Coated pits act as molecular filters

Influenza A virus uncoating

Influenza A virus (IAV) is an enveloped virus of the Orthomyxoviridae with a negative-sense single-stranded RNA genome. During virus cell entry, viral and cellular cues are delivered in a stepwise manner within two distinct cellular compartments-the endosomes and the cytosol. Endosome maturation primes the viral core for uncoating by cytosolic host proteins and host-mediated virus disaggregation is essential for genome import and replication in the nucleus. Recent evidence shows that two well-known cellular proteins-histone deacetylase 6 (HDAC6) and karyopherin-β2 (kapβ2)-uncoat influenza virus. HDAC6 is 1 of 11 HDACs and an X-linked, cytosolic lysine deacetylase. Under normal cellular conditions HDAC6 is the tubulin deacetylase. Under proteasomal stress HDAC6 binds unanchored ubiquitin, dynein and myosin II to sequester misfolded protein aggregates for autophagy. Kapβ2 is a member of the importin β family that transports RNA-binding proteins into the nucleus by binding to disordered nuclear localization signals (NLSs) known as PY-NLS. Kapβ2 is emerging as a universal uncoating factor for IAV and human immunodeficiency virus type 1 (HIV-1). Kapβ2 can also reverse liquid-liquid phase separation (LLPS) of RNA-binding proteins by promoting their disaggregation. Thus, it is becoming evident that key players in the management of cellular condensates and membraneless organelles are potent virus uncoating factors. This emerging concept reveals implications in viral pathogenesis, as well as, the promise for cell-targeted therapeutic strategies to block universal virus uncoating pathways hijacked by enveloped RNA viruses.

Spoiled for Choice: Diverse Endocytic Pathways Function at the Cell Surface

Endocytosis has long been identified as a key cellular process involved in bringing in nutrients, in clearing cellular debris in tissue, in the regulation of signaling, and in maintaining cell membrane compositional homeostasis. While clathrin-mediated endocytosis has been most extensively studied, a number of clathrin-independent endocytic pathways are continuing to be delineated. Here we provide a current survey of the different types of endocytic pathways available at the cell surface and discuss a new classification and plausible molecular mechanisms for some of the less characterized pathways. Along with an evolutionary perspective of the origins of some of these pathways, we provide an appreciation of the distinct roles that these pathways play in various aspects of cellular physiology, including the control of signaling and membrane tension.

Ultrastructural Analysis of Vesicular Transport in Electrotransfection

Emerging evidence from various studies indicates that plasmid DNA (pDNA) is internalized by cells through an endocytosis-like process when it is used for electrotransfection. To provide morphological evidence of the process, we investigated ultrastructures in cells that were associated with the electrotransfected pDNA, using immunoelectron microscopy. The results demonstrate that four endocytic pathways are involved in the uptake of the pDNA, including caveolae- and clathrin-mediated endocytosis, macropinocytosis, and the clathrin-independent carrier/glycosylphosphatidylinositol-anchored protein-enriched early endosomal compartment (CLIC/GEEC) pathway. Among them, macropinocytosis is the most common pathway utilized by cells having various pDNA uptake capacities, and the CLIC/GEEC pathway is observed primarily in human umbilical vein endothelial cells. Quantitatively, the endocytic pathways are more active in easy-to-transfect cells than in hard-to-transfect ones. Taken together, our data provide ultrastructural evidence showing that endocytosis plays an important role in cellular uptake and intracellular transport of electrotransfected pDNA.

Quantum dots implementation as a label for analysis of early stages of EGF receptor endocytosis: a comparative study on cultured cells

EGF complexed to fluorescent photostable quantum dots by biotin-streptavidin system (bEGF-savQD) is attractive for both the basic research and therapeutic application such as targeted drug delivery in EGF-receptor (EGFR) expressing cancers. However, compared to native EGF, the large size of QD and its quasi-multivalency can have unpredictable effects on EGFR endocytosis changing the internalization portal and/or endosomal processing tightly bound to EGF signaling. We have found that bEGF-savQDs enter HeLa cells via the temperature-dependent clathrin-mediated EGF-receptor-specific pathway characteristic for native EGF. We also found that EGF-to-QD concentration ratios used for the complex preparation and the level of EGF receptor expression affect the number and integral densities of the formed endosomes. So, at EGF-to-QD ratio from 4:1 to 12:1 (at nanomolar bEGF concentrations) on average 100 bright endosomes per HeLa cell were formed 15 min after the complex addition, while 1:1 ratio resulted in formation of very few dim endosomes. However, in A431 cells overexpressing EGFR 1:1 ratio was effective. Using dynamin inhibition and Na-acidic washout we showed that bEGF-savQDs bind surface receptors and enter clathrin-coated pits slower than the same ligands without QD. Yet, the bEGF-savQD demonstrated similar to native EGF and bEGF-savCy3 co-localization dynamics with tethering protein EEA1 and HRS, the key component of sorting ESCRT0 complex. In conclusion, our comparative study reveals that in respect to entrapment into coated pits, endosomal recruitment, endosome fusions, and the initial steps of endosomal maturation, bEGF-savQD behaves like native EGF and QD implementation does not affect these important events.

Identifying dynamic membrane structures with atomic-force microscopy and confocal imaging

Combining the biological specificity of fluorescence microscopy with topographical features revealed by atomic force microscopy (AFM) provides new insights into cell biology. However, the lack of systematic alignment capabilities especially in scanning-tip AFM has limited the combined application approach as AFM drift leads to increasing image mismatch over time. We present an alignment correction method using the cantilever tip as a reference landmark. Since the precise tip position is known in both the fluorescence and AFM images, exact re-alignment becomes possible. We used beads to demonstrate the validity of the method in a complex artificial sample. We then extended this method to biological samples to depict membrane structures in fixed and living human fibroblasts. We were able to map nanoscale membrane structures, such as clathrin-coated pits, to their respective fluorescent spots. Reliable alignment between fluorescence signals and topographic structures opens possibilities to assess key biological processes at the cell surface such as endocytosis and exocytosis.

Asymmetry of single cells and where that leads

Most single animal cells have an internal vector that determines where recycling membrane is added to the cell's surface. Because of the specific molecular composition of this added membrane, a dynamic asymmetry is formed on the surface of the cell. The consequences of this dynamic asymmetry are discussed, together with what they imply for how cells move. The polarity of a single-celled embryo, such as that of the nematode Caenorhabditis elegans, is explored in a similar framework.

An alternative mechanism of clathrin-coated pit closure revealed by ion conductance microscopy

Simultaneous ion conductance and confocal microscopy in live cells reveal a new form of asymmetric clathrin-coated pit closure.

Current knowledge of the structural changes taking place during clathrin-mediated endocytosis is largely based on electron microscopy images of fixed preparations and x-ray crystallography data of purified proteins. In this paper, we describe a study of clathrin-coated pit dynamics in living cells using ion conductance microscopy to directly image the changes in pit shape, combined with simultaneous confocal microscopy to follow molecule-specific fluorescence. We find that 70% of pits closed with the formation of a protrusion that grew on one side of the pit, covered the entire pit, and then disappeared together with pit-associated clathrin–enhanced green fluorescent protein (EGFP) and actin-binding protein–EGFP (Abp1-EGFP) fluorescence. This was in contrast to conventionally closing pits that closed and cleaved from flat membrane sheets and lacked accompanying Abp1-EGFP fluorescence. Scission of both types of pits was found to be dynamin-2 dependent. This technique now enables direct spatial and temporal correlation between functional molecule-specific fluorescence and structural information to follow key biological processes at cell surfaces.

Quantitative experimental assessment of macromolecular crowding effects at membrane surfaces

We examined how crowding of the surfaces of lipid vesicles with either grafted polyethyleneglycol (PEG) chains or bilayer-anchored protein molecules affects the binding of soluble proteins to the vesicle surface. Escherichia coli dihydrofolate reductase (DHFR, 18 kDa) or a larger fusion protein, NusA-DHFR (72 kDa), binds reversibly but with high affinity to a methotrexate-modified lipid (MTX-PE) incorporated into large unilamellar vesicles. Incorporation of phosphatidylethanolamine-PEG5000 into the vesicles strongly decreases the affinity of binding of both proteins, to a degree that varies roughly exponentially with the lateral density of the PEG chains. Covalently coupling maltose-binding protein (MBP) to the vesicle surfaces also strongly decreases the affinity of binding of NusDHFR or DHFR, to a degree that likewise varies roughly exponentially with the surface density of anchored MBP. Surface-coupled MBP strongly decreases the rate of binding of NusDHFR to MTX-PE-incorporating vesicles but does not affect the rate of NusDHFR dissociation. The large magnitudes of these effects (easily exceeding an order of magnitude for moderate degrees of surface crowding) support previous theoretical analyses and suggest that surface-crowding effects can markedly influence a variety of important aspects of protein behavior in membranes.

Transmembrane domains control exclusion of membrane proteins from clathrin-coated pits

Efficient sorting of proteins is essential to allow transport between intracellular compartments while maintaining their specific composition. During endocytosis, membrane proteins can be concentrated in endocytic vesicles by specific interactions between their cytoplasmic domains and cytosolic coat proteins. It is, however, unclear whether they can be excluded from transport vesicles and what the determinants for this sorting could be. Here, we show that in the absence of cytosolic sorting signals, transmembrane domains control the access of surface proteins to endosomal compartments. They act in particular by determining the degree of exclusion of membrane proteins from endocytic clathrin-coated vesicles. When cytosolic endocytosis signals are present, it is the combination of cytosolic and transmembrane determinants that ultimately controls the efficiency with which a given transmembrane protein is endocytosed.

Functional roles of short sequence motifs in the endocytosis of membrane receptors

Internalization and trafficking of cell-surface membrane receptors and proteins into subcellular compartments is mediated by specific short-sequence signal motifs, which are usually located within the cytoplasmic domains of these receptor and protein molecules. The signals usually consist of short linear amino acid sequences, which are recognized by adaptor coat proteins along the endocytic and sorting pathways. The complex arrays of signals and recognition proteins ensure the dynamic movement, accurate trafficking, and designated distribution of transmembrane receptors and ligands into intracellular compartments, particularly to the endosomal-lysosomal system. This review summarizes the new information and concepts, integrating them with the current and established views of endocytosis, intracellular trafficking, and sorting of membrane receptors and proteins. Particular emphasis has been given to the functional roles of short-sequence signal motifs responsible for the itinerary and destination of membrane receptors and proteins moving into the subcellular compartments. The specific characteristics and functions of short-sequence motifs, including various tyrosine-based, dileucine-type, and other short-sequence signals in the trafficking and sorting of membrane receptors and membrane proteins are presented and discussed.

Clathrin and LRP-1-independent constitutive endocytosis and recycling of uPAR

Background

The urokinase receptor (uPAR/CD87) is highly expressed in malignant tumours. uPAR, as a GPI anchored protein, is preferentially located at the cell surface, where it interacts with its ligands urokinase (uPA) and the extracellular matrix protein vitronectin, thus promoting plasmin generation, cell-matrix interactions and intracellular signalling events. Interaction with a complex formed by uPA and its inhibitor PAI-1 induces cell surface down regulation and recycling of the receptor via the clathrin-coated pathway, a process dependent on the association to LRP-1.

Methodology/Principal Findings

In this study, we have found that along with the ligand-induced down-regulation, uPAR also internalizes and recycles constitutively through a second pathway that is independent of LRP-1 and clathrin but shares some properties with macropinocytosis. The ligand-independent route is amiloride-sensitive, does not require uPAR partitioning into lipid rafts, is independent of the activity of small GTPases RhoA, Rac1 and Cdc42, and does not require PI3K activity. Constitutively endocytosed uPAR is found in EEA1 positive early/recycling endosomes but does not reach lysosomes in the absence of ligands. Electron microscopy analysis reveals the presence of uPAR in ruffling domains at the cell surface, in macropinosome-like vesicles and in endosomal compartments.

Conclusions/Significance

These results indicate that, in addition to the ligand-induced endocytosis of uPAR, efficient surface expression and membrane trafficking might also be driven by an uncommon macropinocytic mechanism coupled with rapid recycling to the cell surface.

Structure of coated pits and vesicles

Purified coated vesicles typically contain clathrin and auxiliary structural proteins of about 100 000 and 50 000 relative molecular mass (Mr). A model is described for the packing of clathrin trimers into the characteristic pentagons and hexagons of the surface lattices of coats. In a coated vesicle, clathrin appears to bind to an inner 'core' of particles containing the 100 000 and 50 000 Mr proteins. These core proteins may, in turn, bind to receptors in the membrane of the vesicle. The vesicle itself surrounds specific content molecules bound to their respective receptors. Budding coated vesicles are believed to act as molecular filters in cells. The known structural features of coated membranes are discussed in terms of this apparent role.

Endocytosis, the sorting problem and cell locomotion in fibroblasts

Fibroblasts endocytose lipid plus a subset of plasma membrane proteins over their entire surface and reinsert this into the plasma membrane at the cell's leading edge. This process is used to extend the fibroblast forwards. This circulation causes a flow of these endocytosed molecules over the cell's surface. Molecules, such as proteins, sitting in this flow can distribute themselves randomly by Brownian motion, but large objects (or small tethered ones) cannot. These large objects therefore cap. A mechanism is presented whereby this process could be used for locomotion using many weak interactions with the substrate. In addition it is suggested that the observed selectivity of coated pits may be sufficient to sort out proteins during transfer of membrane from one organelle to another so that the specific characters of the parent membranes are maintained.

Endocytosis, membrane recycling and sorting of GPI-anchored proteins: Trypanosoma brucei as a model system

In the flagellated protozoon Trypanosoma brucei, endo- and exocytosis are restricted to a small area of the plasma membrane, the flagellar pocket. All endosomal compartments and the single Golgi complex are located within the posterior part of the cell between the flagellar pocket and the nucleus. The use of reverse genetic tools, including RNA interference, in combination with quantitative 3D-fluorescence and electron microscopic techniques has provided an insight into endosomal membrane traffic, which occurs at a very high rate and appears to exhibit a lower level of complexity than in mammalian cells. The flagellate is an excellent model system for studies on endocytosis, sorting and recycling of glycosylphosphatidylinositol-anchored glycoproteins, because 10(7) molecules of the variant surface glycoprotein form a dense coat at the cell's surface. Because the endocytic rate varies widely at different stages in the parasite's life cycle, trypanosomes may be used for investigating developmental aspects of their endocytic system.

Endocytosis of a glycosylphosphatidylinositol-anchored protein via clathrin-coated vesicles, sorting by default in endosomes, and exocytosis via RAB11-positive carriers

Recently, proteins linked to glycosylphosphatidylinositol (GPI) residues have received considerable attention both for their association with lipid microdomains and for their specific transport between cellular membranes. Basic features of trafficking of GPI-anchored proteins or glycolipids may be explored in flagellated protozoan parasites, which offer the advantage that their surface is dominated by these components. In Trypanosoma brucei, the GPI-anchored variant surface glycoprotein (VSG) is efficiently sorted at multiple intracellular levels, leading to a 50-fold higher membrane concentration at the cell surface compared with the endoplasmic reticulum. We have studied the membrane and VSG flow at an invagination of the plasma membrane, the flagellar pocket, the sole region for endo- and exocytosis in this organism. VSG enters trypanosomes in large clathrin-coated vesicles (135 nm in diameter), which deliver their cargo to endosomes. In the lumen of cisternal endosomes, VSG is concentrated by default, because a distinct class of small clathrin-coated vesicles (50-60 nm in diameter) budding from the cisternae is depleted in VSG. TbRAB11-positive cisternal endosomes, containing VSG, fragment by an unknown process giving rise to intensely TbRAB11- as well as VSG-positive, disk-like carriers (154 nm in diameter, 34 nm in thickness), which are shown to fuse with the flagellar pocket membrane, thereby recycling VSG back to the cell surface.

Endocytosis of lipid rafts: an identity crisis

Membrane rafts enriched in cholesterol and sphingolipids have been hypothesized to be key mediators of sorting and signaling functions of associated molecules. Apart from a limited number of biophysical studies in living cell membranes, raft-association has been defined by a simple biochemical criterion, namely the ability to partition with detergent-resistant membranes (DRMs). Here we examine the evidence for the specification of internalization mechanisms and endocytic pathways by rafts as defined by this criterion. We have surveyed the endocytic trafficking of a variety of molecules such as lipids, toxins, glycosylphosphatidylinositol (GPI)-anchored proteins, and DRM-associated transmembrane proteins.

Polarized targeting of peripheral membrane proteins in neurons

Differential targeting of neuronal proteins to axons and dendrites is essential for directional information flow within the brain, however, little is known about this protein-sorting process. Here, we investigate polarized targeting of lipid-anchored peripheral membrane proteins, postsynaptic density-95 (PSD-95) and growth-associated protein-43 (GAP-43). Whereas the N-terminal palmitoylated motif of PSD-95 is necessary but not sufficient for sorting to dendrites, the palmitoylation motif of GAP-43 is sufficient for axonal targeting and can redirect a PSD-95 chimera to axons. Systematic mutagenesis of the GAP-43 and PSD-95 palmitoylation motifs indicates that the spacing of the palmitoylated cysteines and the presence of nearby basic amino acids determine polarized targeting by these two motifs. Similarly, the axonal protein paralemmin contains a C-terminal palmitoylated domain, which resembles that of GAP-43 and also mediates axonal targeting. These axonally targeted palmitoylation motifs also mediate targeting to detergent-insoluble glycolipid-enriched complexes in heterologous cells, suggesting a possible role for specialized lipid domains in axonal sorting of peripheral membrane proteins.

On the dynamics of TCR:CD3 complex cell surface expression and downmodulation

TCR downmodulation following ligation by MHC:peptide complexes is considered to be a pivotal event in T cell activation. Here, we analyzed the dynamics of TCR:CD3 cell surface expression on resting and antigen-activated T cells. We show that the TCR:CD3 complex is very stable and is rapidly internalized and recycled in resting T cells. Surprisingly, the internalization rate is not increased following TCR ligation by MHC:peptide complexes, despite significant TCR downmodulation, suggesting that constitutive internalization rather than ligation-induced downmodulation serves as the force that drives serial ligation. Furthermore, TCR downmodulation is mediated by the intracellular retention of ligated complexes and degradation by lysosomes and proteasomes. Thus, our data demonstrate that ligation induces TCR downmodulation by preventing recycling rather than inducing internalization.

Clathrin-mediated endocytosis of MUC1 is modulated by its glycosylation state

MUC1 is a mucin-like type 1 transmembrane protein associated with the apical surface of epithelial cells. In human tumors of epithelial origin MUC1 is overexpressed in an underglycosylated form with truncated O-glycans and accumulates in intracellular compartments. To understand the basis for this altered subcellular localization, we compared the synthesis and trafficking of various glycosylated forms of MUC1 in normal (Chinese hamster ovary) cells and glycosylation-defective (ldlD) cells that lack the epimerase to make UDP-Gal/GalNAc from UDP-Glc/GlcNAc. Although the MUC1 synthesized in ldlD cells was rapidly degraded, addition of GalNAc alone to the culture media resulted in stabilization and near normal surface expression of MUC1 with truncated but sialylated O-glycans. Interestingly, the initial rate of endocytosis of this underglycosylated MUC1 was stimulated by twofold compared with fully glycosylated MUC1. However, the half-lives of the two forms were not different, indicating that trafficking to lysosomes was not affected. Both the normal and stimulated internalization of MUC1 could be blocked by hypertonic media, a hallmark of clathrin-mediated endocytosis. MUC1 endocytosis was also blocked by expression of a dominant-negative mutant of dynamin-1 (K44A), and MUC1 was observed in both clathrin-coated pits and vesicles by immunoelectron microscopy of ultrathin cryosections. Our data suggest that the subcellular redistribution of MUC1 in tumor cells could be a direct result of altered endocytic trafficking induced by its aberrant glycosylation; potential models are discussed. These results also implicate a new role for O-glycans on mucin-like membrane proteins entering the endocytic pathway through clathrin-coated pits.

The Rous sarcoma virus Env glycoprotein contains a highly conserved motif homologous to tyrosine-based endocytosis signals and displays an unusual internalization phenotype

The cytoplasmic domains of retroviral transmembrane (TM) glycoproteins contain conserved sequence motifs that resemble tyrosine-based (YXXO-type) endocytosis signals. We have previously described a mutant Rous sarcoma virus (RSV) Env protein, Env-mu26, with an L165R mutation in the membrane-spanning domain (MSD) of TM, that exhibited dramatically decreased steady-state surface expression (G. L. Davis and E. Hunter, J. Cell Biol. 105:1191-1203, 1987; P. B. Johnston, J. Y. Dong, and E. Hunter, Virology 206:353-361, 1995). We now demonstrate that the tyrosine of the Y(190)RKM motif in the RSV TM cytoplasmic domain is crucial for the mu26 phenotype and is part of an efficient internalization signal in the context of a mutant MSD. In contrast, despite the presence of the Y(190)RKM motif, wild-type RSV Env is constitutively internalized at a slow rate (1.1%/min) more characteristic of bulk uptake during membrane turnover than of active clustering into endocytic vesicles. The mu26 mutation and two MSD mutations that abrogate palmitoylation of TM resulted in enhanced Env endocytosis indicative of active concentration into coated pits. Surprisingly, an Env-Y190A mutant was apparently excluded from coated pits since its uptake rate of 0.3%/min was significantly below that expected for the bulk rate. We suggest that in RSV Env an inherently functional endocytosis motif is silenced by a counteracting determinant in the MSD that acts to prevent clustering of Env into endocytic vesicles. Mutations in either the cytoplasmic tail or the MSD that inactivate one of the two counteracting signals would thus render the remaining determinant dominant.

The distribution of circulating receptors on COS 7 cells

Rapidly circulating receptors on the surfaces of migrating or spreading cells are often concentrated toward the cells' leading edges. This polar distribution is the principal evidence that circulating membrane is returned to the surface of these cells at their fronts, where it is proposed to assist the forward extension of the cell. However, when low-density lipoprotein (LDL) receptors are transiently expressed in COS cells-cells which actively spread-their distribution is random, indicating that COS cells do not behave like other motile cells. This anomaly is examined here. I find that COS cell lines expressing either transferrin or LDL receptors do, in fact, usually have polar distributions of both receptors. The transient expression of LDL receptors interferes, in an unknown way, with the polar properties of COS cells. Furthermore, it is shown that whether receptors are located toward the cell's edge or are randomly distributed depends both on the ability of the receptor to circulate and on the state of the particular cell at that time.

Urokinase-type plasminogen activator receptor is internalized by different mechanisms in polarized and nonpolarized Madin-Darby canine kidney epithelial cells

Accumulated data indicate that endocytosis of the glycosylphosphatidyl-inositol-anchored protein urokinase plasminogen activator receptor (uPAR) depends on binding of the ligand uPA:plasminogen activator inhibitor-1 (PAI-1) and subsequent interaction with internalization receptors of the low-density lipoprotein receptor family, which are internalized through clathrin-coated pits. This interaction is inhibited by receptor-associated protein (RAP). We show that uPAR with bound uPA:PAI-1 is capable of entering cells in a clathrin-independent process. First, HeLaK44A cells expressing mutant dynamin efficiently internalized uPA:PAI-1 under conditions in which transferrin endocytosis was blocked. Second, in polarized Madin-Darby canine kidney (MDCK) cells, which expressed human uPAR apically, the low basal rate of uPAR ligand endocytosis, which could not be inhibited by RAP, was increased by forskolin or phorbol ester (phorbol 12-myristate 13-acetate), which selectively up-regulate clathrin-independent endocytosis from the apical domain of epithelial cells. Third, in subconfluent nonpolarized MDCK cells, endocytosis of uPA:PAI-1 was only decreased marginally by RAP. At the ultrastructural level uPAR was largely excluded from clathrin-coated pits in these cells and localized in invaginated caveolae only in the presence of cross-linking antibodies. Interestingly, a larger fraction of uPAR in nonpolarized relative to polarized MDCK cells was insoluble in Triton X-100 at 0 degreesC, and by surface labeling with biotin we also show that internalized uPAR was mainly detergent insoluble, suggesting a correlation between association with detergent-resistant membrane microdomains and higher degree of clathrin-independent endocytosis. Furthermore, by cryoimmunogold labeling we show that 5-10% of internalized uPAR in nonpolarized, but not polarized, MDCK cells is targeted to lysosomes by a mechanism that is regulated by ligand occupancy.

Viral cell entry induced by cross-linked decay-accelerating factor

Decay-accelerating factor (DAF) mediates cellular attachment for many human picornaviruses. In most cases, viral binding to DAF is itself insufficient to permit cell infectivity, with a second, functional internalization receptor being required to facilitate this process. Previously, we postulated that the role of DAF in enterovirus cell infection is as a sequestration receptor, maintaining a reservoir of bound virus in an infectious state, awaiting interaction with functional internalization receptors. Many of these functional receptors possess the capacity to induce relatively rapid changes in capsid conformations, resulting in the formation of altered particles (A-type particles). In this report, we show that antibody-cross-linked DAF, in contrast to endogenous surface-expressed forms, can act as a functional virus receptor to mediate coxsackie A21 virus (CAV21) lytic cell infection. In contrast to the situation with ICAM-1-mediated CAV21 infection, in which high levels of A-type particles are formed, cross-linked DAF-induced CAV21 replication occurs in the absence of detectable A-particle formation.

Cholesterol is required for surface transport of influenza virus hemagglutinin

Transport from the TGN to the basolateral surface involves a rab/N-ethylmaleimide-sensitive fusion protein (NSF)/soluble NSF attachment protein (SNAP)/SNAP receptor (SNARE) mechanism. Apical transport instead is thought to be mediated by detergent-insoluble sphingolipid-cholesterol rafts. By reducing the cholesterol level of living cells by 60-70% with lovastatin and methyl-beta-cyclodextrin, we show that the TGN-to-surface transport of the apical marker protein influenza virus hemagglutinin was slowed down, whereas the transport of the basolateral marker vesicular stomatitis virus glycoprotein as well as the ER-to-Golgi transport of both membrane proteins was not affected. Reduction of transport of hemagglutinin was accompanied by increased solubility in the detergent Triton X-100 and by significant missorting of hemagglutinin to the basolateral membrane. In addition, depletion of cellular cholesterol by lovastatin and methyl-beta-cyclodextrin led to missorting of the apical secretory glycoprotein gp-80, suggesting that gp-80 uses a raft-dependent mechanism for apical sorting. Our data provide for the first time direct evidence for the functional significance of cholesterol in the sorting of apical membrane proteins as well as of apically secreted glycoproteins.

Identification of components of trans-Golgi network-derived transport vesicles and detergent-insoluble complexes by nanoelectrospray tandem mass spectrometry

Epithelial cells have to deliver newly synthesized proteins to the apical and the basolateral plasma membrane domains of the polarized cell surface. Sorting takes place in the trans-Golgi network and at least two vesicular carriers exist for apical and basolateral delivery. After immuno-isolation, the composition of these vesicle preparations was analyzed by two-dimensional (2-D) gel electrophoresis and detergent extraction. In this paper we compare the constituents of detergent-insoluble complexes in different cell lines of polarized or nonpolarized origin and present the identification of five previously uncharacterized proteins. We show that our protein identification strategy can be successfully applied to the problem of small hydrophobic proteins from organisms that have not been substantially sequenced. The high sensitivity of nanoelectrospray tandem mass spectrometry allowed us to identify two proteins that belong to the p23/p24 family of putative cargo receptors for vesicular trafficking. Furthermore we have mapped CD9 and CD81, two members of a large family of proteins consisting of highly hydrophobic four transmembrane proteins. In addition we have identified caveolin-2 as a constituent of basolateral transport vesicles. We have also extended our analysis of immuno-isolated vesicles to a more basic pI range and show that this region on 2-D gels is devoid of proteins. With these approaches and with the previously published data we have now identified most of the major low molecular weight proteins recovered in detergent-insoluble glycolipid-enriched complexes.

Endocytosis and recycling of varicella-zoster virus Fc receptor glycoprotein gE: internalization mediated by a YXXL motif in the cytoplasmic tail

Varicella-zoster virus (VZV) encodes a cell surface Fc receptor, glycoprotein gE. VZV gE has previously been shown to display several features common to nonviral cell surface receptors. Most recently, VZV gE was reported to be tyrosine phosphorylated on a dimeric form (J. K. Olson, G. A. Bishop, and C. Grose, J. Virol. 71:110-119, 1997). Thereafter, attention focused on the ability of VZV gE to undergo receptor-mediated endocytosis. The current transient transfection studies demonstrated by confocal microscopy and internalization assays that VZV gE was endocytosed when expressed in HeLa cells. Endocytosis of gE was shown to be dependent on clathrin-coated vesicle formation within the cells. Subsequent colocalization studies showed that endocytosis of VZV gE closely mimicked endocytosis of the transferrin receptor. The gE cytoplasmic tail and more specifically tyrosine residue 582 were determined by mutagenesis studies to be important for efficient internalization of the protein; this tyrosine residue is part of a conserved YXXL motif. The amount of gE internalized at any given time reached a steady state of 32%. In addition, like the transferrin receptor, internalized gE recycled to the cell surface. The finding of gE endocytosis provided insight into earlier documentation of gE serine/threonine and tyrosine phosphorylation, since these phosphorylation events may serve as sorting signals for internalized receptors. Taken together with the previous discovery that both human and simian immunodeficiency virus envelope proteins can undergo endocytosis, the gE findings suggest that endocytosis of envelope components may be a posttranslational regulatory mechanism among divergent families of enveloped viruses.

Clathrin-coated vesicle formation and protein sorting: an integrated process

Clathrin-coated vesicles were the first discovered and remain the most extensively characterized transport vesicles. They mediate endocytosis of transmembrane receptors and transport of newly synthesized lysosomal hydrolases from the trans-Golgi network to the lysosome. Cell-free assays for coat assembly, membrane binding, and coated vesicle budding have provided detailed functional and structural information about how the major coat constituents, clathrin and the adaptor protein complexes, interact with each other, with membranes, and with the sorting signals found on cargo molecules. Coat constituents not only serve to shape the budding vesicle, but also play a direct role in the packaging of cargo, suggesting that protein sorting and vesicle budding are functionally integrated. The functional interplay between the coated vesicle machinery and its cargo could ensure sorting fidelity and packaging efficiency and might enable modulation of vesicular trafficking in response to demand.

A di-hydrophobic Leu-Val motif regulates the basolateral localization of CD44 in polarized Madin-Darby canine kidney epithelial cells

Both in vivo and in vitro the distribution of the resident plasma membrane adhesion protein, CD44, is restricted to the basolateral domain of polarized epithelial cells, suggesting a role in interepithelial interactions. To determine how this localization might be regulated a range of CD44 cytoplasmic domain mutations were generated and a minimal 5 amino acid sequence, His330-Leu-Val-Asn-Lys334, was identified which when deleted results in expression of CD44 on the apical microvillal membrane. Further mutagenesis throughout this regions pinpointed a critical di-hydrophobic motif, Leu331/Val332. The ability of wild type but not mutant CD44 cytoplasmic domains to redirect an apically targeted protein, placental alkaline phosphatase, to the basolateral plasma membrane demonstrates that this sequence can function as a dominant localization signal. This His330-Lys334 sequence is spatially separate from other CD44 regulatory elements and as discussed here, a comparison with known basolateral sorting sequences identified in other transmembrane proteins suggests that a distinct mechanism operates to retain resident plasma membrane proteins in their correct plasma membrane subdomains.

Identification of a 40-kDa cell surface sialoglycoprotein with the characteristics of a major influenza C virus receptor in a Madin-Darby canine kidney cell line

Infection of cells by influenza C virus is known to be initiated by virus attachment to cell surface glycoconjugates containing N-acetyl-9-O-acetylneuraminic acid. Using an in vitro virus binding assay, we have detected this carbohydrate on several glycoproteins of Madin-Darby canine kidney cells (type I), a polarized epithelial cell line permissive for infection with influenza C virus. Among these proteins, only one was found to be present to a significant extent on the cell surface. This protein, gp40, was characterized as an O-glycosylated (mucin-type) integral membrane protein of 40 kDa, which was predominantly localized on the apical plasma membrane of filter-grown cells. It is a major cell surface sialoglycoprotein in this cell line and was shown to be subject to constitutive and rapid endocytosis. Thus, this glycoprotein can mediate not only the binding of influenza C virus to the cell surface, but also its delivery to endosomes, where penetration occurs by membrane fusion. Other highly sialylated cell surface glycoproteins were also detected but did not mediate influenza C virus binding to a significant extent, indicating that only gp40 contains 9-O-acetylated sialic acids.

The N-terminal domain of a glycolipid-anchored prion protein is essential for its endocytosis via clathrin-coated pits

The cellular prion protein (PrPC) is a glycolipid-anchored protein that is involved in the pathogenesis of fatal spongiform encephalopathies. We have shown previously that, in contrast to several other glycolipid-anchored proteins, chPrP, the chicken homologue of mammalian PrPC, is endocytosed via clathrin-coated pits in cultured neuroblastoma cells, as well as in embryonic neurons and glia (Shyng, S.-L., Heuser, J. E., and Harris, D. A. (1994) J. Cell Biol. 125, 1239-1250). In this study, we have determined that the N-terminal half of the chPrP polypeptide chain is essential for its endocytosis. Deletions within this region reduce the amount of chPrP internalized, as measured by surface iodination or biotinylation, and decrease its concentration in clathrin-coated pits, as determined by quantitative electron microscopic immunogold labeling. Mouse PrP, as well as two mouse PrP/chPrP chimeras, are internalized as efficiently as chPrP, suggesting that conserved features of secondary and tertiary structure are involved in interaction with the endocytic machinery. Our results indicate that the ectodomain of a protein can contain endocytic targeting information, and they strongly support a model in which the polypeptide chain of PrPC binds to the extracellular domain of a transmembrane protein that contains a coated pit localization signal in its cytoplasmic tail.

Epithelial sorting of a glycosylphosphatidylinositol-anchored bacterial protein expressed in polarized renal MDCK and intestinal Caco-2 cells

To evaluate whether a glycosylphosphatidylinositol (GPI) anchor can function as a protein sorting signal in polarized intestinal epithelial cells, the GPI-attachment sequence from Thy-1 was fused to bacterial endoglucanase E' (EGE') from Clostridium thermocellum and polarity of secretion of the chimeric EGE'-GPI protein was evaluated. The chimeric EGE'-GPI protein was shown to be associated with a GPI anchor by TX-114 phase-partitioning and susceptibility to phosphoinositol-specific phospholipase C. In polarized MDCK cells, EGE' was localized almost exclusively to the apical cell surface, while in polarized intestinal Caco-2 cells, although 80% of the extracellular form of the enzyme was routed through the apical membrane over a 24 hour period, EGE' was also detected at the basolateral membrane. Rates of delivery of EGE'-GPI to the two membrane domains in Caco-2 cells, as determined with a biotinylation protocol, revealed apical delivery was approximately 2.5 times that of basolateral. EGE' delivered to the basolateral cell surface was transcytosed to the apical surface. These data indicate that a GPI anchor does represent a dominant apical sorting signal in intestinal epithelial cells. However, the mis-sorting of a proportion of EGE'GPI to the basolateral surface of Caco-2 cells provides an explanation for additional sorting signals in the ectodomain of some endogenous GPI-anchored proteins.

Filipin-sensitive caveolae-mediated transport in endothelium: reduced transcytosis, scavenger endocytosis, and capillary permeability of select macromolecules

Caveolae or noncoated plasmalemmal vesicles found in a variety of cells have been implicated in a number of important cellular functions including endocytosis, transcytosis, and potocytosis. Their function in transport across endothelium has been especially controversial, at least in part because there has not been any way to selectively inhibit this putative pathway. We now show that the ability of sterol binding agents such as filipin to disassemble endothelial noncoated but not coated plasmalemmal vesicles selectively inhibits caveolae-mediated intracellular and transcellular transport of select macromolecules in endothelium. Filipin significantly reduces the transcellular transport of insulin and albumin across cultured endothelial cell monolayers. Rat lung microvascular permeability to albumin in situ is significantly decreased after filipin perfusion. Conversely, paracellular transport of the small solute inulin is not inhibited in vitro or in situ. In addition, we show that caveolae mediate the scavenger endocytosis of conformationally modified albumins for delivery to endosomes and lysosomes for degradation. This intracellular transport is inhibited by filipin both in vitro and in situ. Other sterol binding agents including nystatin and digitonin also inhibit this degradative process. Conversely, the endocytosis and degradation of activated alpha 2-macroglobulin, a known ligand of the clathrin-dependent pathway, is not affected. Interestingly, filipin appears to inhibit insulin uptake by endothelium for transcytosis, a caveolae-mediated process, but not endocytosis for degradation, apparently mediated by the clathrin-coated pathway. Such selective inhibition of caveolae not only provides critical evidence for the role of caveolae in the intracellular and transcellular transport of select macromolecules in endothelium but also may be useful for distinguishing transport mediated by coated versus noncoated vesicles.

Capping and receptor-mediated endocytosis of cell-bound verotoxin (Shiga-like toxin). 1: Chemical identification of an amino acid in the B subunit necessary for efficient receptor glycolipid binding and cellular internalization

The glycolipid globotriaosylceramide (Gb3) is the plasma membrane receptor that mediates the internalization of verotoxin (VT1) into susceptible cells by capping and receptor-mediated endocytosis (RME). Internalization of fluorescein isothiocyanate-conjugated holotoxin into Daudi lymphoma cells was found to be slower than the pentameric receptor binding B subunit alone, suggesting that the A subunit may interact with the membrane to compromise the lateral mobility of the receptor bound B subunit. 3-D reconstruction of fluorescent images by confocal microscopy confirmed the complete internalization of holotoxin. VT1 internalization and cytotoxicity was inhibited by monodansyl cadavarine, which supports a role for clathrin coated pits in the RME of VT1. Biotinylation of the B subunit (in contrast to fluorescein labelling) was found to prevent toxin internalization. This effect correlated with reduced binding of Gb3 and reduced cytotoxicity in vitro. By cleavage of the B subunit at the single tryptophan residue, the reduced Gb3 binding and lack of cellular internalization was shown to be due to the biotinylation of lysine 53 in the VT1 B subunit. This residue was not labelled with fluorescein isothiocyanate in the native protein. This conclusion was confirmed by the finding that biotinylation of VT2c (which contains lys 53) prevented glycolipid receptor binding, whereas biotinylation of VT2e (in which lys 53 is substituted by ile) had no effect.

A glycolipid-anchored prion protein is endocytosed via clathrin-coated pits

The cellular prion protein (PrPc) is a glycolipid-anchored, cell surface protein of unknown function, a posttranslationally modified isoform of which PrPSc is involved in the pathogenesis of Creutzfeldt-Jakob disease, scrapie, and other spongiform encephalopathies. We have shown previously that chPrP, a chicken homologue of mammalian PrPC, constitutively cycles between the cell surface and an endocytic compartment, with a transit time of approximately 60 min in cultured neuroblastoma cells. We now report that endocytosis of chPrP is mediated by clathrin-coated pits. Immunogold labeling of neuroblastoma cells demonstrates that the concentration of chPrP within 0.05 microns of coated pits is 3-5 times higher than over other areas of the plasma membrane. Moreover, gold particles can be seen within coated vesicles and deeply invaginated coated pits that are in the process of pinching off from the plasma membrane. ChPrP is also localized to coated pits in primary cultures of neurons and glia, and is found in coated vesicles purified from chicken brain. Finally, internalization of chPrP is reduced by 70% after neuroblastoma cells are incubated in hypertonic medium, a treatment that inhibits endocytosis by disrupting clathrin lattices. Caveolae, plasmalemmal invaginations in which several other glycolipid-anchored proteins are concentrated, are not seen in neuroblastoma cells analyzed by thin-section or deep-etch electron microscopy. Moreover, these cells do not express detectable levels of caveolin, a caveolar coat protein. Since chPrP lacks a cytoplasmic domain that could interact directly with the intracellular components of clathrin-coated pits, we propose that the polypeptide chain of chPrP associates with the extracellular domain of a transmembrane protein that contains a coated pit internalization signal.

Biosynthesis and intracellular transport of a bile canalicular plasma membrane protein: studies in vivo and in the perfused rat liver

B10 is an integral glycoprotein of the plasma membrane that is exclusively localized to the canalicular (apical) domain in normal rat hepatocytes but may be expressed on the basolateral (sinusoidal and lateral) membrane in pathophysiological situations. To understand how B10 may be localized to the basolateral surface, we studied the biosynthesis and transport of this apical protein. In vivo pulse-chase experiments, followed by subcellular fractionation of the liver and immunoprecipitation, showed that B10 is first synthesized as a high-mannose form of 123 kD and then matured to a complex glycosylated form of 130 kD, which peaks in the Golgi apparatus after 15 min of chase and reaches the plasma membrane with a half-time of 30 to 45 min. Analysis of the protein in plasma membrane domain fractions showed that most of the newly synthesized molecule was localized in basolateral fractions after 30 min of chase and subsequently appeared in apical fractions. After 90 min of chase, most of the radiolabeled protein had reached its steady-state apical distribution. The same experiments performed in the perfused rat liver, in which the chase can be improved, gave similar results, except that the apical distribution of the radioactive molecule was attained more quickly. Thus B10, like all apical plasma membrane proteins studied so far in hepatocytes, is first transported to the basolateral surface and then reaches the membrane of the bile canaliculi. Alterations of the transcytotic step from the basolateral to the apical surfaces may result in abnormal basolateral localization.

Replacement of the phospholipid-anchor in the contact site A glycoprotein of D. discoideum by a transmembrane region does not impede cell adhesion but reduces residence time on the cell surface

The contact site A (csA) glycoprotein of Dictyostelium discoideum, a cell adhesion molecule expressed in aggregating cells, is inserted into the plasma membrane by a ceramide-based phospholipid (PL) anchor. A carboxyterminal sequence of 25 amino acids of the primary csA translation product proved to contain the signal required for PL modification. CsA is known to be responsible for rapid, EDTA-resistant cohesion of cells in agitated suspensions. To investigate the role of the PL modification of this protein, the anchor was replaced by the transmembrane region and short cytoplasmic tail of another plasma membrane protein of D. discoideum. In cells transformed with appropriate vectors, PL-anchored or transmembrane csA was expressed under the control of an actin promoter during growth and development. The transmembrane form enabled the cells to agglutinate in the presence of shear forces, similar to the PL-anchored wild-type form. However, the transmembrane form was much more rapidly internalized and degraded. In comparison to other cell-surface glycoproteins of D. discoideum the internalization rate of the PL-anchored csA was extremely slow, most likely because of its exclusion from the clathrin-mediated pathway of pinocytosis. Thus, our results indicate that the phospholipid modification is not essential for the csA-mediated fast type of cell adhesion but guarantees long persistence of the protein on the cell surface.

Caveolin forms a hetero-oligomeric protein complex that interacts with an apical GPI-linked protein: implications for the biogenesis of caveolae

Glycosyl-phosphatidylinositol (GPI)-linked proteins are transported to the apical surface of epithelial cells where they undergo cholesterol-dependent clustering in membrane micro-invaginations, termed caveolae or plasmalemmal vesicles. However, the sorting machinery responsible for this caveolar-clustering mechanism remains unknown. Using transfected MDCK cells as a model system, we have identified a complex of cell surface molecules (80, 50, 40, 22-24, and 14 kD) that interact in a pH- and cholesterol-dependent fashion with an apical recombinant GPI-linked protein. A major component of this hetero-oligomeric protein complex is caveolin, a type II transmembrane protein. As this hetero-oligomeric caveolin complex is detectable almost immediately after caveolin synthesis, our results suggest that caveolae may assemble intracellularly during transport to the cell surface. As such, our studies have implications for understanding both the intracellular biogenesis of caveolae and their subsequent interactions with GPI-linked proteins in epithelia and other cell types.

The structure, biosynthesis and function of glycosylated phosphatidylinositols in the parasitic protozoa and higher eukaryotes

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The cytoplasmic tail of CD44 is required for basolateral localization in epithelial MDCK cells but does not mediate association with the detergent-insoluble cytoskeleton of fibroblasts

A number of recent reports on the trafficking of receptor proteins in MDCK epithelial cells have provided evidence that delivery to the basolateral domain requires a specific targeting sequence and that deletion of this sequence results in constitutive expression on the apical surface. To date, these studies have concentrated on receptors which are competent for internalization via the clathrin coated pits. We have examined the localization of a resident plasma membrane protein by transfecting human CD44 into MDCK cells. Using human specific and cross-species reactive antibodies, we show that in MDCK cells both the endogenous and transfected wild-type CD44 are found on the basolateral surface where they are restricted to the lateral domain. Deletion of the CD44 cytoplasmic tail reduces the half life of this mutant protein and causes it to be expressed both on the apical surface and to a significant extent within the cell. We have also used biochemical and morphological analysis to investigate the interaction of CD44 with the cytoskeleton in detergent extracted cells. Strikingly different extraction results were obtained between epithelial and fibroblast cells. However, there is no difference in the Triton X-100 solubility of the transfected wild-type and tail-less CD44 in fibroblasts and both forms of the protein remain associated with the cortical cytoskeleton after Triton X-100 extraction. These results demonstrate that the sequence present in the cytoplasmic domain of CD44 responsible for its distribution in epithelial cells is functionally and spatially separate from the ability of this protein to associate with the cytoskeleton.

Structure of clathrin-coated vesicles from small-angle scattering experiments

Previously published small-angle neutron and X-ray scattering data from coated vesicles, reassembled coats, and stripped vesicles have been analyzed in terms of one common model. The neutron data sets include contrast variation measurements at three different D2O solvent concentrations. The model used for interpreting the data has spherical symmetry and explicitly takes into account polydispersity, which is described by a Gaussian distribution. A constant thickness of the clathrin coats is assumed. The fitting of the model shows that the coated vesicles consist of a low-density outer protein shell (clathrin) and a central protein shell (accessory polypeptides and receptors) of approximately six times higher density. For the X-ray scattering and neutron contrast variation data, the polydispersity of the samples is of the order of 90 A (full-width-at-half-maximum value) and the average outer radius is approximately 400 A. The inner high-density shell has inner and outer radii of 115 and 190 A, respectively. A simultaneous fit to the three neutron contrast variation data sets identifies the lipid membrane with a thickness of 40 A and an outer radius of 196 A. Thus, the membrane and the high-density protein shell overlap in space, which shows that the lipid membrane contains protein. The molecular mass of the average particle is 27 x 10(6) Da. The coated vesicles consist, on average, of approximately 85% protein and 15% lipids. About 40% of the protein mass is situated in the central high-density shell, which gives a large amount of protein in the lipid membrane. The densities of the central shell and the lipid membrane show that the hydration is small in the central region. A comparison of the total mass, the mass distribution, and the structure of the average-size particles with the barrel structure shows that the accessory polypeptides are incorporated in the lipid membrane. The results from the neutron data for the reassembled coats show that the structure of these particles is very similar to the structure of the native coats. The main difference is a higher density of the central protein shell, which shows that the membrane is replaced by protein in the reassembled coats.

Clathrin: its role in receptor-mediated vesicular transport and specialized functions in neurons

Clathrin constitutes the coat of vesicles involved in three receptor-mediated intracellular transport pathways; the export of aggregated material from the trans-Golgi network for regulated secretion, the transfer of lysosomal hydrolases from the trans-Golgi network to lysosomes and receptor-mediated endocytosis at the plasma membrane. The clathrin subunits and the other major coat constituents, the adaptor polypeptides, interact in specific ways to build the characteristic polygonal clathrin lattice and to attach the coat to integral membrane receptors. Both clathrin coat assembly and disassembly on the cytoplasmic side of the membrane are multistep processes that are regulated by the coat constituents themselves and by cytosolic proteins and factors. Neurons represent a cell type with distinct morphology and special demands on exocytic and endocytic pathways that requires neuron-specific constituents and modifications of clathrin-coated vesicles.