms that use sorting information within both the cytoplasmic tail and the lumenal domain of the receptor. In contrast, CD63 is recruited to already-budded WPB by an AP-3-dependent route. These different mechanisms of recruitment lead to the presence of distinct immature and mature populations of WPB in human umbilical vein endothelial cells (HUVEC).

Constitutive sharing of recycling synaptic vesicles between presynaptic boutons

The synaptic vesicle cycle is vital for sustained neurotransmitter release. It has been assumed that functional synaptic vesicles are replenished autonomously at individual presynaptic terminals. Here we tested this assumption by using FM dyes in combination with fluorescence recovery after photobleaching and correlative light and electron microscopy in cultured rat hippocampal neurons. After photobleaching, synapses acquired recently recycled FM dye-labeled vesicles originating from nonphotobleached synapses by a process requiring dynamic actin turnover. The imported vesicles entered the functional pool at their host synapses, as revealed by the exocytic release of the dye upon stimulation. FM1-43 photoconversion and ultrastructural analysis confirmed the incorporation of imported vesicles into the presynaptic terminal, where they mixed with the native vesicle pools. Our results demonstrate that synaptic vesicle recycling is not confined to individual presynaptic terminals as is widely believed; rather, a substantial proportion of recycling vesicles are shared constitutively between boutons.

Depletion of TSG101 forms a mammalian `Class E' compartment: a multicisternal early endosome with multiple sorting defects

The early endosome comprises morphologically distinct regions specialised in sorting cargo receptors. A central question is whether receptors move through a predetermined structural pathway, or whether cargo selection contributes to the generation of endosome morphology and membrane flux. Here, we show that depletion of tumour susceptibility gene 101 impairs the selection of epidermal growth factor receptor away from recycling receptors within the limiting membrane of the early endosome. Consequently, epidermal growth factor receptor sorting to internal vesicles of the multivesicular body and cargo recycling to the cell surface or Golgi complex are inhibited. These defects are accompanied by disruption of bulk flow transport to the lysosome and profound structural rearrangement of the early endosome. The pattern of tubular and vacuolar domains is replaced by enlarged vacuoles, many of which are folded into multicisternal structures resembling the `Class E' compartments that define several Saccharomyces cerevisiae vacuolar protein sorting mutants. The cisternae are interleaved by a fine matrix but lack other surface elaborations, most notably clathrin.

Dicumarol, an inhibitor of ADP-ribosylation of CtBP3/BARS, fragments golgi non-compact tubular zones and inhibits intra-golgi transport

Dicumarol (3,3'-methylenebis[4-hydroxycoumarin]) is an inhibitor of brefeldin-A-dependent ADP-ribosylation that antagonises brefeldin-A-dependent Golgi tubulation and redistribution to the endoplasmic reticulum. We have investigated whether dicumarol can directly affect the morphology of the Golgi apparatus. Here we show that dicumarol induces the breakdown of the tubular reticular networks that interconnect adjacent Golgi stacks and that contain either soluble or membrane-associated cargo proteins. This results in the formation of 65-120-nm vesicles that are sometimes invaginated. In contrast, smaller vesicles (45-65 nm in diameter, a size consistent with that of coat-protein-I-dependent vesicles) that excluded cargo proteins from their lumen are not affected by dicumarol. All other endomembranes are largely unaffected by dicumarol, including Golgi stacks, the ER, multivesicular bodies and the trans-Golgi network. In permeabilized cells, dicumarol activity depends on the function of CtBP3/BARS protein and pre-ADP-ribosylation of cytosol inhibits the breakdown of Golgi tubules by dicumarol. In functional experiments, dicumarol markedly slows down intra-Golgi traffic of VSV-G transport from the endoplasmic reticulum to the medial Golgi, and inhibits the diffusional mobility of both galactosyl transferase and VSV-G tagged with green fluorescent protein. However, it does not affect: transport from the trans-Golgi network to the cell surface; Golgi-to-endoplasmic reticulum traffic of ERGIC58; coat-protein-I-dependent Golgi vesiculation by AlF4 or ADP-ribosylation factor; or ADP-ribosylation factor and beta-coat protein binding to Golgi membranes. Thus the ADP-ribosylation inhibitor dicumarol induces the selective breakdown of the tubular components of the Golgi complex and inhibition of intra-Golgi transport. This suggests that lateral diffusion between adjacent stacks has a role in protein transport through the Golgi complex.

Golgi enzymes are enriched in perforated zones of golgi cisternae but are depleted in COPI vesicles

In the most widely accepted version of the cisternal maturation/progression model of intra-Golgi transport, the polarity of the Golgi complex is maintained by retrograde transport of Golgi enzymes in COPI-coated vesicles. By analyzing enzyme localization in relation to the three-dimensional ultrastructure of the Golgi complex, we now observe that Golgi enzymes are depleted in COPI-coated buds and 50- to 60-nm COPI-dependent vesicles in a variety of different cell types. Instead, we find that Golgi enzymes are concentrated in the perforated zones of cisternal rims both in vivo and in a cell-free system. This lateral segregation of Golgi enzymes is detectable in some stacks during steady-state transport, but it was significantly prominent after blocking endoplasmic reticulum-to-Golgi transport. Delivery of transport carriers to the Golgi after the release of a transport block leads to a diminution in Golgi enzyme concentrations in perforated zones of cisternae. The exclusion of Golgi enzymes from COPI vesicles and their transport-dependent accumulation in perforated zones argues against the current vesicle-mediated version of the cisternal maturation/progression model.

Farnesyltransferase inhibitors disrupt EGF receptor traffic through modulation of the RhoB GTPase

The Rho family of small GTPases play a pivotal role in the dynamic regulation of the actin cytoskeleton. Recent studies have suggested that these signalling proteins also have wide-ranging functions in membrane trafficking pathways. The Rho family member RhoB was shown to localise to vesicles of the endocytic compartment, suggesting a potential function in regulation of endocytic traffic. In keeping with this, we have previously shown that expression of active RhoB causes a delay in the intracellular trafficking of the epidermal growth factor (EGF) receptor; however, the site of action of RhoB within the endocytic pathway is still unknown. RhoB exists as two prenylated forms in cells: geranylgeranylated RhoB (RhoB-GG) and farnesylated RhoB (RhoB-F). Here we use farnesyltransferase inhibitors (FTIs) to show that prenylation specifies the cellular localisation of RhoB. RhoB-GG localises to multivesicular late endosomes and farnesylated RhoB (RhoB-F) localises to the plasma membrane. The gain of endosomal RhoB-GG elicited by FTI treatment reduces sorting of EGF receptor to the lysosome and increases recycling to the plasma membrane. Ultrastructural analysis shows that activation of RhoB through drug treatment or mutation has no effect the sorting of receptor into late endosomes, but instead inhibits the subsequent transfer of late endosomal receptor to the lysosome.

Annexin 11 is required for midbody formation and completion of the terminal phase of cytokinesis

Annexins are Ca(2+)-binding, membrane-fusogenic proteins with diverse but poorly understood functions. Here, we show that during cell cycle progression annexin 11 translocates from the nucleus to the spindle poles in metaphase and to the spindle midzone in anaphase. Annexin 11 is recruited to the midbody in late telophase, where it forms part of the detergent-resistant matrix that also contains CHO1. To investigate the significance of these observations, we used RNA interference to deplete cells of annexin 11. A combination of confocal and video time-lapse microscopy revealed that cells lacking annexin 11 fail to establish a functional midbody. Instead, daughter cells remain connected by intercellular bridges that contain bundled microtubules and cytoplasmic organelles but exclude normal midbody components such as MKLP1 and Aurora B. Annexin 11-depleted cells failed to complete cytokinesis and died by apoptosis. These findings demonstrate an essential role for annexin 11 in the terminal phase of cytokinesis.

Active and specific recruitment of a soluble cargo protein for endoplasmic reticulum exit in the absence of functional COPII component Sec24p

Exit of proteins from the yeast endoplasmic reticulum (ER) is thought to occur in vesicles coated by four proteins, Sec13p, Sec31p, Sec23p and Sec24p, which assemble at ER exit sites to form the COPII coat. Sec13p may serve a structural function, whereas Sec24p has been suggested to operate in selection of cargo proteins into COPII vesicles. We showed recently that the soluble glycoprotein Hsp150 exited the ER in the absence of Sec13p function. Here we show that its ER exit did not require functional Sec24p. Hsp150 was secreted to the medium in a sec24-1 mutant at restrictive temperature 37 degrees C, while cell wall invertase and vacuolar carboxypeptidase Y remained in the ER. The determinant guiding Hsp150 to this transport route was mapped to the C-terminal domain of 114 amino acids by deletion analysis, and by an HRP fusion protein-based EM technology adapted here for yeast. This domain actively mediated ER exit of Sec24p-dependent invertase in the absence of Sec24p function. However, the domain was entirely dispensable for ER exit when Sec24p was functional. The Sec24p homolog Sfb2p was shown not to compensate for nonfunctional Sec24p in ER exit of Hsp150. Our data show that a soluble cargo protein, Hsp150, is selected actively and specifically to budding sites lacking normal Sec24p by a signature residing in its C-terminal domain.

Membrane targeting of Rab GTPases is influenced by the prenylation motif

Rab GTPases are regulators of membrane traffic. Rabs specifically associate with target membranes via the attachment of (usually) two geranylgeranyl groups in a reaction involving Rab escort protein and Rab geranylgeranyl transferase. In contrast, related GTPases are singly prenylated by CAAX prenyl transferases. We report that di-geranylgeranyl modification is important for targeting of Rab5a and Rab27a to endosomes and melanosomes, respectively. Transient expression of EGFP-Rab5 mutants containing two prenylatable cysteines (CGC, CC, CCQNI, and CCA) in HeLa cells did not affect endosomal targeting or function, whereas mono-cysteine mutants (CSLG, CVLL, or CVIM) were mistargeted to the endoplasmic reticulum (ER) and were nonfunctional. Similarly, Rab27aCVLL mutant is also mistargeted to the ER and transgenic expression on a Rab27a null background (Rab27aash) did not rescue the coat color phenotype, suggesting that Rab27aCVLL is not functional in vivo. CAAX prenyl transferase inhibition and temperature-shift experiments further suggest that Rabs, singly or doubly modified are recruited to membranes via a Rab escort protein/Rab geranylgeranyl transferase-dependent mechanism that is distinct from the insertion of CAAX-containing GTPases. Finally, we show that both singly and doubly modified Rabs are extracted from membranes by RabGDIalpha and propose that the mistargeting of Rabs to the ER results from loss of targeting information.

Selective and signal-dependent recruitment of membrane proteins to secretory granules formed by heterologously expressed von Willebrand factor

von Willebrand factor (vWF) is a large, multimeric protein secreted by endothelial cells and involved in hemostasis. When expressed in AtT-20 cells, vWF leads to the de novo formation of cigar-shaped organelles similar in appearance to the Weibel-Palade bodies of endothelial cells in which vWF is normally stored before regulated secretion. The membranes of this vWF-induced organelle, termed the pseudogranule, are uncharacterized. We have examined the ability of these pseudogranules, which we show are secretagogue responsive, to recruit membrane proteins. Coexpression experiments show that the Weibel-Palade body proteins P-selectin and CD63, as well as the secretory organelle membrane proteins vesicle-associated membrane protein-2 and synaptotagmin I are diverted away from the endogenous adrenocorticotropic hormone-containing secretory granules to the vWF-containing pseudogranules. However, transferrin receptor, lysosomal-associated membrane protein 1, and sialyl transferase are not recruited. The recruitment of P-selectin is dependent on a tyrosine-based motif within its cytoplasmic domain. Our data show that vWF pseudogranules specifically recruit a subset of membrane proteins, and that in a process explicitly driven by the pseudogranule content (i.e., vWF), the active recruitment of at least one component of the pseudogranule membrane (i.e., P-selectin) is dependent on residues of P-selectin that are cytosolic and therefore unable to directly interact with vWF.

Human VPS34 is required for internal vesicle formation within multivesicular endosomes

After internalization from the plasma membrane, activated EGF receptors (EGFRs) are delivered to multivesicular bodies (MVBs). Within MVBs, EGFRs are removed from the perimeter membrane to internal vesicles, thereby being sorted from transferrin receptors, which recycle back to the plasma membrane. The phosphatidylinositol (PI) 3'-kinase inhibitor, wortmannin, inhibits internal vesicle formation within MVBs and causes EGFRs to remain in clusters on the perimeter membrane. Microinjection of isotype-specific inhibitory antibodies demonstrates that the PI 3'-kinase required for internal vesicle formation is hVPS34. In the presence of wortmannin, EGFRs continue to be delivered to lysosomes, showing that their removal from the recycling pathway and their delivery to lysosomes does not depend on inward vesiculation. We showed previously that tyrosine kinase-negative EGFRs fail to accumulate on internal vesicles of MVBs but are recycled rather than delivered to lysosomes. Therefore, we conclude that selection of EGFRs for inclusion on internal vesicles requires tyrosine kinase but not PI 3'-kinase activity, whereas vesicle formation requires PI 3'-kinase activity. Finally, in wortmannin-treated cells there is increased EGF-stimulated tyrosine phosphorylation when EGFRs are retained on the perimeter membrane of MVBs. Therefore, we suggest that inward vesiculation is involved directly with attenuating signal transduction.

Assembly and maturation of the flavivirus Kunjin virus appear to occur in the rough endoplasmic reticulum and along the secretory pathway, respectively

The intracellular assembly site for flaviviruses in currently not known but is presumed to be located within the lumen of the rough endoplasmic reticulum (RER). Building on previous studies involving immunofluorescence (IF) and cryoimmunoelectron microscopy of Kunjin virus (KUN)-infected cells, we sought to identify the steps involved in the assembly and maturation of KUN. Thus, using antibodies directed against envelope protein E in IF analysis, we found the accumulation of E within regions coincident with the RER and endosomal compartments. Immunogold labeling of cryosections of infected cells indicated that E and minor envelope protein prM were localized to reticulum membranes continuous with KUN-induced convoluted membranes (CM) or paracrystalline arrays (PC) and that sometimes the RER contained immunogold-labeled virus particles. Both proteins were also observed to be labeled in membranes at the periphery of the induced CM or PC structures, but the latter were very seldom labeled internally. Utilizing drugs that inhibit protein and/or membrane traffic throughout the cell, we found that the secretion of KUN particles late in infection was significantly affected in the presence of brefeldin A and that the infectivity of secreted particles was severely affected in the presence of monensin and N-nonyl-deoxynojirimycin. Nocodazole did not appear to affect maturation, suggesting that microtubules play no role in assembly or maturation processes. Subsequently, we showed that the exit of intact virions from the RER involves the transport of individual virions within individual vesicles en route to the Golgi apparatus. The results suggest that the assembly of virions occurs within the lumen of the RER and that subsequent maturation occurs via the secretory pathway.

Golgi clusters and vesicles mediate mitotic inheritance independently of the endoplasmic reticulum

We have examined the fate of Golgi membranes during mitotic inheritance in animal cells using four-dimensional fluorescence microscopy, serial section reconstruction of electron micrographs, and peroxidase cytochemistry to track the fate of a Golgi enzyme fused to horseradish peroxidase. All three approaches show that partitioning of Golgi membranes is mediated by Golgi clusters that persist throughout mitosis, together with shed vesicles that are often found associated with spindle microtubules. We have been unable to find evidence that Golgi membranes fuse during the later phases of mitosis with the endoplasmic reticulum (ER) as a strategy for Golgi partitioning (Zaal, K.J., C.L. Smith, R.S. Polishchuk, N. Altan, N.B. Cole, J. Ellenberg, K. Hirschberg, J.F. Presley, T.H. Roberts, E. Siggia, et al. 1999. Cell. 99:589-601) and suggest that these results, in part, are the consequence of slow or abortive folding of GFP-Golgi chimeras in the ER. Furthermore, we show that accurate partitioning is accomplished early in mitosis, by a process of cytoplasmic redistribution of Golgi fragments and vesicles yielding a balance of Golgi membranes on either side of the metaphase plate before cell division.

Overexpression of auxin-binding protein enhances the sensitivity of guard cells to auxin

To explore the role of auxin-binding protein (ABP1) in planta, a number of transgenic tobacco (Nicotiana tabacum) lines were generated. The wild-type KDEL endoplasmic reticulum targeting signal was mutated to HDEL, another common retention sequence in plants, and to KEQL or KDELGL to compromise its activity. The auxin-binding kinetics of these forms of ABP1 were found to be similar to those of ABP1 purified from maize (Zea mays). To test for a physiological response mediated by auxin, intact guard cells of the transgenic plants were impaled with double-barreled microelectrodes, and auxin-dependent changes in K(+) currents were recorded under voltage clamp. Exogenous auxin affected inwardly and outwardly rectifying K(+) currents in a dose-dependent manner. Auxin sensitivity was markedly enhanced in all plants overexpressing ABP1, irrespective of the form present. Immunogold electron microscopy was used to investigate the localization of ABP1 in the transgenic plants. All forms were detected in the endoplasmic reticulum and the KEQL and KDELGL forms passed further across the Golgi stacks than KDEL and HDEL forms. However, neither electron microscopy nor silver-enhanced immunogold epipolarization microscopy revealed differences in cell surface ABP1 abundance for any of the plants, including control plants, which indicated that overexpression of ABP1 alone was sufficient to confer increased sensitivity to added auxin. Jones et al. ([1998] Science 282: 1114-1117) found increased cell expansion in transgenic plants overexpressing wild-type ABP1. Single cell recordings extend this observation, with the demonstration that the auxin sensitivity of guard cell K(+) currents is mediated, at least in part, by ABP1.

The p58-positive pre-golgi intermediates consist of distinct subpopulations of particles that show differential binding of COPI and COPII coats and contain vacuolar H(+)-ATPase

We have studied the structural and functional properties of the pre-Golgi intermediate compartment (IC) in normal rat kidney cells using analytical cell fractionation with p58 as the principal marker. The sedimentation profile (sediterm) of p58, obtained by analytical differential centrifugation, revealed in steady-state cells the presence of two main populations of IC elements whose average sedimentation coefficients, s(H)=1150+/-58S (‘heavy’) and s(L)=158+/-8S (‘light’), differed from the s-values obtained for elements of the rough and smooth endoplasmic reticulum. High resolution analysis of these subpopulations in equilibrium density gradients further revealed that the large difference in their s-values was mainly due to particle size. The ‘light’ particle population contained the bulk of COPI and COPII coats, and redistribution of p58 to these particles was observed in transport-arrested cells, showing that the two types of elements are also compositionally distinct and have functional counterparts in intact cells. Using a specific antibody against the 16 kDa proteolipid subunit of the vacuolar H(+)-ATPase, an enrichment of the V(o)domain of the ATPase was observed in the p58-positive IC elements. Interestingly, these elements could contain both COPI and COPII coats and their density distribution was markedly affected by GTP(γ)S. Together with morphological observations, these results demonstrate that, in addition to clusters of small tubules and vesicles, the IC also consists of large-sized structures and corroborate the proposal that the IC elements contain an active vacuolar H(+)-ATPase.

Cytoskeleton mediating transport between the ER system and the Golgi apparatus in the green alga Scenedesmus acutus

In the green alga Scenedesmus acutus, Golgi bodies are located near the nucleus and supplied with transition vesicles that bud from the outer nuclear envelope membrane. Using this alga, we have shown previously that thiamine pyrophosphatase (TPPase), a marker enzyme of Golgi bodies, migrates in vesicles from the Golgi bodies to the ER via the nuclear envelope in the presence of BFA (Noguchi et al., Protoplasma 201, 202-212, 1998). In this study we demonstrate that both cytochalasin B and oryzalin (microtubule-disrupting agent) inhibit the BFA-induced migration of TPPase from Golgi bodies to the nuclear envelope. However, only actin filaments--not microtubules--can be detected between the nuclear envelope and the Golgi bodies in both BFA-treated and untreated cells. These observations suggest that actin filaments mediate the BFA-induced retrograde transport of vesicles. This mechanism differs from that found in mammalian cells, in which microtubules mediate BFA-induced retrograde transport by the elongation of membrane tubules from the Golgi cisternae. We also discuss the non-participation of the cytoskeleton in anterograde transport from the nuclear envelope to the Golgi bodies.

Dynamics of transitional endoplasmic reticulum sites in vertebrate cells

A typical vertebrate cell contains several hundred sites of transitional ER (tER). Presumably, tER sites generate elements of the ER-Golgi intermediate compartment (ERGIC), and ERGIC elements then generate Golgi cisternae. Therefore, characterizing the mechanisms that influence tER distribution may shed light on the dynamic behavior of the Golgi. We explored the properties of tER sites using Sec13 as a marker protein. Fluorescence microscopy confirmed that tER sites are long-lived ER subdomains. tER sites proliferate during interphase but lose Sec13 during mitosis. Unlike ERGIC elements, tER sites move very little. Nevertheless, when microtubules are depolymerized with nocodazole, tER sites redistribute rapidly to form clusters next to Golgi structures. Hence, tER sites have the unusual property of being immobile, yet dynamic. These findings can be explained by a model in which new tER sites are created by retrograde membrane traffic from the Golgi. We propose that the tER-Golgi system is organized by mutual feedback between these two compartments.

Secretory lysosome biogenesis in cytotoxic T lymphocytes from normal and Chediak Higashi syndrome patients

The lytic proteins mediating target cell killing are stored in the lysosomes of activated cytotoxic T lymphocytes (CTL) and are secreted upon recognition of a target cell. These secretory lysosomes cannot be detected in resting T lymphocytes. Interaction of a resting cell with a target cell activates de novo formation of secretory lysosomes. CTL clones in culture mimic this behaviour, and so provide an ideal system for studying secretory lysosome biogenesis and maturation. In the genetic disease, Chediak Higashi syndrome (CHS), all lysosomes in the cells are enlarged and reduced in number compared with wild-type (WT) cells. We have used CTL from this disease to study secretory lysosome biogenesis and maturation. We show that at early stages after activation the secretory lysosomes are identical in WT and mutant cells, and that delivery of proteins to the secretory lysosome along the biosynthetic and endocytic pathways is normal in the mutant cells. With time, the lysosomes in the mutant cells aggregate, become larger and fewer in number and eventually form giant structures. Our results show that the initial steps of secretory lysosome formation are normal in CHS, but that the organelles subsequently fuse together during cell maturation to form the giant secretory lysosomes.