Distinct reading of different structural determinants modulates the dileucine-mediated transport steps of the lysosomal membrane protein LIMPII and the insulin-sensitive glucose transporter GLUT4

Lysosomal membrane proteins LAMP1 and LIMP2 are segregated in the Golgi apparatus independently of their clathrin adaptor binding motif

To reach the lysosome, lysosomal membrane proteins (LMPs) are translocated in the endoplasmic reticulum after synthesis and then transported to the Golgi apparatus. The existence of a direct transport from the Golgi apparatus to the endosomes but also of an indirect route through the plasma membrane has been described. Clathrin adaptor binding motifs contained in the cytosolic tail of LMPs have been described as key players in their intracellular trafficking. Here we used the RUSH assay to synchronize the biosynthetic transport of multiple LMPs. After exiting the Golgi apparatus, RUSH-synchronized LAMP1 was addressed to the cell surface both after overexpression or at endogenous level. Its YXXΦ motif was not involved in the transport from the Golgi apparatus to the plasma membrane but in its endocytosis. LAMP1 and LIMP2 were sorted from each other after reaching the Golgi apparatus. LIMP2 was incorporated in punctate structures for export from the Golgi apparatus from which LAMP1 is excluded. LIMP2-containing post-Golgi transport intermediates did not rely neither on its adaptor binding signal nor on its C-terminal cytoplasmic domain.

Equilibrative nucleotide transporter ENT3 (SLC29A3): A unique transporter for inherited disorders and cancers

As a crucial gene associated with diseases, the SLC29A3 gene encodes the equilibrative nucleoside transporter 3 (ENT3). ENT3 plays an essential regulatory role in transporting intracellular hydrophilic nucleosides, nucleotides, hydrophilic anticancer and antiviral nucleoside drugs, energy metabolism, subcellular localization, protein stability, and signal transduction. The mutation and inactivation of SLC29A3 are intimately linked to the occurrence, development, and prognosis of various human tumors. Moreover, many hereditary human diseases, such as H syndrome, pigmentary hypertrichosis and non-autoimmune insulin-dependent diabetes mellitus (PHID) syndrome, Faisalabad histiocytosis (FHC), are related to SLC29A3 mutations. This review explores the mechanisms of SLC29A3 mutations and expression alterations in inherited disorders and cancers. Additionally, we compile studies on the inhibition of ENT3, which may serve as an effective strategy to potentiate the anticancer activity of chemotherapy. Thus, the synopsis of genetics, permeant function and drug therapy of ENT3 provides a new theoretical and empirical foundation for the diagnosis, prognosis of evaluation and treatment of various related diseases.

Equilibrative nucleoside transporter 3 promotes the progression of hepatocellular carcinoma by regulating the AKT/mTOR signaling pathway

Equilibrative nucleoside transporter 3 (ENT3) belongs to the solute carrier family 29. Nucleoside transporters encoded by ENT3 play an important role in the uptake of nucleosides, nucleobases, and their nucleoside analogs, as well as participate in and regulate several physiological activities. However, no study has so far reported the role of ENT3 in hepatocellular carcinoma (HCC). We employed bioinformatics to analyze the expression, prognosis, and mechanism of ENT3 in HCC, as well as verified the same through biological experiments including cell proliferation, cell migration and invasion, and cell cycle and apoptosis, along with the detection of the AKT/mTOR protein expression in the pathway by Western blotting. ENT3 was widely and highly expressed in pan-cancer and upregulated in HCC. The upregulated ENT3 was related to the poor prognosis and clinical features in HCC patients. ENT3 knockdown inhibited cell proliferation, migration, and invasion and promoted cell apoptosis. ENT3 knockdown reduced the p-AKT and p-mTOR protein phosphorylation level, inhibited p-p70S6K1 and increased the p-4EBP1-the downstream effector of the AKT/mTOR pathway-protein phosphorylation level. Our study findings demonstrated that the expression of ENT3 was upregulated in HCC, which represents a poor prognosis. Thus, ENT3 promotes the progression of HCC through the AKT/mTOR signaling pathway.

Differential barcoding of opioid receptors trafficking

Over the past several years, studies have highlighted the δ-opioid receptor (DOPr) as a promising therapeutic target for chronic pain management. While exhibiting milder undesired effects than most currently prescribed opioids, its specific agonists elicit effective analgesic responses in numerous animal models of chronic pain, including inflammatory, neuropathic, diabetic, and cancer-related pain. However, as compared with the extensively studied μ-opioid receptor, the molecular mechanisms governing its trafficking remain elusive. Recent advances have denoted several significant particularities in the regulation of DOPr intracellular routing, setting it apart from the other members of the opioid receptor family. Although they share high homology, each opioid receptor subtype displays specific amino acid patterns potentially involved in the regulation of its trafficking. These precise motifs or "barcodes" are selectively recognized by regulatory proteins and therefore dictate several aspects of the itinerary of a receptor, including its anterograde transport, internalization, recycling, and degradation. With a specific focus on the regulation of DOPr trafficking, this review will discuss previously reported, as well as potential novel trafficking barcodes within the opioid and nociceptin/orphanin FQ opioid peptide receptors, and their impact in determining distinct interactomes and physiological responses.

Glucose Transport and Transporters in the Endomembranes

Glucose is a basic nutrient in most of the creatures; its transport through biological membranes is an absolute requirement of life. This role is fulfilled by glucose transporters, mediating the transport of glucose by facilitated diffusion or by secondary active transport. GLUT (glucose transporter) or SLC2A (Solute carrier 2A) families represent the main glucose transporters in mammalian cells, originally described as plasma membrane transporters. Glucose transport through intracellular membranes has not been elucidated yet; however, glucose is formed in the lumen of various organelles. The glucose-6-phosphatase system catalyzing the last common step of gluconeogenesis and glycogenolysis generates glucose within the lumen of the endoplasmic reticulum. Posttranslational processing of the oligosaccharide moiety of glycoproteins also results in intraluminal glucose formation in the endoplasmic reticulum (ER) and Golgi. Autophagic degradation of polysaccharides, glycoproteins, and glycolipids leads to glucose accumulation in lysosomes. Despite the obvious necessity, the mechanism of glucose transport and the molecular nature of mediating proteins in the endomembranes have been hardly elucidated for the last few years. However, recent studies revealed the intracellular localization and functional features of some glucose transporters; the aim of the present paper was to summarize the collected knowledge.

Two di-leucine motifs regulate trafficking and function of mouse ASIC2a

BACKGROUND

Acid-sensing ion channels (ASICs) are proton-gated cation channels that mediate acid-induced responses in neurons. ASICs are important for mechanosensation, learning and memory, fear, pain, and neuronal injury. ASIC2a is widely expressed in the nervous system and modulates ASIC channel trafficking and activity in both central and peripheral systems. Here, to better understand mechanisms regulating ASIC2a, we searched for potential protein motifs that regulate ASIC2a trafficking.

RESULTS AND CONCLUSIONS

We identified a LLDLL sequence in the C-terminal juxtamembrane region of ASIC2a. Deleting or mutating the LLDLL sequence increased total expression and surface levels of ASIC2a in CHO cells. Mutating either of the two LL motifs had a similar effect. We further assessed ASIC2a localization in organotypic hippocampal slice neurons. The LL motif mutants exhibited increased dendritic trafficking and elevated targeting to dendritic spines. Consistent with an efficient trafficking, the LL motif mutants increased acid-activated current density. In addition, mutating the second LL motif increased pH sensitivity of the channel. These data identify the LL motifs as a negative regulator of ASIC2a trafficking and function, and suggest novel regulatory mechanisms in acid signaling.

Equilibrative nucleoside transporter 3 depletion in β-cells impairs mitochondrial function and promotes apoptosis: Relationship to pigmented hypertrichotic dermatosis with insulin-dependent diabetes

Loss of function recessive mutations in the SLC29A3 gene that encodes human equilibrative nucleoside transporter 3 (ENT3) have been identified in patients with pigmented hypertrichotic dermatosis with insulin-dependent diabetes (PHID). ENT3 is a member of the equilibrative nucleoside transporter (ENT) family whose primary function is mediating transport of nucleosides and nucleobases. The aims of this study were to characterise ENT3 expression in islet β-cells and identify the effects of its depletion on β-cell mitochondrial activity and apoptosis. RT-PCR amplification identified ENT3 expression in human and mouse islets and exocrine pancreas, and in MIN6 β-cells. Immunohistochemistry using human and mouse pancreas sections exhibited extensive ENT3 immunostaining of β-cells, which was confirmed by co-staining with an anti-insulin antibody. In addition, exposure of dispersed human islet cells and MIN6 β-cells to MitoTracker and an ENT3 antibody showed co-localisation of ENT3 to β-cell mitochondria. Consistent with this, Western blot analysis confirmed enhanced ENT3 immunoreactivity in β-cell mitochondria-enriched fractions. Furthermore, ENT3 depletion in β-cells increased mitochondrial DNA content and promoted an energy crisis characterised by enhanced ATP-linked respiration and proton leak. Finally, inhibition of ENT3 activity by dypridamole and depletion of ENT3 by siRNA-induced knockdown resulted in increased caspase 3/7 activities in β-cells. These observations demonstrate that ENT3 is predominantly expressed by islet β-cells where it co-localises with mitochondria. Depletion of ENT3 causes mitochondrial dysfunction which is associated with enhanced β-cell apoptosis. Thus, apoptotic loss of islet β-cells may contribute to the occurrence of autoantibody-negative insulin-dependent diabetes in individuals with non-functional ENT3 mutations.

DKWSLLL, a versatile DXXXLL-type signal with distinct roles in the Cu(+)-regulated trafficking of ATP7B

In the liver, the P-type ATPase and membrane pump ATP7B plays a crucial role in Cu(+) donation to cuproenzymes and in the elimination of excess Cu(+). ATP7B is endowed with a COOH-cytoplasmic (DE)XXXLL-type traffic signal. We find that accessory (Lys -3, Trp -2, Ser -1 and Leu +2) and canonical (D -4, Leu 0 and Leu +1) residues confer the DKWSLLL signal with the versatility required for the Cu(+)-regulated cycling of ATP7B between the trans-Golgi network (TGN) and the plasma membrane (PM). The separate mutation of these residues caused a disruption of the signal, resulting in different ATP7B distribution phenotypes. These phenotypes indicate the key roles of specific residues at separate steps of ATP7B trafficking, including sorting at the TGN, transport from the TGN to the PM and its endocytosis, and recycling to the TGN and PM. The distinct roles of ATP7B in the TGN and PM and the variety of phenotypes caused by the mutation of the canonical and accessory residues of the DKWSLLL signal can explain the separate or joined presentation of Wilson's cuprotoxicosis and the dysfunction of the cuproenzymes that accept Cu(+) at the TGN.

Lysosomal integral membrane protein-2: a new player in lysosome-related pathology

Lysosomes require the presence of many specialized proteins to facilitate their roles in cellular maintenance. One such protein that has proven to be an important player in the lysosomal field is lysosomal integral membrane protein-2 (LIMP-2), encoded by the gene SCARB2. LIMP-2 is required for the normal biogenesis and maintenance of lysosomes and endosomes and has been identified as the specific receptor for glucocerebrosidase, the enzyme deficient in Gaucher disease. Research into LIMP-2 and the SCARB2 gene indicate that it may be a factor contributing to the clinical heterogeneity seen among patients with Gaucher disease. Mutations in SCARB2 have also been identified as the cause of action myoclonus renal failure (AMRF), and in some cases progressive myoclonic epilepsy. A total of 14 disease-causing SCARB2 mutations have been identified to date. The role of LIMP-2 in human pathology has expanded with its identification as a component of the intercalated disk in cardiac muscle and as a receptor for specific enteroviruses, two unanticipated findings that reaffirm the myriad roles of lysosomal proteins. Studies into the full impact of LIMP-2 deficiency and the LIMP2/glucocerebrosidase molecular pathway will lead to a better understanding of disease pathogenesis in Gaucher disease and AMRF, and to new insights into lysosomal processing, trafficking and function.

Coxiella burnetii effector protein subverts clathrin-mediated vesicular trafficking for pathogen vacuole biogenesis

Successful macrophage colonization by Coxiella burnetii, the cause of human Q fever, requires pathogen-directed biogenesis of a large, growth-permissive parasitophorous vacuole (PV) with phagolysosomal characteristics. The vesicular trafficking pathways co-opted by C. burnetii for PV development are poorly defined; however, it is predicted that effector proteins delivered to the cytosol by a defective in organelle trafficking/intracellular multiplication (Dot/Icm) type 4B secretion system are required for membrane recruitment. Here, we describe involvement of clathrin-mediated vesicular trafficking in PV generation and the engagement of this pathway by the C. burnetii type 4B secretion system substrate Coxiella vacuolar protein A (CvpA). CvpA contains multiple dileucine [DERQ]XXXL[LI] and tyrosine (YXXΦ)-based endocytic sorting motifs like those recognized by the clathrin adaptor protein (AP) complexes AP1, AP2, and AP3. A C. burnetii ΔcvpA mutant exhibited significant defects in replication and PV development, confirming the importance of CvpA in infection. Ectopically expressed mCherry-CvpA localized to tubular and vesicular domains of pericentrosomal recycling endosomes positive for Rab11 and transferrin receptor, and CvpA membrane interactions were lost upon mutation of endocytic sorting motifs. Consistent with CvpA engagement of the endocytic recycling system, ectopic expression reduced uptake of transferrin. In pull-down assays, peptides containing CvpA-sorting motifs and full-length CvpA interacted with AP2 subunits and clathrin heavy chain. Furthermore, depletion of AP2 or clathrin by siRNA treatment significantly inhibited C. burnetii replication. Thus, our results reveal the importance of clathrin-coated vesicle trafficking in C. burnetii infection and define a role for CvpA in subverting these transport mechanisms.

Sorting of the vesicular GABA transporter to functional vesicle pools by an atypical dileucine-like motif

Increasing evidence indicates that individual synaptic vesicle proteins may use different signals, endocytic adaptors, and trafficking pathways for sorting to distinct pools of synaptic vesicles. Here, we report the identification of a unique amino acid motif in the vesicular GABA transporter (VGAT) that controls its synaptic localization and activity-dependent recycling. Mutational analysis of this atypical dileucine-like motif in rat VGAT indicates that the transporter recycles by interacting with the clathrin adaptor protein AP-2. However, mutation of a single acidic residue upstream of the dileucine-like motif leads to a shift to an AP-3-dependent trafficking pathway that preferentially targets the transporter to the readily releasable and recycling pool of vesicles. Real-time imaging with a VGAT-pHluorin fusion provides a useful approach to explore how unique sorting sequences target individual proteins to synaptic vesicles with distinct functional properties.

GLUT4 traffic through an ESCRT-III-dependent sorting compartment in adipocytes

In insulin target tissues, GLUT4 is known to traffic through multiple compartments that may involve ubiquitin- and/or SUMO-dependent targeting. During these trafficking steps, GLUT4 is sorted into a storage reservoir compartment that is acutely released by insulin signalling processes that are downstream of PI 3-kinase associated changes in inositol phospholipids. As ESCRT components have recently been found to influence cellular sorting processes that are related to changes in both ubiquitination and inositol phospholipids, we have examined whether GLUT4 traffic is routed through ESCRT dependent sorting steps. Introduction of the dominant negative inhibitory constructs of the ESCRT-III components CHMP3 (CHMP3(1–179)) and Vps4 (GFP-Vps4^E235Q) into rat adipocytes leads to the accumulation of GLUT4 in large, coalesced and extended vesicles structures that co-localise with the inhibitory constructs over large parts of the extended structure. A new swollen hybrid and extensively ubiquitinated compartment is produced in which GLUT4 co-localises more extensively with the endosomal markers including EEA1 and transferrin receptors but also with the TGN marker syntaxin6. These perturbations are associated with failure of insulin action on GLUT4 traffic to the cell surface and suggest impairment in an ESCRT-dependent sorting step used for GLUT4 traffic to its specialised reservoir compartment.

A tandem Di-hydrophobic motif mediates clathrin-dependent endocytosis via direct binding to the AP-2 ασ2 subunits

Select plasma membrane proteins can be marked as cargo for inclusion into clathrin-coated pits by common internalization signals (e.g. YXXΦ, dileucine motifs, NPXY) that serve as universal recognition sites for the AP-2 adaptor complex or other clathrin-associated sorting proteins. However, some surface proteins, such as the Kir2.3 potassium channel, lack canonical signals but are still targeted for clathrin-dependent endocytosis. Here, we explore the mechanism. We found an unusual endocytic signal in Kir2.3 that is based on two consecutive pairs of hydrophobic residues. Characterized by the sequence ΦΦXΦΦ (a tandem di-hydrophobic (TDH) motif, where Φ is a hydrophobic amino acid), the signal shows no resemblance to other endocytic motifs, yet it directly interacts with AP-2 to target the Kir2.3 potassium channel into the endocytic pathway. We found that the tandem di-hydrophobic motif directly binds to the ασ2 subunits of AP-2, interacting within a large hydrophobic cleft that encompasses part of the docking site for di-Leu signals, but includes additional structures. These observations expand the repertoire of clathrin-dependent internalization signals and the ways in which AP-2 can coordinate endocytosis of cargo proteins.

The role of insulin-regulated aminopeptidase in MHC class I antigen presentation

Production of MHC-I ligands from antigenic proteins generally requires multiple proteolytic events. While the proteolytic steps required for antigen processing in the endogenous pathway are clearly established, persisting gaps of knowledge regarding putative cross-presentation compartments have made it difficult to map the precise proteolytic events required for generation of cross-presented antigens. It is only in the past decade that the importance of aminoterminal trimming as the final step in the endogenous presentation pathway has been recognized and that the corresponding enzymes have been described. This review focuses on the aminoterminal trimming of exogenous cross-presented peptides, with particular emphasis on the identification of insulin responsive aminopeptidase (IRAP) as the principal trimming aminopeptidase in endosomes and phagosomes.

Two phosphatidylinositol 4-kinases control lysosomal delivery of the Gaucher disease enzyme, β-glucocerebrosidase

Trafficking of glucocerebrosidase (GBA) enzyme from the endoplasmic reticulum to the lysosome requires lysosomal integral membrane protein type 2 (LIMP-2), which is a receptor for GBA. This study shows that phosphatidylinositol 4-kinase (PI4K) type IIIβ controls the exit of LIMP-2/GBA complex from the Golgi, while PI4KIIα is required for the post-Golgi trafficking of the complex via the late endosomes.

Gaucher disease is a lysosomal storage disorder caused by a defect in the degradation of glucosylceramide catalyzed by the lysosomal enzyme β-glucocerebrosidase (GBA). GBA reaches lysosomes via association with its receptor, lysosomal integral membrane protein type 2 (LIMP-2). We found that distinct phosphatidylinositol 4-kinases (PI4Ks) play important roles at multiple steps in the trafficking pathway of the LIMP-2/GBA complex. Acute depletion of phosphatidylinositol 4-phosphate in the Golgi caused accumulation of LIMP-2 in this compartment, and PI4KIIIβ was found to be responsible for controlling the exit of LIMP-2 from the Golgi. In contrast, depletion of PI4KIIα blocked trafficking at a post-Golgi compartment, leading to accumulation of LIMP-2 in enlarged endosomal vesicles. PI4KIIα depletion also caused secretion of missorted GBA into the medium, which was attenuated by limiting LIMP-2/GBA exit from the Golgi by PI4KIIIβ inhibitors. These studies identified PI4KIIIβ and PI4KIIα as important regulators of lysosomal delivery of GBA, revealing a new element of control to sphingolipid homeostasis by phosphoinositides.

Dynamic redistribution of calcium sensitive potassium channels (hK(Ca)3.1) in migrating cells

Calcium-sensitive potassium channels (K(Ca)3.1) are expressed in virtually all migrating cells. Their activity is required for optimal cell migration so that their blockade leads to slowing down. K(Ca)3.1 channels must be inserted into the plasma membrane in order to elicit their physiological function. However, the plasma membrane of migrating cells is subject to rapid recycling by means of endo- and exocytosis. Here, we focussed on the endocytic internalization and the intracellular transport of the human isoform hK(Ca)3.1. A hK(Ca)3.1 channel construct with an HA-tag in the extracellularly located S3-S4 linker was transfected into migrating transformed renal epithelial MDCK-F cells. Channel internalization was visualized and quantified with immunofluorescence and a cell-based ELISA. Movement of hK(Ca)3.1 channel containing vesicles as well as migration of MDCK-F cells were monitored by means of time lapse video microscopy. hK(Ca)3.1 channels are endocytosed during migration. Most of the hK(Ca)3.1 channel containing vesicles are moving at a speed of up to 2 µm/sec in a microtubule-dependent manner towards the front of MDCK-F cells. Our experiments indicate that endocytosis of hK(Ca)3.1 channels is clathrin-dependent since they colocalize with clathrin adaptor proteins and since it is impaired when a C-terminal dileucine motif is mutated. The C-terminal dileucine motif is also important for the subcellular localization of hK(Ca)3.1 channels in migrating cells. Mutated channels are no longer concentrated at the leading edge. We therefore propose that recycling of hK(Ca)3.1 channels contributes to their characteristic subcellular distribution in migrating cells.

Routes to the tonoplast: the sorting of tonoplast transporters in Arabidopsis mesophyll protoplasts

Vacuoles perform a multitude of functions in plant cells, including the storage of amino acids and sugars. Tonoplast-localized transporters catalyze the import and release of these molecules. The mechanisms determining the targeting of these transporters to the tonoplast are largely unknown. Using the paralogous Arabidopsis thaliana inositol transporters INT1 (tonoplast) and INT4 (plasma membrane), we performed domain swapping and mutational analyses and identified a C-terminal di-leucine motif responsible for the sorting of higher plant INT1-type transporters to the tonoplast in Arabidopsis mesophyll protoplasts. We demonstrate that this motif can reroute other proteins, such as INT4, SUCROSE TRANSPORTER2 (SUC2), or SWEET1, to the tonoplast and that the position of the motif relative to the transmembrane helix is critical. Rerouted INT4 is functionally active in the tonoplast and complements the growth phenotype of an int1 mutant. In Arabidopsis plants defective in the β-subunit of the AP-3 adaptor complex, INT1 is correctly localized to the tonoplast, while sorting of the vacuolar sucrose transporter SUC4 is blocked in cis-Golgi stacks. Moreover, we demonstrate that both INT1 and SUC4 trafficking to the tonoplast is sensitive to brefeldin A. Our data show that plants possess at least two different Golgi-dependent targeting mechanisms for newly synthesized transporters to the tonoplast.

The polymorphic HCMV glycoprotein UL20 is targeted for lysosomal degradation by multiple cytoplasmic dileucine motifs

Human cytomegalovirus (HCMV) is a widespread and persistent beta-herpesvirus. The large DNA genome of HCMV encodes many proteins that are non-essential for viral replication including numerous proteins subverting host immunosurveillance. One of them is the barely characterized UL20, which is encoded adjacent to the well-defined immunoevasins UL16 and UL18. UL20 is a type I transmembrane glycoprotein with an immunoglobulin-like ectodomain that is highly polymorphic among HCMV strains. Here, we show that the homodimeric UL20, by virtue of its cytoplasmic domain, does not reach the cell surface but is targeted to endosomes and lysosomes. Accordingly, UL20 exhibits a short half-life because of rapid lysosomal degradation. Trafficking of UL20 to lysosomes is determined by several, independently functioning dileucine-based sorting motifs in the cytoplasmic domain of UL20 and involves the adaptor protein (AP) complex AP-1. Combined substitution of three dileucine motifs allowed strong cell surface expression of UL20 comparable to UL20 mutants lacking the cytoplasmic tail. Finally, we show that the intracellularly located UL20 also is subject to lysosomal degradation in the context of viral infection. Altogether, from these data, we hypothesize that UL20 is destined to efficiently sequester yet-to-be defined cellular proteins for degradation in lysosomes.

LIMP-2 links late phagosomal trafficking with the onset of the innate immune response to Listeria monocytogenes: a role in macrophage activation

The innate immune response to Listeria monocytogenes depends on phagosomal bacterial degradation by macrophages. Here, we describe the role of LIMP-2, a lysosomal type III transmembrane glycoprotein and scavenger-like protein, in Listeria phagocytosis. LIMP-2-deficient mice display a macrophage-related defect in Listeria innate immunity. They produce less acute phase pro-inflammatory cytokines/chemokines, MCP-1, TNF-α, and IL-6 but normal levels of IL-12, IL-10, and IFN-γ and a 25-fold increase in susceptibility to Listeria infection. This macrophage defect results in a low listericidal potential, poor response to TNF-α activation signals, impaired phago-lysosome transformation into antigen-processing compartments, and uncontrolled LM cytosolic growth that fails to induce normal levels of acute phase pro-inflammatory cytokines. LIMP-2 transfection of CHO cells confirmed that LIMP-2 participates in the degradation of Listeria within phagosomes, controls the late endosomal/lysosomal fusion machinery, and is linked to the activation of Rab5a. Therefore, the role of LIMP-2 appears to be connected to the TNF-α-dependent and early activation of Listeria macrophages through internal signals linking the regulation of late trafficking events with the onset of the innate Listeria immune response.

The cytosolic tail dipeptide Ile-Met of the pea receptor BP80 is required for recycling from the prevacuole and for endocytosis

Pea (Pisum sativum) BP80 is a vacuolar sorting receptor for soluble proteins and has a cytosolic domain essential for its intracellular trafficking between the trans-Golgi network and the prevacuole. Based on mammalian knowledge, we introduced point mutations in the cytosolic region of the receptor and produced chimeras of green fluorescent protein fused to the transmembrane domain of pea BP80 along with the modified cytosolic tails. By analyzing the subcellular location of these chimera, we found that mutating Glu-604, Asp-616, or Glu-620 had mild effects, whereas mutating the Tyr motif partially redistributed the chimera to the plasma membrane. Replacing both Ile-608 and Met-609 by Ala (IMAA) led to a massive redistribution of fluorescence to the vacuole, indicating that recycling is impaired. When the chimera uses the alternative route, the IMAA mutation led to a massive accumulation at the plasma membrane. Using Arabidopsis thaliana plants expressing a fluorescent reporter with the full-length sequence of At VSR4, we demonstrated that the receptor undergoes brefeldin A-sensitive endocytosis. We conclude that the receptors use two pathways, one leading directly to the lytic vacuole and the other going via the plasma membrane, and that the Ileu-608 Met-609 motif has a role in the retrieval step in both pathways.

Membrane trafficking of large conductance calcium-activated potassium channels is regulated by alternative splicing of a transplantable, acidic trafficking motif in the RCK1-RCK2 linker

Trafficking of the pore-forming alpha-subunits of large conductance calcium- and voltage-activated potassium (BK) channels to the cell surface represents an important regulatory step in controlling BK channel function. Here, we identify multiple trafficking signals within the intracellular RCK1-RCK2 linker of the cytosolic C terminus of the channel that are required for efficient cell surface expression of the channel. In particular, an acidic cluster-like motif was essential for channel exit from the endoplasmic reticulum and subsequent cell surface expression. This motif could be transplanted onto a heterologous nonchannel protein to enhance cell surface expression by accelerating endoplasmic reticulum export. Importantly, we identified a human alternatively spliced BK channel variant, hSloDelta(579-664), in which these trafficking signals are excluded because of in-frame exon skipping. The hSloDelta(579-664) variant is expressed in multiple human tissues and cannot form functional channels at the cell surface even though it retains the putative RCK domains and downstream trafficking signals. Functionally, the hSloDelta(579-664) variant acts as a dominant negative subunit to suppress cell surface expression of BK channels. Thus alternative splicing of the intracellular RCK1-RCK2 linker plays a critical role in determining cell surface expression of BK channels by controlling the inclusion/exclusion of multiple trafficking motifs.

Lysosomal targeting of the CLN7 membrane glycoprotein and transport via the plasma membrane require a dileucine motif

CLN7 is a polytopic lysosomal membrane protein deficient in variant late infantile neuronal ceroid lipofuscinosis, a neurodegenerative lysosomal storage disorder. In this study fluorescence protease protection assays and mutational analyses revealed the N- and C-terminal tails of CLN7 in the cytosol and two N-glycosylation sites at N371 and N376. Both partially and non-glycosylated CLN7 were correctly transported to lysosomes. To identify lysosomal targeting motifs, we generated CD4-chimera fused to the N- and C-terminal domains of CLN7. Lysosomal localization of the chimeric proteins requires a consensus acidic dileucine-based motif in the N-terminus and two tandem tyrosine-based signals in the C-terminus. Mutation of these sorting motifs resulted in cell surface redistribution of CD4 chimeras. However, the dileucine-based motif is of critical importance for lysosomal localization of the full-length CLN7 in different cell lines. Cell surface biotinylation revealed that at equilibrium 22% of total CLN7 is localized at the plasma membrane. Mutation of the dileucine motif or the co-expression of dominant-negative mutant dynamin K44A led to a further increase of CLN7 at the plasma membrane. Our data demonstrate that CLN7 contains several cytoplasmic lysosomal targeting signals of which the N-terminal dileucine-based motif is required for the predominant lysosomal targeting along the indirect pathway and clathrin-mediated endocytosis of CLN7.

Mutating the dileucine motif of the human beta(2)-adrenoceptor reduces the high initial rate of receptor phosphorylation by GRK without affecting postendocytic sorting

The internalization of beta(2)-adrenoceptors after agonist activation results in a desensitized and phosphorylated receptor that either resensitizes by recycling to the cell surface or becomes degraded by postendocytic sorting to lysosomes. The duration and physiological effects of agonists therefore depend on beta(2)-adrenoceptor sorting, highlighting the importance of sorting signals. Dileucine motifs within other membrane proteins act as signals for endocytosis and/or postendocytic sorting, and the beta(2)-adrenoceptor has a dileucine motif within helix 8 that might play a role in efficient receptor recycling and/or downregulation. beta(2)-adrenoceptor internalization and sorting were studied in HEK293 cells stably expressing wild type or mutant dialanine L339A,L340A beta(2)-adrenoceptors. The mutant beta(2)-adrenoceptors showed a significantly lower initial rate of phosphorylation at the prominent G-protein coupled receptor kinase (GRK) sites Ser355 and 356 compared to wild type beta(2)-adrenoceptors. Furthermore, the agonist-induced endocytic rate constant for L339A,L340A beta(2)-adrenoceptors was reduced to approximately 25% that of wild type beta(2)-adrenoceptors, which resulted in a similar reduction in agonist-induced downregulation. Internalized L339A,L340A beta(2)-adrenoceptors recycled to the surface with a rate and extent similar to that of wild type beta(2)-adrenoceptors. Therefore, although the role of L339,L340 in beta(2)-adrenoceptor recycling or postendocytic sorting seems minimal, we conclude that L339,L340 is required for the initial high rate of phosphorylation by G-protein coupled receptor kinases at Ser355,356, which in turn is required for efficient beta(2)-adrenoceptors endocytosis.

GLUT8, the enigmatic intracellular hexose transporter

GLUT8 is a class III sugar transporter predominantly expressed in testis and brain. In contrast to the class I and class II transporters, hydrophobicity plots predict a short extracellular loop between transmembrane domain (TM)1 and TM2 and a long extracellular loop between TM9 and TM10 that contains the only N-glycosylation site. In vitro translated GLUT8 migrates as a 35-kDa protein that is glycosylated in the presence of microsomal membranes. In heterologous expression systems, glucose transport activity (Km of 2 mM) was inhibited by fructose and galactose. The transporter carries an NH2-terminal endosomal/lysosomal targeting motif ([DE]XXXL[LI]). Accordingly, constitutive GLUT8 has been found to be associated with endosomes and lysosomes but also with membranes of the endoplasmic reticulum. A similar distribution was detected after overexpression of wild-type or tagged GLUT8 in different cell systems. In these cells, none of the conventional signals tested induced a translocation of GLUT8 to the plasma membrane. Therefore, GLUT8 appears to catalyze transport of sugars or sugar derivatives through intracellular membranes. Slc2a8 knockout mice were viable, developed normally, and showed mild alterations in brain (increased proliferation of neuronal cells in dentate gyrus of the hippocampus, hyperactivity), heart (impaired transmission of electrical wave through the atrium), and sperm cells (reduced number of motile sperm cells associated with reduced mitochondrial membrane potential and ATP levels in sperm). The links between molecular function, cellular localization and phenotype of the knockout mouse is unclear and remains to be determined.

The C-terminus of GLUT4 targets the transporter to the perinuclear compartment but not to the insulin-responsive vesicles

Postprandial blood glucose clearance is mediated by GLUT4 (glucose transporter 4) which is translocated from an intracellular storage pool to the plasma membrane in response to insulin. The nature of the intracellular storage pool of GLUT4 is not well understood. Immunofluorescence staining shows that, under basal conditions, the major population of GLUT4 resides in the perinuclear compartment. At the same time, biochemical fractionation reveals that GLUT4 is localized in IRVs (insulin-responsive vesicles). The relationship between the perinuclear GLUT4 compartment and the IRVs is not known. In the present study, we have exchanged the C-termini of GLUT4 and cellugyrin, another vesicular protein that is not localized in the IRVs and has no insulin response. We have found that GLUT4 with the cellugyrin C-terminus loses its specific perinuclear localization, whereas cellugyrin with the GLUT4 C-terminus acquires perinuclear localization and becomes co-localized with GLUT4. This, however, is not sufficient for the effective entry of the latter chimaera into the IRVs as only a small fraction of cellugyrin with the GLUT4 C-terminus is targeted to the IRVs and is translocated to the plasma membrane in response to insulin stimulation. We suggest that the perinuclear GLUT4 storage compartment comprises the IRVs and the donor membranes from which the IRVs originate. The C-terminus of GLUT4 is required for protein targeting to the perinuclear donor membranes, but not to the IRVs.

Human equilibrative nucleoside transporter (ENT) family of nucleoside and nucleobase transporter proteins

1. The human (h) SLC29 family of integral membrane proteins is represented by four members, designated equilibrative nucleoside transporters (ENTs) because of the properties of the first-characterized family member, hENT1. They belong to the widely distributed eukaryotic ENT family of equilibrative and concentrative nucleoside/nucleobase transporter proteins. 2. A predicted topology of eleven transmembrane helices has been experimentally confirmed for hENT1. The best-characterized members of the family, hENT1 and hENT2, possess similar broad permeant selectivities for purine and pyrimidine nucleosides, but hENT2 also efficiently transports nucleobases. hENT3 has a similar broad permeant selectivity for nucleosides and nucleobases and appears to function in intracellular membranes, including lysosomes. 3. hENT4 is uniquely selective for adenosine, and also transports a variety of organic cations. hENT3 and hENT4 are pH sensitive, and optimally active under acidic conditions. ENTs, including those in parasitic protozoa, function in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis and, in humans, are also responsible for the cellular uptake of nucleoside analogues used in the treatment of cancers and viral diseases. 4. By regulating the concentration of adenosine available to cell surface receptors, mammalian ENTs additionally influence physiological processes ranging from cardiovascular activity to neurotransmission.

Sorting of lysosomal proteins

Lysosomes are composed of soluble and transmembrane proteins that are targeted to lysosomes in a signal-dependent manner. The majority of soluble acid hydrolases are modified with mannose 6-phosphate (M6P) residues, allowing their recognition by M6P receptors in the Golgi complex and ensuing transport to the endosomal/lysosomal system. Other soluble enzymes and non-enzymatic proteins are transported to lysosomes in an M6P-independent manner mediated by alternative receptors such as the lysosomal integral membrane protein LIMP-2 or sortilin. Sorting of cargo receptors and lysosomal transmembrane proteins requires sorting signals present in their cytosolic domains. These signals include dileucine-based motifs, DXXLL or [DE]XXXL[LI], and tyrosine-based motifs, YXXØ, which interact with components of clathrin coats such as GGAs or adaptor protein complexes. In addition, phosphorylation and lipid modifications regulate signal recognition and trafficking of lysosomal membrane proteins. The complex interaction of both luminal and cytosolic signals with recognition proteins guarantees the specific and directed transport of proteins to lysosomes.

A dileucine signal situated in the C-terminal tail of the lysosomal membrane protein p40 is responsible for its targeting to lysosomes

Transport of newly synthesized lysosomal membrane proteins from the TGN (trans-Golgi network) to the lysosomes is due to the presence of specific signals in their cytoplasmic domains that are recognized by cytosolic adaptors. p40, a hypothetical transporter of 372 amino acids localized in the lysosomal membrane, contains four putative lysosomal sorting motifs in its sequence: three of the YXXphi-type (Y(6)QLF, Y(106)VAL, Y(333)NGL) and one of the [D/E]XXXL[L/I]-type (EQERL(360)L(361)). To test the role of these motifs in the biosynthetic transport of p40, we replaced the most critical residues of these consensus sequences, the tyrosine residue or the leucine-leucine pair, by alanine or alanine-valine respectively. We analysed the subcellular localization of the mutated p40 proteins in transfected HeLa cells by confocal microscopy and by biochemical approaches (subcellular fractionation on self-forming Percoll density gradients and cell surface biotinylation). The results of the present study show that p40 is mistargeted to the plasma membrane when its dileucine motif is disrupted. No role of the tyrosine motifs could be put forward. Taken together, our results provide evidence that the sorting of p40 from the TGN to the lysosomes is directed by the dileucine EQERL(360)L(361) motif situated in its C-terminal tail.

Trafficking of vesicular neurotransmitter transporters

Vesicular neurotransmitter transporters are required for the storage of all classical and amino acid neurotransmitters in secretory vesicles. Transporter expression can influence neurotransmitter storage and release, and trafficking targets the transporters to different types of secretory vesicles. Vesicular transporters traffic to synaptic vesicles (SVs) as well as large dense core vesicles and are recycled to SVs at the nerve terminal. Some of the intrinsic signals for these trafficking events have been defined and include a dileucine motif present in multiple transporter subtypes, an acidic cluster in the neural isoform of the vesicular monoamine transporter (VMAT) 2 and a polyproline motif in the vesicular glutamate transporter (VGLUT) 1. The sorting of VMAT2 and the vesicular acetylcholine transporter to secretory vesicles is regulated by phosphorylation. In addition, VGLUT1 uses alternative endocytic pathways for recycling back to SVs following exocytosis. Regulation of these sorting events has the potential to influence synaptic transmission and behavior.

Role of the carboxyl terminal di-leucine in phosphorylation and internalization of C5a receptor

The carboxyl tail of G protein-coupled receptors contains motifs that regulate receptor interactions with intracellular partners. Activation of the human neutrophil complement fragment C5a receptor (C5aR) is terminated by phosphorylation of the carboxyl tail followed by receptor internalization. In this study, we demonstrated that bulky hydrophobic residues in the membrane-proximal region of the C5aR carboxyl tail play an important role in proper structure and function of the receptor: Substitution of leucine 319 with alanine (L319A) resulted in receptor retention in the endoplasmic reticulum, whereas a L318A substitution allowed receptor transport to the cell surface, but showed slow internalization upon activation, presumably due to a defect in phosphorylation by both PKC and GRK. Normal agonist-induced activation of ERK1/2 and intracellular calcium release suggested that the L318A mutation did not affect receptor signaling. Binding of GRK2 and PKCbetaII to intracellular loop 3 of C5aR in vitro indicated that mutagenesis of L318 did not affect kinase binding. Limited proteolysis with trypsin revealed a conformational difference between wild type and mutant receptor. Our studies support a model in which the L318/L319 stabilizes an amphipathic helix (Q305-R320) in the membrane-proximal region of C5aR.

The mass action hypothesis: formation of Glut4 storage vesicles, a tissue-specific, regulated exocytic compartment

Insulin stimulates glucose uptake into the target tissues of fat and muscle by recruiting or translocating Glut4 glucose transport proteins to their functional location at the cell surface. In the basal state, Glut4 is sequestered intracellularly in several vesicular compartments, one of which has come to be known as Glut4 storage vesicles (GSVs). The GSVs represent a tissue-specific compartment that is an ultimate target of the insulin signalling cascade. Glut4 translocation has been extensively studied because of its intrinsic scientific importance to cell biology as well as its relevance to the pathology of type 2 diabetes mellitus. I review herein the ontogeny of GSVs and their composition as it relates to a tissue-specific, hormone-sensitive exocytic compartment and propose a mechanism for their formation.

Fish glucose transporter (GLUT)-4 differs from rat GLUT4 in its traffic characteristics but can translocate to the cell surface in response to insulin in skeletal muscle cells

In mammals, glucose transporter (GLUT)-4 plays an important role in glucose homeostasis mediating insulin action to increase glucose uptake in insulin-responsive tissues. In the basal state, GLUT4 is located in intracellular compartments and upon insulin stimulation is recruited to the plasma membrane, allowing glucose entry into the cell. Compared with mammals, fish are less efficient restoring plasma glucose after dietary or exogenous glucose administration. Recently our group cloned a GLUT4-homolog in skeletal muscle from brown trout (btGLUT4) that differs in protein motifs believed to be important for endocytosis and sorting of mammalian GLUT4. To study the traffic of btGLUT4, we generated a stable L6 muscle cell line overexpressing myc-tagged btGLUT4 (btGLUT4myc). Insulin stimulated btGLUT4myc recruitment to the cell surface, although to a lesser extent than rat-GLUT4myc, and enhanced glucose uptake. Interestingly, btGLUT4myc showed a higher steady-state level at the cell surface under basal conditions than rat-GLUT4myc due to a higher rate of recycling of btGLUT4myc and not to a slower endocytic rate, compared with rat-GLUT4myc. Furthermore, unlike rat-GLUT4myc, btGLUT4myc had a diffuse distribution throughout the cytoplasm of L6 myoblasts. In primary brown trout skeletal muscle cells, insulin also promoted the translocation of endogenous btGLUT4 to the plasma membrane and enhanced glucose transport. Moreover, btGLUT4 exhibited a diffuse intracellular localization in unstimulated trout myocytes. Our data suggest that btGLUT4 is subjected to a different intracellular traffic from rat-GLUT4 and may explain the relative glucose intolerance observed in fish.

The GLUT4 glucose transporter

Few physiological parameters are more tightly and acutely regulated in humans than blood glucose concentration. The major cellular mechanism that diminishes blood glucose when carbohydrates are ingested is insulin-stimulated glucose transport into skeletal muscle. Skeletal muscle both stores glucose as glycogen and oxidizes it to produce energy following the transport step. The principal glucose transporter protein that mediates this uptake is GLUT4, which plays a key role in regulating whole body glucose homeostasis. This review focuses on recent advances on the biology of GLUT4.

The gamma/sigma1 and alpha/sigma2 hemicomplexes of clathrin adaptors AP-1 and AP-2 harbor the dileucine recognition site

The clathrin adaptors AP-1 and AP-2 bind cargo proteins via two types of motifs: tyrosine-based Yxx phi and dileucine-based [DE]XXXL[LI]. Although it is well established that Yxx phi motifs bind to the mu subunits of AP-1 or AP-2, dileucine motifs have been reported to bind to either the mu or beta subunits of these adaptors as well as the gamma/sigma1 hemicomplex of AP-1. To clarify this controversy, the various subunits of AP-1 and AP-2 were expressed individually and in hemicomplex form in insect cells, and they were used in glutathione S-transferase pull-down assays to determine their binding properties. We report that the gamma/sigma1 or alpha/sigma2 hemicomplexes bound the dileucine-based motifs of several proteins quite strongly, whereas binding by the beta1/mu1 and beta2/mu2 hemicomplexes, and the individual beta or mu subunits, was extremely weak or undetectable. The gamma/sigma1 and alpha/sigma2 hemicomplexes displayed substantial differences in their preference for particular dileucine-based motifs. Most strikingly, an aspartate at position -4 compromised binding to the gamma/sigma1 hemicomplex, whereas minimally affecting binding to alpha/sigma2. There was an excellent correlation between binding to the alpha/sigma2 hemicomplex and in vivo internalization mediated by the dileucine-based sorting signals. These findings provide new insights into the trafficking mechanisms of D/EXXXL[LI]-mediated sorting signals.

A Specific Dileucine Motif Is Required for the GGA-dependent Entry of Newly Synthesized Insulin-responsive Aminopeptidase into the Insulin-responsive Compartment

In muscle and adipose cells, the insulin-responsive aminopeptidase (IRAP) is localized to intracellular storage sites and undergoes insulin-dependent redistribution to the cell surface. Following expression, the newly synthesized IRAP protein traffics to the perinuclear insulin-sensitive compartment and acquires insulin sensitivity 6-9 h following biosynthesis. Knockdown of GGA1 by RNA interference prevented IRAP from entering, but not exiting, the insulin-responsive compartment. Mutation of the dileucine motif at positions 76 and 77 (EGFP-IRAP/AA(76,77)), but not the dileucine motif at positions 53 and 54, resulted in the rapid default of the reporter to the cell surface beginning at 3 h following biosynthesis. Alanine substitution of 9 residues amino- or carboxyl-terminal to LL(76,77) did not perturb basal intracellular sequestration or abrogate insulin-stimulated IRAP translocation. Moreover, a dominant interfering GGA mutant (VHS-GAT) potently inhibited insulin-stimulated translocation of EGFP-IRAP/WT but did not block the constitutive exocytotic trafficking of EGFP-IRAP/AA(76,77). In addition, the EGFP-IRAP/WT and EGFP-IRAP/AA(76,77) constructs occupied morphologically distinct tubulovesicular compartments in the perinuclear region. Taken together, these data indicate that LL(76,77) functions during the GGA-dependent sorting of newly made IRAP into the insulin-responsive storage compartment.

Role of conserved intracellular motifs in Serrate signalling, cis-inhibition and endocytosis

Notch is the receptor in a signalling pathway that operates in a diverse spectrum of developmental processes. Its ligands (e.g. Serrate) are transmembrane proteins whose signalling competence is regulated by the endocytosis-promoting E3 ubiquitin ligases, Mindbomb1 and Neuralized. The ligands also inhibit Notch present in the same cell (cis-inhibition). Here, we identify two conserved motifs in the intracellular domain of Serrate that are required for efficient endocytosis. The first, a dileucine motif, is dispensable for trans-activation and cis-inhibition despite the endocytic defect, demonstrating that signalling can be separated from bulk endocytosis. The second, a novel motif, is necessary for interactions with Mindbomb1/Neuralized and is strictly required for Serrate to trans-activate and internalise efficiently but not for it to inhibit Notch signalling. Cis-inhibition is compromised when an ER retention signal is added to Serrate, or when the levels of Neuralized are increased, and together these data indicate that cis-inhibitory interactions occur at the cell surface. The balance of ubiquitinated/unubiquitinated ligand will thus affect the signalling capacity of the cell at several levels.

Sequences in the N-terminal cytoplasmic domain of Saccharomyces cerevisiae maltose permease are required for vacuolar degradation but not glucose-induced internalization

In Saccharomyces cerevisiae, glucose addition to maltose fermenting cells causes a rapid loss of maltose transport activity and ubiquitin-mediated vacuolar proteolysis of maltose permease. GFP-tagged Mal61 maltose permease was used to explore the role of the N-terminal cytoplasmic domain in glucose-induced inactivation. In maltose-grown cells, Mal61/HA-GFP localizes to the cell surface and, surprisingly, to the vacuole. Studies of end3Delta and doa4Delta mutants indicate that a slow constitutive internalization of Mal61/HA-GFP is required for its vacuolar localization. Site-specific mutagenesis of multiple serine/threonine residues in a putative PEST sequence of the N-terminal cytoplasmic domain of maltose permease blocks glucose-induced Mal61p degradation but does not affect the rapid loss of maltose transport activity associated with glucose-induced internalization. The internalized multiple Ser/Thr mutant protein co-localizes with Snf7p in a putative late endosome or E-compartment. Further, alteration of a putative dileucine [D/EExxxLL/I] motif at residues 64-70 causes a significant defect in maltose transport activity and mislocalization to an E-compartment but appears to have little impact on glucose-induced internalization. We conclude that the N-terminal cytoplasmic domain of maltose permease is not the target of the signaling pathways leading to glucose-induced internalization of Mal61 permease but is required for its subsequent delivery to the vacuole for degradation.

Modulation of cellular protein trafficking by human immunodeficiency virus type 1 Nef: role of the acidic residue in the ExxxLL motif

The nef gene contributes to the replication of primate lentiviruses by altering the trafficking of cellular proteins involved in adaptive immunity (class I and II major histocompatibility complex [MHC]) and viral transmission (CD4 and DC-SIGN). A conserved acidic leucine-based sequence (E(160)xxxLL) within human immunodeficiency virus type 1 (HIV-1) Nef binds to the cellular adaptor protein (AP) complexes, which mediate protein sorting into endosomal vesicles. The leucine residues in this motif are required for the down-regulation of CD4 and for the up-regulation of DC-SIGN and the invariant chain of MHC class II, but the role of the acidic residue is unclear. Here, substitution of E160 with uncharged residues impaired the ability of Nef to up-regulate the expression of the invariant chain and DC-SIGN at the cell surface, whereas substitution with a basic residue was required for a similar effect on the down-regulation of CD4. All substitutions of E160 relieved the Nef-mediated block to transferrin uptake. E160 was required for the efficient interaction of Nef with AP-1 and AP-3 and for the stabilization of these complexes on endosomal membranes in living cells. Systematic mutation of the ExxxLL sequence together with correlation of binding and functional data leads to the hypotheses that AP-1 and AP-3 are major cofactors for the effect of Nef on the trafficking of transferrin, are less important but contribute to the modulation of the invariant chain and DC-SIGN, and are least critical for the modulation of CD4. The data suggest that the E160 residue plays a differential role in the modulation of leucine-dependent Nef-targets and support a model in which distinct AP complexes are used by Nef to modulate different cellular proteins.

Three Dileucine-like Motifs within the Interbox1/2 Region of the Human Oncostatin M Receptor Prevent Efficient Surface Expression in the Absence of an Associated Janus Kinase

The oncostatin M receptor (OSMR) is part of receptor complexes for oncostatin M and interleukin-31. Signaling events are triggered by Jaks (Janus kinases) that constitutively bind to membrane-proximal receptor regions. Besides their established role in signaling, Jaks are involved in the regulation of the surface expression of several cytokine receptors. Here, we analyzed the structural requirements within the human OSMR that underlie its limited surface expression in the absence of associated Jaks. We identified three dileucine-like motifs within the Jak-binding region of the OSMR that control receptor surface and overall expression. A receptor mutant in which all three motifs were mutated to alanine displayed markedly increased surface expression. Although the surface half-life of this mutant was increased compared with that of the wild-type receptor, no difference in the internalization rate was detectable, implying that these receptors differ in their post-endocytic fate. The protein stability of the wild-type receptor was markedly lower than that of mutant receptors, but could be strongly increased in the presence of the lysosomal inhibitor chloroquine. Our data are consistent with the dileucine motifs being involved in destabilization of receptors devoid of associated Jaks as part of a quality control ensuring signaling competence of OSMRs.

GLUT8 Contains a [DE]XXXL[LI] Sorting Motif and Localizes to a Late Endosomal/Lysosomal Compartment

Glucose transporter 8 (GLUT8) contains a cytoplasmic N-terminal dileucine motif and localizes to a thus far unidentified intracellular compartment. Translocation of GLUT8 to the plasma membrane (PM) was found in insulin-treated mouse blastocysts. Using overexpression of GLUT8 in adipocytes and neuronal cells however, insulin treatment or depolarization of the cells did not lead to GLUT8 PM translocation in other studies. In addition, other experiments showing dynamin-dependent endocytosis of GLUT8 suggested that GLUT8 recycles between an endosomal compartment and the PM. To reveal the functional/physiological role of GLUT8, we studied its subcellular localization in 3T3L1, HEK293 and CHO cells. We show that GLUT8 does not co-localize with GLUT4 and does not redistribute to the PM after treatment with insulin, ionophores or okadaic acid in these cell lines. Once endocytosed, GLUT8 does not recycle to the PM. GLUT8 localizes to late endosomes and lysosomes. An interspecies GLUT8 - sequence alignment revealed the presence of a highly conserved late endosomal/lysosomal-targeting motif ([DE]XXXL[LI]). Changing the glutamate to arginine as found in GLUT4 (RRXXXLL) alters GLUT8 endocytosis and retains the transporter at the PM. Furthermore, sorting GLUT8 to late endosomes/lysosomes does not require prior presence of GLUT8 at the PM followed by its endocytosis. In summary, GLUT8 does not reside in a recycling vesicle pool and is distinct from GLUT4. From our data, we postulate a role for GLUT8 in transport of hexoses across intracellular membranes, for example in specific compartments of GLUT8 expression such as the acrosome of mature spermatozoa or secretory granules in neurons. Furthermore, a role for GLUT8 in hexose transport across the lysosomal membrane, a transport mechanism that has long been suggested but unexplained, is discussed.

Multiple motifs regulate the trafficking of GABA(B) receptors at distinct checkpoints within the secretory pathway

gamma-Aminobutyric acid type B receptors (GABA(B)) are G-protein-coupled receptors that mediate GABAergic inhibition in the brain. Their functional expression is dependent upon the formation of heterodimers between GABA(B)R1 and GABA(B)R2 subunits, a process that occurs within the endoplasmic reticulum (ER). However, the mechanisms that regulate receptor surface expression remain largely unknown. Here, we demonstrate that access to the cell surface for GABA(B)R1 is sequentially controlled by an RSR(R) motif and a LL motif within its cytoplasmic domain. In addition, we reveal that msec7-1, a guanine-nucleotide-exchange factor (GEF) for the ADP-ribosylation factor (ARF) family of GTPases, critical regulators of vesicular membrane trafficking, interacts with GABA(B)R1 via the LL motif in this subunit. Finally, we establish that msec7-1 modulates the cell surface expression of GABA(B) receptors, a process that is dependent upon the integrity of the LL motif in GABA(B)R1. Together, our results demonstrate that the cell surface expression of the GABA(B)R1 subunit is regulated by multiple motifs, which act at distinct checkpoints in the secretory pathway, and also suggest a novel role for msec7-1 in regulating the membrane trafficking of GABA(B)R1 subunits.

Targeting of NPC1 to late endosomes involves multiple signals, including one residing within the putative sterol-sensing domain

The NPC1 protein is a multipass transmembrane protein whose deficiency causes the autosomal recessive lipid storage disorder Niemann-Pick type C1. NPC1 localizes predominantly to late endosomes and has a dileucine motif located within a small cytoplasmic tail thought to target the protein to this location. Our data have suggested previously that the protein can reach its correct location in the absence of its cytoplasmic tail, suggesting that other signals contribute to NPC1 targeting. By using various FLAG-tagged and CD32-NPC1 chimeric fusion constructs, we show that multiple signals are responsible for the trafficking of NPC1 to the endosomal compartment, including the dileucine motif and a previously unidentified signal residing within the putative sterol-sensing domain transmembrane domain 3. Neither region alone was capable of directing heterologous CD32 fusions to late endosomes exclusively via the trans-Golgi network to the late endosome route taken by wild-type NPC1; transmembrane domain 3 was unable to maintain CD32 in late endosomes, indicating that two or more signals work in concert to target and retain NPC1 in this compartment. In addition we confirm that the tail dileucine motif is not essential for NPC1 targeting to late endosomes, and we discuss the implications of this finding along with the previously unappreciated role for transmembrane domain 3 in NPC1 localization and function.

Varied mechanisms underlie the free sialic acid storage disorders

Salla disease and infantile sialic acid storage disorder are autosomal recessive neurodegenerative diseases characterized by loss of a lysosomal sialic acid transport activity and the resultant accumulation of free sialic acid in lysosomes. Genetic analysis of these diseases has identified several unique mutations in a single gene encoding a protein designated sialin (Verheijen, F. W., Verbeek, E., Aula, N., Beerens, C. E., Havelaar, A. C., Joosse, M., Peltonen, L., Aula, P., Galjaard, H., van der Spek, P. J., and Mancini, G. M. (1999) Nat. Genet. 23, 462-465; Aula, N., Salomaki, P., Timonen, R., Verheijen, F., Mancini, G., Mansson, J. E., Aula, P., and Peltonen, L. (2000) Am. J. Hum. Genet. 67, 832-840). From the biochemical phenotype of the diseases and the predicted polytopic structure of the protein, it has been suggested that sialin functions as a lysosomal sialic acid transporter. Here we directly demonstrate that this activity is mediated by sialin and that the recombinant protein has functional characteristics similar to the native lysosomal sialic acid transport system. Furthermore, we describe the effect of disease-causing mutations on the protein. We find that the majority of the mutations are associated with a complete loss of activity, while the mutations associated with the milder forms of the disease lead to reduced, but residual, function. Thus, there is a direct correlation between sialin function and the disease state. In addition, we find with one mutation that the protein is retained in the endoplasmic reticulum, indicating that altered trafficking of sialin is also associated with disease. This analysis of the molecular mechanism of sialic acid storage disorders is a further step in identifying therapeutic approaches to these diseases.

Dileucine and YXXL motifs in the cytoplasmic tail of the bovine leukemia virus transmembrane envelope protein affect protein expression on the cell surface

Several retroviruses downmodulate the cell surface expression of envelope (Env) proteins through peptide sequences located in the cytoplasmic tail of the transmembrane (TM) subunit. We investigated whether cell surface expression of a chimeric protein containing the cytoplasmic domain of the TM protein (CTM) of bovine leukemia virus (BLV) was regulated by two membrane-proximal dileucine motifs or by tyrosine Y487 or Y498 in YXXL motifs. A chimeric protein composed of the extracellular and membrane-spanning portions of human CD8-alpha plus a wild-type (wt) BLV CTM was detectable on the surface of only 40% of the cells in which it was transiently expressed. Replacement of either dileucine pair with alanines increased the level of surface display of chimeric proteins. Nearly all cells became surface positive when both dileucine motifs were altered simultaneously and when either an N-terminal segment containing both dileucine motifs or a C-terminal segment containing all YXXL motifs was deleted. In contrast, replacement of Y487 or Y498 with alanine or phenylalanine enabled only small increases in surface display compared with wt levels. Chimeric proteins had similar stabilities but were downmodulated from the cell surface at three different rates. Point mutants segregated into each of the three groups of proteins categorized according to these different rates. Interestingly, Y487 mutants were downmodulated less efficiently than Y498 mutants, which behaved like wt. CD8-CTM chimeric proteins were phosphorylated on serine residues, but the native BLV Env protein was not phosphorylated either in transfected cells or in a lymphoid cell line constitutively producing BLV. Thus, both dileucine and YXXL motifs within the BLV CTM contribute to downmodulation of a protein containing this domain. Interactions with other proteins may influence surface exposure of Env protein complexes in virus-infected cells, assisting in viral evasion of adaptive immunity.

Signals regulating trafficking of Menkes (MNK; ATP7A) copper-translocating P-type ATPase in polarized MDCK cells

The Menkes protein (MNK; ATP7A) functions as a transmembrane copper-translocating P-type ATPase and plays a vital role in systemic copper absorption in the gut and copper reabsorption in the kidney. Polarized epithelial cells such as Madin-Darby canine kidney (MDCK) cells are a physiologically relevant model for systemic copper absorption and reabsorption in vivo. In this study, cultured MDCK cells were used to characterize MNK trafficking and enabled the identification of signaling motifs required to target the protein to specific membranes. Using confocal laser scanning microscopy and surface biotinylation we demonstrate that MNK relocalizes from the Golgi to the basolateral (BL) membrane under elevated copper conditions. As previously shown in nonpolarized cells, the metal binding sites in the NH2-terminal domain of MNK were found to be required for copper-regulated trafficking from the Golgi to the plasma membrane. These data provide molecular evidence that is consistent with the presumed role of this protein in systemic copper absorption in the gut and reabsorption in the kidney. Using site-directed mutagenesis, we identified a dileucine motif proximal to the COOH terminus of MNK that was critical for correctly targeting the protein to the BL membrane and a putative PDZ target motif that was required for localization at the BL membrane in elevated copper.

Luteinizing Hormone and Follicle-Stimulating Hormone Exhibit Different Secretion Patterns from Cultured Madin-Darby Canine Kidney Cells

LH, FSH, and chorionic gonadotropin (CG) are comprised of a common alpha subunit and a hormone-specific beta subunit. Using Madin-Darby canine kidney (MDCK) epithelial cells to examine the polarized secretion of human CG/LH, we previously reported that CG and LH were detected in the apical and basolateral compartments, respectively, and the carboxyl terminal end of the CGbeta subunit contains a strong apical signal. Here we show that the carboxyl seven amino acids in the LHbeta subunit contribute to the basolateral secretion of LH, and an LH chimera bearing the CGbeta apical signal is redirected from the basolateral to the apical compartments. Because LH and FSH are synthesized in the same cell, we also compared the secretion polarity of LH with FSH. MDCK cells expressing the FSH dimer displayed an almost equal distribution of protein into the apical and basolateral compartments. Given that the LHbeta and CGbeta carboxy terminal sequences, which differ from that in the FSHbeta subunit, occupy a pivotal role in their polarized behavior, the results support the hypothesis that pituitary exit of LH and FSH occur via different secretion pathways, and are released spatially from the pituitary via different circulatory routes.

The equilibrative nucleoside transporter family, SLC29

The human SLC29 family of proteins contains four members, designated equilibrative nucleoside transporters (ENTs) because of the properties of the first-characterised family member, hENT1. They belong to the widely-distributed eukaryotic ENT family of equilibrative and concentrative nucleoside/nucleobase transporters and are distantly related to a lysosomal membrane protein, CLN3, mutations in which cause neuronal ceroid lipofuscinosis. A predicted topology of 11 transmembrane helices with a cytoplasmic N-terminus and an extracellular C-terminus has been experimentally confirmed for hENT1. The best-characterised members of the family, hENT1 and hENT2, possess similar broad substrate specificities for purine and pyrimidine nucleosides, but hENT2 in addition efficiently transports nucleobases. The ENT3 and ENT4 isoforms have more recently also been shown to be genuine nucleoside transporters. All four isoforms are widely distributed in mammalian tissues, although their relative abundance varies: ENT2 is particularly abundant in skeletal muscle. In polarised cells ENT1 and ENT2 are found in the basolateral membrane and, in tandem with concentrative transporters of the SLC28 family, may play a role in transepithelial nucleoside transport. The transporters play key roles in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis, and are also responsible for the cellular uptake of nucleoside analogues used in the treatment of cancers and viral diseases. In addition, by regulating the concentration of adenosine available to cell surface receptors, they influence many physiological processes ranging from cardiovascular activity to neurotransmission.

Immune Evasion by Adenovirus E3 Proteins: Exploitation of Intracellular Trafficking Pathways

Adenoviruses (Ads) are nonenveloped viruses which replicate and assemble in the nucleus. Therefore, viral membrane proteins are not directly required for their multiplication. Yet, all human Ads encode integral membrane proteins in the early transcription unit 3 (E3). Previous studies on subgenus C Ads demonstrated that most E3 proteins exhibit immunomodulatory functions. In this review we focus on the E3 membrane proteins, which appear to be primarily devoted to remove critical recognition structures for the host immune system from the cell surface. The molecular mechanism for removal depends on the E3 protein involved: E3/19K prevents expression of newly synthesized MHC molecules by inhibition of ER export, whereas E3/10.4-14.5K down-regulate apoptosis receptors by rerouting them into lysosomes. The viral proteins mediating these processes contain typical transport motifs, such as KKXX, YXXphi, or LL. E3/49K, another recently discovered E3 protein, may require such motifs to reach a processing compartment essential for its presumed immunomodulatory activity. Thus, E3 membrane proteins exploit the intracellular trafficking machinery for immune evasion. Conspicuously, many E3 membrane proteins from Ads other than subgenus C also contain putative transport motifs. Close inspection of surrounding amino acids suggests that many of these are likely to be functional. Therefore, Ads might harbor more E3 proteins that exploit intracellular trafficking pathways as a means to manipulate immunologically important key molecules. Differential expression of such functions by Ads of different subgenera may contribute to their differential pathogenesis. Thus, an unexpected link emerges between viral manipulation of intracellular transport pathways and immune evasion.

Two motifs target Batten disease protein CLN3 to lysosomes in transfected nonneuronal and neuronal cells

Batten disease is a neurodegenerative disorder resulting from mutations in CLN3, a polytopic membrane protein, whose predominant intracellular destination in nonneuronal cells is the lysosome. The topology of CLN3 protein, its lysosomal targeting mechanism, and the development of Batten disease are poorly understood. We provide experimental evidence that both the N and C termini and one large loop domain of CLN3 face the cytoplasm. We have identified two lysosomal targeting motifs that mediate the sorting of CLN3 in transfected nonneuronal and neuronal cells: an unconventional motif in the long C-terminal cytosolic tail consisting of a methionine and a glycine separated by nine amino acids [M(X)9G], and a more conventional dileucine motif, located in the large cytosolic loop domain and preceded by an acidic patch. Each motif on its own was sufficient to mediate lysosomal targeting, but optimal efficiency required both. Interestingly, in primary neurons, CLN3 was prominently seen both in lysosomes in the cell body and in endosomes, containing early endosomal antigen-1 along neuronal processes. Because there are few lysosomes in axons and peripheral parts of dendrites, the presence of CLN3 in endosomes of neurons may be functionally important. Endosomal association of the protein was independent of the two lysosomal targeting motifs.

Identification of a murine CD28 dileucine motif that suppresses single-chain chimeric T-cell receptor expression and function

Recent preclinical and clinical trials have demonstrated the therapeutic potential of T lymphocytes redirected with genetically engineered T-cell receptor (TCR) surrogates against infected, cancerous, or autoreactive cells. These surrogate TCRs link a ligand-recognition domain to signaling regions from the TCR. We previously compared the function of surrogate TCRs that include TCR or TCR and CD28 signaling regions. We found that primary murine T cells modified to specifically target Kb-restricted CD8+ T cells using either Kb-zeta or Kb-CD28-zeta receptors had similar functional activities, although the CD28-zeta receptor showed a 2-fold to 4-fold decreased expression. We have now identified a previously unrecognized dileucine motif in the murine CD28 signaling domain that accounts for this reduced expression. Inactivation of this motif increased chimeric receptor surface expression 2- to 5-fold. T cells expressing the dileucine-mutated CD28-zeta chimeric receptor demonstrated enhanced proliferation, cytokine production, and cytolytic activities. Further, cells expressing this dileucine-mutated receptor were highly effective in eliminating antigen-specific CD8+ T lymphocytes in vivo. These results therefore identify a critical motif limiting the function of receptor-modified T lymphocytes, demonstrate that inactivation of this motif enhances chimeric receptor function, and illustrate a potential novel application of receptor-modified T lymphocytes in the induction of immune tolerance.