Protein–lipid interactions in membrane trafficking at the Golgi complex

ADP-Ribosylation Factor Family of Small GTP-Binding Proteins: Their Membrane Recruitment, Activation, Crosstalk and Functions

Members of the ADP-ribosylation factor (ARF) family of guanine-nucleotide binding proteins play critical roles in various cellular processes, especially in regulating the secretory, and endocytic pathways. The fidelity of intracellular vesicular trafficking depends on proper activations and precise subcellular distributions of ARF family proteins regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Here we review recent progress in understanding the membrane recruitment, activation, crosstalk, and functions of ARF family proteins.

Oncogenic Roles of GOLPH3 in the Physiopathology of Cancer

Golgi phosphoprotein 3 (GOLPH3), a Phosphatidylinositol 4-Phosphate [PI(4)P] effector at the Golgi, is required for Golgi ribbon structure maintenance, vesicle trafficking and Golgi glycosylation. GOLPH3 has been validated as an oncoprotein through combining integrative genomics with clinopathological and functional analyses. It is frequently amplified in several solid tumor types including melanoma, lung cancer, breast cancer, glioma, and colorectal cancer. Overexpression of GOLPH3 correlates with poor prognosis in multiple tumor types including 52% of breast cancers and 41% to 53% of glioblastoma. Roles of GOLPH3 in tumorigenesis may correlate with several cellular activities including: (i) regulating Golgi-to-plasma membrane trafficking and contributing to malignant secretory phenotypes; (ii) controlling the internalization and recycling of key signaling molecules or increasing the glycosylation of cancer relevant glycoproteins; and (iii) influencing the DNA damage response and maintenance of genomic stability. Here we summarize current knowledge on the oncogenic pathways involving GOLPH3 in human cancer, GOLPH3 influence on tumor metabolism and surrounding stroma, and its possible role in tumor metastasis formation.

Activators and Effectors of the Small G Protein Arf1 in Regulation of Golgi Dynamics During the Cell Division Cycle

When eukaryotic cells divide, they must faithfully segregate not only the genetic material but also their membrane-bound organelles into each daughter cell. To assure correct partitioning of cellular contents, cells use regulatory mechanisms to verify that each stage of cell division has been correctly accomplished before proceeding to the next step. A great deal is known about mechanisms that regulate chromosome segregation during cell division, but we know much less about the mechanisms by which cellular organelles are partitioned, and how these processes are coordinated. The Golgi apparatus, the central sorting and modification station of the secretory pathway, disassembles during mitosis, a process that depends on Arf1 and its regulators and effectors. Prior to total disassembly, the Golgi ribbon in mammalian cells, composed of alternating cisternal stacks and tubular networks, undergoes fission of the tubular networks to produce individual stacks. Failure to carry out this unlinking leads to cell division arrest at late G2 prior to entering mitosis, an arrest that can be relieved by inhibition of Arf1 activation. The level of active Arf1-GTP drops during mitosis, due to inactivation of the major Arf1 guanine nucleotide exchange factor at the Golgi, GBF1. Expression of constitutively active Arf1 prevents Golgi disassembly, and leads to defects in chromosome segregation and cytokinesis. In this review, we describe recent advances in understanding the functions of Arf1 regulators and effectors in the crosstalk between Golgi structure and cell cycle regulation.

Super-resolution Stimulated Emission Depletion-Fluorescence Correlation Spectroscopy Reveals Nanoscale Membrane Reorganization Induced by Pore-Forming Proteins

Membrane-protein interactions play a central role in membrane mediated cellular processes ranging from signaling, budding, and fusion, to transport across the cell membrane. Of particular significance is the process of efficient protein olgomerization and transmembrane pore formation on the membrane surface; the primary virulent pathway for the action of antimicrobial peptides and pore forming toxins (PFTs). The suggested nanoscopic length scales and dynamic nature of such membrane lipid-protein interactions makes their detection extremely challenging. Using a combination of super-resolution stimulated emission depletion nanoscopy with fluorescence correlation spectroscopy (STED-FCS) we unravel the emergence of nanoscale lateral heterogeneity in supported bilayer membranes made up of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol upon interaction with the PFT, listeriolysin O (LLO). A distinct length scale-dependent dynamical crossover (<200 nm) from a Brownian diffusive regime is observed at 33 and 50% cholesterol compositions, indicating the partitioning of lipids into domains with variable cholesterol content. At 25% cholesterol content, this dyamical crossover is observed only in bilayers incubated with LLO providing evidence for the existence of sub ∼100 nm dynamical lipid nanodomains bound to LLO pore assemblies. By introducing asymmetry in cholesterol composition across the bilayer leaflets we infer that this domain formation is driven largely due to active cholesterol sequestration and transient trapping of lipids to the membrane bound motifs present in the toxins, en route to LLO oligomerization and subsequent pore formation. Bilayers prepared with labeled lipids present in either the proximal or distal leaflet allow us to track the dynamical perturbation in a leaflet-dependent manner upon LLO incubation. From the differences in the extent and intensity of the dynamical crossover as observed with STED-FCS, these experiments reveal that the affinity for cholesterol in the membrane binding motifs of the LLO subdomains induce cholesterol and lipid reorganization to a greater extent in the distal (upper) leaflet when compared with the proximal (lower) leaflet. The observed length scale-dependent membrane reorganization that occurs due to invasion by LLO could be generalized to other cholesterol-dependent cytolysins and emphasizes the significant advantage of using super-resolution STED nanoscopy to unravel complex lipid-protein interactions in membrane and cellular biophysics.

Phenomenology based multiscale models as tools to understand cell membrane and organelle morphologies

An intriguing question in cell biology is "how do cells regulate their shape?" It is commonly believed that the observed cellular morphologies are a result of the complex interaction among the lipid molecules (constituting the cell membrane), and with a number of other macromolecules, such as proteins. It is also believed that the common biophysical processes essential for the functioning of a cell also play an important role in cellular morphogenesis. At the cellular scale-where typical dimensions are in the order of micrometers-the effects arising from the molecular scale can either be modeled as equilibrium or non-equilibrium processes. In this chapter, we discuss the dynamically triangulated Monte Carlo technique to model and simulate membrane morphologies at the cellular scale, which in turn can be used to investigate several questions related to shape regulation in cells. In particular, we focus on two specific problems within the framework of isotropic and anisotropic elasticity theories: namely, (i) the origin of complex, physiologically relevant, membrane shapes due to the interaction of the membrane with curvature remodeling proteins, and (ii) the genesis of steady state cellular shapes due to the action of non-equilibrium forces that are generated by the fission and fusion of transport vesicles and by the binding and unbinding of proteins from the parent membrane.

Arfs at a glance

The Arf small G proteins regulate protein and lipid trafficking in eukaryotic cells through a regulated cycle of GTP binding and hydrolysis. In their GTP-bound form, Arf proteins recruit a specific set of protein effectors to the membrane surface. These effectors function in vesicle formation and tethering, non-vesicular lipid transport and cytoskeletal regulation. Beyond fundamental membrane trafficking roles, Arf proteins also regulate mitosis, plasma membrane signaling, cilary trafficking and lipid droplet function. Tight spatial and temporal regulation of the relatively small number of Arf proteins is achieved by their guanine nucleotide-exchange factors (GEFs) and GTPase-activating proteins (GAPs), which catalyze GTP binding and hydrolysis, respectively. A unifying function of Arf proteins, performed in conjunction with their regulators and effectors, is sensing, modulating and transporting the lipids that make up cellular membranes. In this Cell Science at a Glance article and the accompanying poster, we discuss the unique features of Arf small G proteins, their functions in vesicular and lipid trafficking in cells, and how these functions are modulated by their regulators, the GEFs and GAPs. We also discuss how these Arf functions are subverted by human pathogens and disease states.

Arf Proteins and Their Regulators: At the Interface Between Membrane Lipids and the Protein Trafficking Machinery

The Arf small GTP-binding (G) proteins regulate membrane traffic and organelle structure in eukaryotic cells through a regulated cycle of GTP binding and hydrolysis. The first function identified for Arf proteins was recruitment of cytosolic coat complexes to membranes to mediate vesicle formation. However, subsequent studies have uncovered additional functions, including roles in plasma membrane signalling pathways, cytoskeleton regulation, lipid droplet function, and non-vesicular lipid transport. In contrast to other families of G proteins, there are only a few Arf proteins in each organism, yet they function specifically at many different cellular locations. Part of this specificity is achieved by formation of complexes with their guanine nucleotide-exchange factors (GEFs) and GTPase activating proteins (GAPs) that catalyse GTP binding and hydrolysis, respectively. Because these regulators outnumber their Arf substrates by at least 3-to-1, an important aspect of understanding Arf function is elucidating the mechanisms by which a single Arf protein is incorporated into different GEF, GAP, and effector complexes. New insights into these mechanisms have come from recent studies showing GEF–effector interactions, Arf activation cascades, and positive feedback loops. A unifying theme in the function of Arf proteins, carried out in conjunction with their regulators and effectors, is sensing and modulating the properties of the lipids that make up cellular membranes.

Generation of unique poliovirus RNA replication organelles

Poliovirus (PV), a model for interactions of picornaviruses with host cells, replicates its genomic RNA in association with cellular membranes. The origin of PV replication membranes has not been determined. Hypotheses about the origin of replication membranes, based largely on localization of viral proteins, include modification of coat protein complex I (COPI) and/or COPII secretory pathway vesicles and subversion of autophagic membranes. Here, we use an antibody against double-stranded RNA (dsRNA) to identify replication complexes by detection of dsRNA replication intermediates. dsRNA signal is dependent on virus genome replication and colocalizes with the viral integral membrane protein 3A, which is part of the RNA replication complex. We show that early in infection, dsRNA does not colocalize with a marker for autophagic vesicles, making it unlikely that autophagosomes contribute to the generation of PV RNA replication membranes. We also find that dsRNA does not colocalize with a marker of the COPII coat, Sec31, and, in fact, we demonstrate proteasome-dependent loss of full-length Sec31 during PV infection. These data indicate that COPII vesicles are an unlikely source of PV replication membranes. We show that the Golgi resident G-protein Arf1 and its associated guanine nucleotide exchange factor (GEF), GBF1, transiently colocalize with dsRNA early in infection. In uninfected cells, Arf1 nucleates COPI coat formation, although during infection the COPI coat itself does not colocalize with dsRNA. Phosphatidylinositol-4-phosphate, which is associated with enterovirus-induced vesicles, tightly colocalizes with Arf1/GBF1 throughout infection. Our data point to a noncanonical role for some of the COPI-generating machinery in producing unique replication surfaces for PV RNA replication. IMPORTANCE Picornaviruses are a diverse and major cause of human disease, and their genomes replicate in association with intracellular membranes. There are multiple hypotheses to explain the nature and origin of these membranes, and a complete understanding of the host requirements for membrane rearrangement would provide novel drug targets essential for viral genome replication. Here, we study the model picornavirus, poliovirus, and show that some, but not all, components of the cellular machinery required for retrograde traffic from the Golgi apparatus to the endoplasmic reticulum are transiently present at the sites of viral RNA replication. We also show that the full-length Sec31 protein, which has been suggested to be present on PV RNA replication membranes, is lost during infection in a proteasome-dependent manner. This study helps to reconcile multiple hypotheses about the origin of poliovirus replication membranes and points to known host cell protein complexes that would make likely drug targets to inhibit picornavirus infections.

ARF family G proteins and their regulators: roles in membrane transport, development and disease

The ADP-ribosylation factor (ARF) family of guanine-nucleotide-binding (G) proteins, including the ARF proteins, ARF-like (ARL) proteins and SAR1, regulates membrane traffic and organelle structure, and each family member is regulated through a cycle of GTP binding and GTP hydrolysis, which activate and inactivate, respectively, the G protein.

Traditionally, ARFs have been characterized for their immediate effects in the recruitment of coat proteins to drive cargo sorting, the recruitment of enzymes that can alter membrane lipid composition and the regulation of cytoskeletal factors. Now, new roles for ARFs have been discovered at the Golgi complex, for example in driving lipid transport. ARL proteins are also being increasingly linked to coordination of trafficking with cytoskeletal processes, for example during ciliogenesis.

There is particular interest in the mechanisms that control recruitment of the ARF guanine nucleotide exchange factors (GEFs) that mediate GTP binding to ARFs and, in the case of the cytohesin (also known as ARNO) GEF, membrane recruitment is coupled to relief of autoinhibition. GEFs such as cytohesin may also participate in a cascade of activation between particular pairs of ARFs.

Traditionally, G protein signalling has been viewed as a linear pathway, with the GDP-bound form of an ARF protein being inactive; however, more recent studies have highlighted novel roles for these GDP-bound forms and have also shown that GEFs and GTPase-activating proteins (GAPs) themselves can engage in distinct signalling responses through scaffolding functions.

The ADP-ribosylation factor (ARF) and ARF-like (ARL) family of G proteins, which are known to regulate membrane traffic and organelle structure, are emerging as regulators of diverse processes, including lipid and cytoskeletal transport. Although traditionally viewed as part of a linear signalling pathway, ARFs and their regulators must now be considered to exist within functional networks, in which both the 'inactive' ARF and the regulators themselves can mediate distinct effects.

Members of the ADP-ribosylation factor (ARF) family of guanine-nucleotide-binding (G) proteins, including the ARF-like (ARL) proteins and SAR1, regulate membrane traffic and organelle structure by recruiting cargo-sorting coat proteins, modulating membrane lipid composition, and interacting with regulators of other G proteins. New roles of ARF and ARL proteins are emerging, including novel functions at the Golgi complex and in cilia formation. Their function is under tight spatial control, which is mediated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that catalyse GTP exchange and hydrolysis, respectively. Important advances are being gained in our understanding of the functional networks that are formed not only by the GEFs and GAPs themselves but also by the inactive forms of the ARF proteins.

The diverse functions of oxysterol-binding proteins

Oxysterol-binding protein (OSBP)-related proteins (ORPs) are lipid-binding proteins that are conserved from yeast to humans. They are implicated in many cellular processes including signaling, vesicular trafficking, lipid metabolism, and nonvesicular sterol transport. All ORPs contain an OSBP-related domain (ORD) that has a hydrophobic pocket that binds a single sterol. ORDs also contain additional membrane-binding surfaces, some of which bind phosphoinositides and may regulate sterol binding. Studies in yeast suggest that ORPs function as sterol transporters, perhaps in regions where organelle membranes are closely apposed. Yeast ORPs also participate in vesicular trafficking, although their role is unclear. In mammalian cells, some ORPs function as sterol sensors that regulate the assembly of protein complexes in response to changes in cholesterol levels. This review will summarize recent advances in our understanding of how ORPs bind lipids and membranes and how they function in diverse cellular processes.

Conserved molecular mechanisms underlying homeostasis of the Golgi complex

The Golgi complex performs a central function in the secretory pathway in the sorting and sequential processing of a large number of proteins destined for other endomembrane organelles, the plasma membrane, or secretion from the cell, in addition to lipid metabolism and signaling. The Golgi apparatus can be regarded as a self-organizing system that maintains a relatively stable morphofunctional organization in the face of an enormous flux of lipids and proteins. A large number of the molecular players that operate in these processes have been identified, their functions and interactions defined, but there is still debate about many aspects that regulate protein trafficking and, in particular, the maintenance of these highly dynamic structures and processes. Here, we consider how an evolutionarily conserved underlying mechanism based on retrograde trafficking that uses lipids, COPI, SNAREs, and tethers could maintain such a homeodynamic system.

Phosphoinositide phosphatases in cell biology and disease

Phosphoinositides are essential signaling molecules linked to a diverse array of cellular processes in eukaryotic cells. The metabolic interconversions of these phospholipids are subject to exquisite spatial and temporal regulation executed by arrays of phosphatidylinositol (PtdIns) and phosphoinositide-metabolizing enzymes. These include PtdIns- and phosphoinositide-kinases that drive phosphoinositide synthesis, and phospholipases and phosphatases that regulate phosphoinositide degradation. In the past decade, phosphoinositide phosphatases have emerged as topics of particular interest. This interest is driven by the recent appreciation that these enzymes represent primary mechanisms for phosphoinositide degradation, and because of their ever-increasing connections with human diseases. Herein, we review the biochemical properties of six major phosphoinositide phosphatases, the functional involvements of these enzymes in regulating phosphoinositide metabolism, the pathologies that arise from functional derangements of individual phosphatases, and recent ideas concerning the involvements of phosphoinositide phosphatases in membrane traffic control.

Screening for the drug-phospholipid interaction: correlation to phospholipidosis

Phospholipid bilayers represent a complex, anisotropic environment fundamentally different from bulk oil or octanol, for instance. Even "simple" drug association to phospholipid bilayers can only be fully understood if the slab-of-hydrocarbon approach is abandoned and the complex, anisotropic properties of lipid bilayers reflecting the chemical structures and organization of the constituent phospholipids are considered. The interactions of drugs with phospholipids are important in various processes, such as drug absorption, tissue distribution, and subcellular distribution. In addition, drug-lipid interactions may lead to changes in lipid-dependent protein activities, and further, to functional and morphological changes in cells, a prominent example being the phospholipidosis (PLD) induced by cationic amphiphilic drugs. Herein we briefly review drug-lipid interactions in general and the significance of these interactions in PLD in particular. We also focus on a potential causal connection between drug-induced PLD and steatohepatitis, which is induced by some cationic amphiphilic drugs.

Nuclear envelope formation: mind the gaps

During mitosis in metazoans, the nuclear envelope (NE) breaks down at prophase and reassembles at telophase. The regulation of NE assembly is essential to correct cell functioning. The complex issue of the regulation of NE formation remains to be solved. It is still uncertain that a single mechanism depicts NE formation during mitosis. The aim of this review is to address some of the cytological, biophysical, and molecular aspects of models of NE formation. Our emphasis is on the role of lipids and their modifying enzymes in envelope assembly. We consider how the NE can be used as a model in characterizing membrane dynamics during membrane fusion. Fusion mechanisms that give insight into the formation of the double membrane of the envelope are summarized. We speculate on the possible roles of phosphoinositides in membrane fusion and NE formation.

Toxoplasma gondii Hsp20 is a stripe-arranged chaperone-like protein associated with the outer leaflet of the inner membrane complex

BACKGROUND INFORMATION

Toxoplasma gondii is among the most successful parasites, with nearly half of the human population chronically infected. T. gondii has five sHsps [small Hsps (heat-shock proteins)] located in different subcellular compartments. Among them, Hsp20 showed to be localized at the periphery of the parasite body. sHsps are widespread, constituting the most poorly conserved family of molecular chaperones. The presence of sHsps in membrane structures is unusual.

RESULTS

The localization of Hsp20 was further analysed using high-resolution fluorescent light microscopy as well as electron microscopy, which revealed that Hsp20 is associated with the outer surface of the IMC (inner membrane complex), in a set of discontinuous stripes following the same spiralling trajectories as the subpellicular microtubules. The detergent extraction profile of Hsp20 was similar to that of GAP45 [45 kDa GAP (gliding-associated protein)], a glideosome protein associated with the IMC, but was different from that of IMC1 protein. Although we were unable to detect interacting protein partners of Hsp20 either in normal or stressed tachyzoites, an interaction of Hsp20 with phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate phospholipids could be observed.

CONCLUSIONS

Hsp20 was shown to be associated with a specialized membranous structure of the parasite, the IMC. This discontinuous striped-arrangement is unique in T. gondii, indicating that the topology of the outer leaflet of the IMC is not homogeneous.

Morphogenesis of post-Golgi transport carriers

The trans-Golgi network (TGN) is one of the main, if not the main, sorting stations in the process of intracellular protein trafficking. It is therefore of central importance to understand how the key players in the TGN-based sorting and delivery process, the post-Golgi carriers (PGCs), form and function. Over the last few years, modern morphological approaches have generated new insights into the questions of PGC biogenesis, structure and dynamics. Here, we present a view by which the “lifecycle” of a PGC consists of several distinct stages: the formation of TGN tubular export domains (where different cargoes are segregated from each other and from the Golgi enzymes); the docking of these tubular domains onto molecular motors and their extrusion towards the cell periphery along microtubules; the fission of the forming PGC from the donor membrane; and the delivery of the newly formed PGC to its specific acceptor organelle. It is now important to add the many molecular machineries that have been described as operating at the TGN to this “morphofunctional map” of the TGN export process.

AP-2-dependent internalization of potassium channel Kir2.3 is driven by a novel di-hydrophobic signal

The localization and density of Kir2.3 channels are influenced by the balance between PDZ protein interaction at the cell surface and routing into the endocytic pathway. Here, we explore mechanisms by which the Kir2.3 channel is directed into the endocytic pathway. We found that Kir2.3 channels are constitutively internalized from the cell surface in a dynamin-dependent manner, indicative of vesicle-mediated endocytosis. The rate of Kir2.3 endocytosis was dramatically attenuated following RNA interference-mediated knockdown of either alpha adaptin (AP-2 clathrin adaptor) or clathrin heavy chain, revealing that Kir2.3 is internalized by an AP-2 clathrin-dependent mechanism. Structure-rationalized mutagenesis studies of a number of different potential AP-2 interaction motifs indicate that internalization of Kir2.3 is largely dependent on a non-canonical di-isoleucine motif (II413) embedded within the C terminus. Internalization assays using CD4-Kir2.3 chimeras demonstrate that the di-isoleucine signal acts in an autonomous and transplantable manner. Kir2.3 co-immunoprecipitates with alpha adaptin, and disruption of the di-isoleucine motif decreased interaction of the channel with AP-2. Replacement of the di-isoleucine motif with a canonical di-leucine internalization signal actually blocked Kir2.3 endocytosis. Moreover, in yeast three-hybrid studies, the Kir2.3 di-isoleucine motif does not bind the AP-2 alphaC-sigma2 hemicomplex in the way that has been recently observed for canonical di-leucine signals. Altogether, the results indicate that Kir2.3 channels are marked for clathrin-dependent internalization from the plasma membrane by a novel AP-2-dependent signal.

Glycosphingolipid synthesis requires FAPP2 transfer of glucosylceramide

The molecular machinery responsible for the generation of transport carriers moving from the Golgi complex to the plasma membrane relies on a tight interplay between proteins and lipids. Among the lipid-binding proteins of this machinery, we previously identified the four-phosphate adaptor protein FAPP2, the pleckstrin homology domain of which binds phosphatidylinositol 4-phosphate and the small GTPase ARF1. FAPP2 also possesses a glycolipid-transfer-protein homology domain. Here we show that human FAPP2 is a glucosylceramide-transfer protein that has a pivotal role in the synthesis of complex glycosphingolipids, key structural and signalling components of the plasma membrane. The requirement for FAPP2 makes the whole glycosphingolipid synthetic pathway sensitive to regulation by phosphatidylinositol 4-phosphate and ARF1. Thus, by coupling the synthesis of glycosphingolipids with their export to the cell surface, FAPP2 emerges as crucial in determining the lipid identity and composition of the plasma membrane.

Variable actin dynamics requirement for the exit of different cargo from thetrans-Golgi network

Efficient post-Golgi trafficking depends on microtubules, but actin filaments and actin-associated proteins are also postulated. Here we examined, by inverse fluorescence recovery after photobleaching, the role of actin dynamics in the exit from the TGN of fluorescent-tagged apical or basolateral and raft or non-raft-associated cargoes. Either the actin-stabilizing jasplakinolide or the actin-depolymerising latrunculin B variably but significantly inhibited post-Golgi traffic of non-raft associated apical p75NTR and basolateral VSV-G cargoes. The TGN-exit of the apical-destined VSV-G mutant was impaired only by latrunculin B. Strikingly, the raft-associated GPI-anchor protein was not affected by either actin toxin. Results indicate that actin dynamics participates in the TGN egress of both apical- and basolateral-targeted proteins but is not needed for apical raft-associated cargo.

Diacylglycerol is required for the formation of COPI vesicles in the Golgi-to-ER transport pathway

Diacylglycerol is necessary for trans-Golgi network (TGN) to cell surface transport, but its functional relevance in the early secretory pathway is unclear. Although depletion of diacylglycerol did not affect ER-to-Golgi transport, it led to a redistribution of the KDEL receptor to the Golgi, indicating that Golgi-to-ER transport was perturbed. Electron microscopy revealed an accumulation of COPI-coated membrane profiles close to the Golgi cisternae. Electron tomography showed that the majority of these membrane profiles originate from coated buds, indicating a block in membrane fission. Under these conditions the Golgi-associated pool of ARFGAP1 was reduced, but there was no effect on the binding of coatomer or the membrane fission protein CtBP3/BARS to the Golgi. The addition of 1,2-dioctanoyl-sn-glycerol or the diacylglycerol analogue phorbol 12,13-dibutyrate reversed the effects of endogenous diacylglycerol depletion. Our findings implicate diacylglycerol in the retrograde transport of proteins from Golgi to the ER and suggest that it plays a critical role at a late stage of COPI vesicle formation.

Cargos and genes: insights into vesicular transport from inherited human disease

Many cellular functions depend on the correct delivery of proteins to specific intracellular destinations. Mutations that alter protein structure and disrupt trafficking of the protein (the "cargo") occur in many genetic disorders. In addition, an increasing number of disorders have been linked to mutations in the genes encoding components of the vesicular transport machinery responsible for normal protein trafficking. We review the clinical phenotypes and molecular pathology of such inherited "protein-trafficking disorders", which provide seminal insights into the molecular mechanisms of protein trafficking. Further characterisation of this expanding group of disorders will provide a basis for developing new diagnostic techniques and treatment strategies and offer insights into the molecular pathology of common multifactorial diseases that have been linked to disordered trafficking mechanisms.

Glycolipid transfer proteins

Glycolipid transfer proteins (GLTPs) are small (24 kDa), soluble, ubiquitous proteins characterized by their ability to accelerate the intermembrane transfer of glycolipids in vitro. GLTP specificity encompasses both sphingoid- and glycerol-based glycolipids, but with a strict requirement that the initial sugar residue be beta-linked to the hydrophobic lipid backbone. The 3D architecture of GLTP reveals liganded structures with unique lipid-binding modes. The biochemical properties of GLTP action at the membrane surface have been studied rather comprehensively, but the biological role of GLTP remains enigmatic. What is clear is that GLTP differs distinctly from other known glycolipid-binding proteins, such as nonspecific lipid transfer proteins, lysosomal sphingolipid activator proteins, lectins, lung surfactant proteins as well as other lipid-binding/transfer proteins. Based on the unique conformational architecture that targets GLTP to membranes and enables glycolipid binding, GLTP is now considered the prototypical and founding member of a new protein superfamily in eukaryotes.

New Insights into Membrane Trafficking and Protein Sorting

Protein transport in the secretory and endocytic pathways is a multistep process involving the generation of transport carriers loaded with defined sets of cargo, the shipment of the cargo-loaded transport carriers between compartments, and the specific fusion of these transport carriers with a target membrane. The regulation of these membrane-mediated processes involves a complex array of protein and lipid interactions. As the machinery and regulatory processes of membrane trafficking have been defined, it is increasingly apparent that membrane transport is intimately connected with a number of other cellular processes, such as quality control in the endoplasmic reticulum (ER), cytoskeletal dynamics, receptor signaling, and mitosis. The fidelity of membrane trafficking relies on the correct assembly of components on organelles. Recruitment of peripheral proteins plays a critical role in defining organelle identity and the establishment of membrane subdomains, essential for the regulation of vesicle transport. The molecular mechanisms for the biogenesis of membrane subdomains are also central to understanding how cargo is sorted and segregated and how different populations of transport carriers are generated. In this review we will focus on the emerging themes of organelle identity, membrane subdomains, regulation of Golgi trafficking, and advances in dissecting pathways in physiological systems.

Phosphoinositide synthesis and degradation in isolated rat liver peroxisomes

Analyzing peroxisomal phosphoinositide (PId(#)) synthesis in highly purified rat liver peroxisomes we found synthesis of phosphatidylinositol 4-phosphate (PtdIns4P), PtdIns(4,5)P(2) and PtdIns(3,5)P(2). PtdIns3P was hardly detected in vitro, however, was observed in vivo after [(32)P]-phosphate labeling of primary rat hepatocytes. In comparison with other subcellular organelles peroxisomes revealed a unique PId pattern suggesting peroxisomal specificity of the observed synthesis. Use of phosphatase inhibitors enhanced the amount of PtdIns4P. The results obtained provide evidence that isolated rat liver peroxisomes synthesize PIds and suggest the association of PId 4-kinase and PId 5-kinase and PId 4-phosphatase activities with the peroxisomal membrane.

Sorting nexin 10 induces giant vacuoles in mammalian cells

Eukaryotic cells maintain a sophisticated network of intracellular membranous system to ensure the proper distribution and compartmentalization of cellular proteins critical for diverse functions such as cell division or cell-cell communication. Yet, little is known about the mechanism that regulates the homeostasis of this system. While analyzing the impact of sorting nexins on the trafficking of membrane type matrix metalloproteinases, we unexpectedly discovered that the expression of SNX10 induced the formation of giant vacuoles in mammalian cells. This vacuolizing activity is sensitive to mutations at the putative phosphoinositide 3-phosphate binding residue Arg(53). Domain-swap experiments with SNX3 demonstrate that the PX domain of SNX10 alone is insufficient to generate vacuoles and the downstream C-terminal domain is required for vacuolization. Brefeldin A, a chemical known to block the endoplasmic reticulum to Golgi transport, inhibited the vacuolization process. Together, these results suggest that SNX10 activity may be involved in the regulation of endosome homeostasis.

Vesicle formation at the plasma membrane and trans-Golgi network: the same but different

An elaborate vesicle transport system supports the active exchange of membranes and protein cargo between the plasma membrane and the trans-Golgi network. Many observations suggest that highly conserved mechanisms are used in vesicle formation and scission. Such similarity is found both at the level of the receptor-ligand sequestration process that uses clathrin and associated polymeric and monomeric adaptor proteins, and in the machinery used to deform and vesiculate lipid membranes.

Phospho-specific binding of 14-3-3 proteins to phosphatidylinositol 4-kinase III β protects from dephosphorylation and stabilizes lipid kinase activity

Phosphatidylinositol-4-kinase-IIIβ (PI4KIIIβ) is activated at the Golgi compartment by PKD-mediated phosphorylation. Subsequent mechanisms responsible for continuous PtdIns(4)P production at Golgi membranes and potential interaction partners of activated PI4KIIIβ are unknown. Here we identify phosphoserine/-threonine binding 14-3-3 proteins as novel regulators of PI4KIIIβ activity downstream of this phosphorylation. The PI4KIIIβ-14-3-3 interaction, evident from GST pulldowns, co-immunoprecipitations and bimolecular fluorescence complementation, was augmented by phosphatase inhibition with okadaic acid. Binding of 14-3-3 proteins to PI4KIIIβ involved the PKD phosphorylation site Ser294, evident from reduced 14-3-3 binding to a S294A PI4KIIIβ mutant. Expression of dominant negative 14-3-3 proteins resulted in decreased PI4KIIIβ Ser294 phosphorylation, whereas wildtype 14-3-3 proteins increased phospho-PI4KIIIβ levels. This was because of protection of PI4KIIIβ Ser294 phosphorylation from phosphatase-mediated dephosphorylation. The functional significance of the PI4KIIIβ-14-3-3 interaction was evident from a reduction of PI4KIIIβ activity upon dominant negative 14-3-3 protein expression. We propose that 14-3-3 proteins function as positive regulators of PI4KIIIβ activity by protecting the lipid kinase from active site dephosphorylation, thereby ensuring a continuous supply of PtdIns(4)P at the Golgi compartment.

Indirect evidence of submicroscopic pores in giant unilamellar [correction of unilamelar] vesicles

Formation of pore-like structures in cell membranes could participate in exchange of matter between cell compartments and modify the lipid distribution between the leaflets of a bilayer. We present experiments on two model systems in which major lipid redistribution is attributed to few submicroscopic transient pores. The first kind of experiments consists in destabilizing the membrane of a giant unilamellar vesicle by inserting conic-shaped fluorescent lipids from the outer medium. The inserted lipids (10% of the vesicle lipids) should lead to membrane rupture if segregated on the outer leaflet. We show that a 5-nm diameter pore is sufficient to ease the stress on the membrane by redistributing the lipids. The second kind of experiments consists in forcing giant vesicles containing functionalized lipids to adhere. This adhesion leads to hemifusion (merging of the outer leaflets). In certain cases, the formation of pores in one of the vesicles is attested by contrast loss on this vesicle and redistribution of fluorescent labels between the leaflets. The kinetics of these phenomena is compatible with transient submicroscopic pores and long-lived membrane defects.

Membrane metabolism mediated by Sec14 family members influences Arf GTPase activating protein activity for transport from the trans-Golgi

The budding yeast Saccharomyces cerevisiae contains a family of Arf (ADP-ribosylation factor) GTPase activating protein (GAP) proteins with the Gcs1 + Age2 ArfGAP pair providing essential overlapping function for the movement of transport vesicles from the trans-Golgi network. We have generated a temperature-sensitive but stable version of the Gcs1 protein that is impaired only for trans-Golgi transport and find that deleterious effects of this enfeebled Gcs1-4 mutant protein are relieved by increased gene dosage of the gcs1-4 mutant gene itself or by the SFH2 gene (also called CSR1), encoding a phosphatidylinositol transfer protein (PITP). This effect was not seen for the SEC14 gene, encoding the founding member of the yeast PITP protein family, even though the Gcs1 and Age2 ArfGAPs are known to be downstream effectors of Sec14-mediated activity for trans-Golgi transport. Sfh2-mediated suppression of inadequate Gcs1-4 function depended on phospholipase D, whereas inadequate Gcs1-4 activity was relieved by increasing levels of diacylglycerol (DAG). Recombinant Gcs1 protein was found to bind certain phospholipids but not DAG. Our findings favor a model of Gcs1 localization through binding to specific phospholipids and activation of ArfGAP activity by DAG-mediated membrane curvature as the transport vesicle is formed. Thus, ArfGAPs are subject to both temporal and spatial regulation that is facilitated by Sfh2-mediated modulation of the lipid environment.

Point mutational analysis of the liganding site in human glycolipid transfer protein. Functionality of the complex

Mammalian glycolipid transfer proteins (GLTPs) facilitate the selective transfer of glycolipids between lipid vesicles in vitro. Recent structural determinations of the apo- and glycolipid-liganded forms of human GLTP have provided the first insights into the molecular architecture of the protein and its glycolipid binding site (Malinina, L., Malakhova, M. L., Brown, R. E., and Patel, D. J. (2004) Nature 430, 1048-1053). In the present study, we have evaluated the functional consequences of point mutation of the glycolipid liganding site of human GLTP within the context of a carrier-based mechanism of glycolipid intermembrane transfer. Different approaches were developed to rapidly and efficiently assess the uptake and release of glycolipid by GLTP. They included the use of glass-immobilized, glycolipid films to load GLTP with glycolipid and separation of GLTP/glycolipid complexes from vesicles containing glycolipid (galactosylceramide or lactosylceramide) or from monosialoganglioside dispersions by employing nickel-nitrilotriacetic acid-based affinity or gel filtration strategies. Point mutants of the sugar headgroup recognition center (Trp-96, Asp-48, Asn-52) and of the ceramide-accommodating hydrophobic tunnel (Phe-148, Phe-183, Leu-136) were analyzed for their ability to acquire and release glycolipid ligand. Two manifestations of point mutation within the liganding site were apparent: (i) impaired formation of the GLTP/glycolipid complex; (ii) impaired acquisition and release of bound glycolipid by GLTP. The results are consistent with a carrier-based mode of GLTP action to accomplish the intermembrane transfer of glycolipid. Also noteworthy was the inefficient release of glycolipid by wtGLTP into phosphatidylcholine acceptor vesicles, raising the possibility of a function other than intermembrane glycolipid transfer in vivo.

Large-scale profiling of Rab GTPase trafficking networks: the membrome

Rab GTPases and SNARE fusion proteins direct cargo trafficking through the exocytic and endocytic pathways of eukaryotic cells. We have used steady state mRNA expression profiling and computational hierarchical clustering methods to generate a global overview of the distribution of Rabs, SNAREs, and coat machinery components, as well as their respective adaptors, effectors, and regulators in 79 human and 61 mouse nonredundant tissues. We now show that this systems biology approach can be used to define building blocks for membrane trafficking based on Rab-centric protein activity hubs. These Rab-regulated hubs provide a framework for an integrated coding system, the membrome network, which regulates the dynamics of the specialized membrane architecture of differentiated cells. The distribution of Rab-regulated hubs illustrates a number of facets that guides the overall organization of subcellular compartments of cells and tissues through the activity of dynamic protein interaction networks. An interactive website for exploring datasets comprising components of the Rab-regulated hubs that define the membrome of different cell and organ systems in both human and mouse is available at http://www.membrome.org/.

Multiple activities for Arf1 at the Golgi complex

The Arf family of GTPases regulates membrane traffic and organelle structure. At the Golgi complex, Arf proteins facilitate membrane recruitment of many cytoplasmic coat proteins to allow sorting of membrane proteins for transport, stimulate the activity of enzymes that modulate the lipid composition of the Golgi, and assemble a cytoskeletal scaffold on the Golgi. Arf1 is the Arf family member most closely studied for its function at the Golgi complex. A number of regulators that activate and inactivate Arf1 on the Golgi have been described that localize to different regions of the organelle. This spatial distribution of Arf regulators may facilitate the recruitment of the coat proteins and other Arf effectors to different regions of the Golgi complex.