Dileucine-based sorting signals bind to the beta chain of AP-1 at a site distinct and regulated differently from the tyrosine-based motif-binding site

A dileucine motif targets E-cadherin to the basolateral cell surface in Madin-Darby canine kidney and LLC-PK1 epithelial cells

E-cadherin is a major adherens junction protein of epithelial cells, with a central role in cell-cell adhesion and cell polarity. Newly synthesized E-cadherin is targeted to the basolateral cell surface. We analyzed targeting information in the cytoplasmic tail of E-cadherin by utilizing chimeras of E-cadherin fused to the ectodomain of the interleukin-2alpha (IL-2alpha) receptor expressed in Madin-Darby canine kidney and LLC-PK(1) epithelial cells. Chimeras containing the full-length or membrane-proximal half of the E-cadherin cytoplasmic tail were correctly targeted to the basolateral domain. Sequence analysis of the membrane-proximal tail region revealed the presence of a highly conserved dileucine motif, which was analyzed as a putative targeting signal by mutagenesis. Elimination of this motif resulted in the loss of Tac/E-cadherin basolateral localization, pinpointing this dileucine signal as being both necessary and sufficient for basolateral targeting of E-cadherin. Truncation mutants unable to bind beta-catenin were correctly targeted, showing, contrary to current understanding, that beta-catenin is not required for basolateral trafficking. Our results also provide evidence that dileucine-mediated targeting is maintained in LLC-PK(1) cells despite the altered polarity of basolateral proteins with tyrosine-based signals in this cell line. These results provide the first direct insights into how E-cadherin is targeted to the basolateral membrane.

Intracellular transport of MHC class II and associated invariant chain in antigen presenting cells from AP-3-deficient mocha mice

MHC class II-restricted antigen presentation requires trafficking of newly synthesized class II-invariant chain complexes from the trans-Golgi network to endosomal, peptide-loading compartments. This transport is mediated by dileucine-like motifs within the cytosolic tail of the invariant chain. Although these signals have been well characterized, the cytosolic proteins that interact with these dileucine signals and mediate Golgi sorting and endosomal transport have not been identified. Recently, an adaptor complex, AP-3, has been identified that interacts with dileucine motifs and mediates endosomal/lysosomal transport in yeast, Drosophila, and mammals. In this report, we have assessed class II-invariant chain trafficking in a strain of mice (mocha) which lacks expression of AP-3. Our studies demonstrate that the lack of AP-3 does not affect the kinetics of invariant chain degradation, the route of class II-invariant chain transport, or the rate and extent of class II-peptide binding as assessed by the generation of SDS-stable dimers. The possible role of other known or unknown adaptor complexes in class II-invariant chain transport is discussed.

Yeast Gga coat proteins function with clathrin in Golgi to endosome transport

Gga proteins represent a newly recognized, evolutionarily conserved protein family with homology to the "ear" domain of the clathrin adaptor AP-1 gamma subunit. Yeast cells contain two Gga proteins, Gga1p and Gga2p, that have been proposed to act in transport between the trans-Golgi network and endosomes. Here we provide genetic and physical evidence that yeast Gga proteins function in trans-Golgi network clathrin coats. Deletion of Gga2p (gga2Delta), the major Gga protein, accentuates growth and alpha-factor maturation defects in cells carrying a temperature-sensitive allele of the clathrin heavy chain gene. Cells carrying either gga2Delta or a deletion of the AP-1 beta subunit gene (apl2Delta) alone are phenotypically normal, but cells carrying both gga2Delta and apl2Delta are defective in growth, alpha-factor maturation, and transport of carboxypeptidase S to the vacuole. Disruption of both GGA genes and APL2 results in cells so severely compromised in growth that they form only microcolonies. Gga proteins can bind clathrin in vitro and cofractionate with clathrin-coated vesicles. Our results indicate that yeast Gga proteins play an important role in cargo-selective clathrin-mediated protein traffic from the trans-Golgi network to endosomes.

Internalization-competent influenza hemagglutinin mutants form complexes with clathrin-deficient multivalent AP-2 oligomers in live cells

Most membrane proteins are endocytosed through clathrin-coated pits via AP-2 adaptor complexes. However, little is known about the interaction of internalization signals with AP-2 in live cells in the absence of clathrin lattices. To investigate this issue, we employed cells cotransfected with pairs of antigenically distinct influenza hemagglutinin (HA) mutants containing different internalization signals of the YXXZ family. To enable studies on the possible association of the naturally trimeric HAs into higher order complexes via binding to AP-2, we exploited the inability of HAs from different influenza strains to form mutual trimers. Thus, we coexpressed HA pairs from different strains (Japan and X:31) bearing similar cytoplasmic tails mutated to include internalization signals. Using antibody-mediated immunofluorescence co-patching on live cells, we demonstrate that internalization-competent HA mutants form higher order complexes and that this clustering depends on the strength of the internalization signal. The clustering persisted in cells treated with hypertonic medium to disperse the clathrin lattices, as validated by co-immunoprecipitation experiments. The clustering of HAs bearing strong internalization signals appears to be mediated via binding to AP-2, as indicated by (i) the coprecipitation of alpha-adaptin with these HAs, even in hypertonically treated cells; (ii) the co-localization (after hypertonic treatment) of AP-2 with antibody-mediated patches of these mutants; and (iii) the dispersal of the higher order HA complexes following chlorpromazine treatment, which removes AP-2 from the plasma membrane. These results suggest that even in the absence of clathrin lattices, AP-2 exists in multivalent complexes capable of simultaneously binding several internalization signals from the same family.

PACS-1 binding to adaptors is required for acidic cluster motif-mediated protein traffic

PACS-1 is a cytosolic protein involved in controlling the correct subcellular localization of integral membrane proteins that contain acidic cluster sorting motifs, such as furin and human immunodeficiency virus type 1 (HIV-1) NEF: We have now investigated the interaction of PACS-1 with heterotetrameric adaptor complexes. PACS-1 associates with both AP-1 and AP-3, but not AP-2, and forms a ternary complex between furin and AP-1. A short sequence within PACS-1 that is essential for binding to AP-1 has been identified. Mutation of this motif yielded a dominant-negative PACS-1 molecule that can still bind to acidic cluster motifs on cargo proteins but not to adaptor complexes. Expression of dominant-negative PACS-1 causes a mislocalization of both furin and mannose 6-phosphate receptor from the trans-Golgi network, but has no effect on the localization of proteins that do not contain acidic cluster sorting motifs. Furthermore, expression of dominant-negative PACS-1 inhibits the ability of HIV-1 Nef to downregulate MHC-I. These studies demonstrate the requirement for PACS-1 interactions with adaptor proteins in multiple processes, including secretory granule biogenesis and HIV-1 pathogenesis.

A single internalization signal from the di-leucine family is critical for constitutive endocytosis of the type II TGF-(β) receptor

Endocytosis has an important contribution to the regulation of the surface expression levels of many receptors. In spite of the central role of the transforming growth factor (β) (TGF-(β)) receptors in numerous cellular and physiological processes, their endocytosis is largely unexplored. Current information on TGF-(β) receptor endocytosis relies exclusively on studies with chimeric constructs containing the extracellular domain of the GM-CSF receptors, following the internalization of the GM-CSF ligand; the conformation and interactions of the chimeric receptors (and therefore their endocytosis) may differ considerably from those of the native TGF-(β) receptors. Furthermore, there are no data on the potential endocytosis motif(s) of the TGF-(β) receptors or other receptor Ser/Thr kinases. Here, we report the use of type II TGF-(β) receptors, myc-tagged at their extracellular terminus, to investigate their endocytosis. Employing fluorescent antibody fragments to label exclusively the cell surface myc-tagged receptors exposed to the external milieu, made it possible to follow the internalization of the receptors, without the complications that render labeling with TGF-(β) (which binds to many cellular proteins) unsuitable for such studies. The results demonstrate that the full-length type II TGF-(β) receptor undergoes constitutive endocytosis via clathrin-coated pits. Using a series of truncation and deletion mutants of this receptor, we identified a short peptide sequence (I(218)I(219)L(220)), which conforms to the consensus of internalization motifs from the di-leucine family, as the major endocytosis signal of the receptor. The functional importance of this sequence in the full-length receptor was validated by the near complete loss of internalization upon mutation of these three amino acids to alanine.

Signal-binding specificity of the mu4 subunit of the adaptor protein complex AP-4

The medium (mu) chains of the adaptor protein (AP) complexes AP-1, AP-2, and AP-3 recognize distinct subsets of tyrosine-based (YXXphi) sorting signals found within the cytoplasmic domains of integral membrane proteins. Here, we describe the signal-binding specificity and affinity of the medium subunit mu4 of the recently described adaptor protein complex AP-4. To elucidate the determinants of specificity, we screened a two-hybrid combinatorial peptide library using mu4 as a selector protein. Statistical analyses of the results revealed that mu4 prefers aspartic acid at position Y+1, proline or arginine at Y+2, and phenylalanine at Y-1 and Y+3 (phi). In addition, we examined the interaction of mu4 with naturally occurring YXXphi signals by both two-hybrid and in vitro binding analyses. These experiments showed that mu4 recognized the tyrosine signal from the human lysosomal protein LAMP-2, HTGYEQF. Using surface plasmon resonance measurements, we determined the apparent dissociation constant for the mu4-YXXphi interaction to be in the micromolar range. To gain insight into a possible role of AP-4 in intracellular trafficking, we constructed a Tac chimera bearing a mu4-specific YXXphi signal. This chimera was targeted to the endosomal-lysosomal system without being internalized from the plasma membrane.

Multiple endocytic signals in the C-terminal tail of the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-dependent protein kinase (PKA)-activated chloride channel that is localized to the plasma membrane and endosomal compartment. Endosomal targeting of CFTR is attributed to the Tyr(1424)-based internalization signal, identified in the C-terminal tail of the channel. Mutation of the Tyr(1424) residue could partly inhibit the endocytosis of CFTR and its association with the adapter protein AP-2. To reveal additional endosomal targeting signals, site-directed mutagenesis of both a chimaera, composed of a truncated form of interleukin 2 receptor alpha chain (TacT) and the C-terminal tail of CFTR (Ct), and the full-length CFTR was performed. Morphological and functional assays revealed the presence of multiple internalization motifs at the C-terminus, consisting of a phenylalanine-based motif (Phe(1413)) and a bipartite endocytic signal, comprising a tyrosine (Tyr(1424)) and a di-Leu-based (Leu(1430)-Leu) motif. Whereas the replacement of any one of the three internalization motifs with alanine prevented the endocytosis of the TacT-Ct chimaera, mutagenesis of Phe(1413)-Leu impaired the biosynthetic processing of CFTR, indicating that Phe(1413) is indispensable for the native structure of CFTR. In contrast, replacement of Leu(1430)-Leu- and Tyr(1424)-based signals with alanine increased the cell-surface density of both the chimaeras and CFTR in an additive manner. These results suggest that the internalization of CFTR is regulated by multiple endocytic sorting signals.

The highly conserved C-terminal dileucine motif in the cytosolic domain of the human immunodeficiency virus type 1 envelope glycoprotein is critical for its association with the AP-1 clathrin adaptor [correction of adapter]

Short amino acid sequences in the cytosolic domains of transmembrane proteins are recognized by specialized adaptor [corrected] proteins which are part of coated vesicles utilized to transport membrane proteins between the trans-Golgi network (TGN) and the plasma membrane (forward and backward). Previously, we and others reported that the membrane-proximal tyrosine residues Y712 (human immunodeficiency virus [HIV]) and Y721 (simian immunodeficiency virus [SIV]) in the envelope glycoprotein (Env) of the primate lentiviruses are crucial for the association of Env with clathrin-associated adaptor [corrected] complex AP-2. The same tyrosine-based endocytosis motifs in the cytosolic domains (EnvCD) of transmembrane gp41 of HIV type 1 (HIV-1) and SIV, respectively, were also shown to modulate the interaction with TGN- and endosome-based clathrin-associated complex AP-1. Our findings suggested that EnvCD binding to AP-1, unlike the association of EnvCD with AP-2, is dependent largely on residues other than Y712 and Y721. Here, we tested if motifs downstream of Y712 affect HIV-1 EnvCD-AP-1 binding and Env trafficking. Mutational analysis revealed that the C-terminal leucine-based motif in Env was crucial for the recruitment of AP-1 in vitro and in Env-expressing cells. In addition to affecting Env-AP-1 association, mutations at the C terminus of Env also altered the subcellular localization of Env, suggesting that proper post-Golgi routing of Env depends on its recruitment of AP-1. Finally, the C-terminal dileucine was shown to assist the membrane-proximal Y712 motif in restricting the cell surface expression of Env.

Clathrin

Clathrin was discovered nearly 25 years ago. Since then, a large number of other proteins that participate in the process by which clathrin-coated vesicles retrieve synaptic membranes or take up endocytic receptors have been identified. The functional relationships among these disparate components remain, in many cases, obscure. High-resolution structures of parts of clathrin, determined by X-ray crystallography, and lower-resolution images of assembled coats, determined by electron cryomicroscopy, now provide the information necessary to integrate various lines of evidence and to design experiments that test specific mechanistic notions. This review summarizes and illustrates the recent structural results and outlines what is known about coated-vesicle assembly in the context of this information.

Binding of AP2 to sorting signals is modulated by AP2 phosphorylation

The two clathrin-associated adaptor complexes AP1 and AP2 are known to participate in the formation of clathrin-coated vesicles at the trans-Golgi network and at the plasma membrane. During this process adaptors are involved in the sequestration of vesicle cargo by binding to the sorting signals within the cytoplasmic domains of the cargo proteins and in the recruitment of the clathrin coat. After budding of the clathrin-coated vesicles, the clathrin and adaptors dissociate from the vesicles. Here we show that in vitro binding of AP2 to sorting signals, which is one of the initial steps in receptor-mediated endocytosis, is modulated by adaptor phosphorylation. AP2 was phosphorylated by incubating purified AP2 in the presence of ATP and dephosphorylated by incubation with alkaline phosphatase. Affinity for tyrosine-, leucine-based and noncanonical sorting motifs was 15-33 times higher for phosphorylated than for dephosphorylated AP2. Also the binding of AP2 to membranes was regulated by adaptor phosphorylation/dephosphorylation and was about 8-fold higher for phosphorylated than for dephosphorylated AP2. Moreover, AP2 isolated from cytosol is higher phosphorylated than membrane-extracted and exhibits a 5-fold higher binding affinity than AP2 extracted from membranes. Taken together these data point to a cycle of phosphorylation/dephosphorylation as a mechanism for regulating the reversible association of AP2 with membranes and sorting signals during the process of receptor-mediated endocytosis.

Multiple C-terminal motifs of the 46-kDa mannose 6-phosphate receptor tail contribute to efficient binding of medium chains of AP-2 and AP-3

The interaction of adaptor protein (AP) complexes with signal structures in the cytoplasmic domains of membrane proteins is required for intracellular sorting. Tyrosine- or dileucine-based motifs have been reported to bind to medium chain subunits (mu) of AP-1, AP-2, or AP-3. In the present study, we have examined the interaction of the entire 67-amino acid cytoplasmic domain of the 46-kDa mannose 6-phosphate receptor (MPR46-CT) containing tyrosine- as well as dileucine-based motifs with mu2 and mu3A chains using the yeast two-hybrid system. Both mu2 and mu3A bind specifically to the MPR46-CT. In contrast, mu3A fails to bind to the cytoplasmic domain of the 300-kDa mannose 6-phosphate receptor. Mutational analysis of the MPR46-CT revealed that the tyrosine-based motif and distal sequences rich in acidic amino acid residues are sufficient for effective binding to mu2. However, the dileucine motif was found to be one part of a consecutive complex C-terminal structure comprising tyrosine and dileucine motifs as well as clusters of acidic residues necessary for efficient binding of mu3A. Alanine substitution of 2 or 4 acidic amino acid residues of this cluster reduces the binding to mu3A much more than to mu2. The data suggest that the MPR46 is capable of interacting with different AP complexes using multiple partially overlapping sorting signals, which might depend on posttranslational modifications or subcellular localization of the receptor.

Negative factor from SIV binds to the catalytic subunit of the V-ATPase to internalize CD4 and to increase viral infectivity

The accessory protein negative factor (Nef) from human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) is required for optimal viral infectivity and the progression to acquired immunodeficiency syndrome (AIDS). Nef interacts with the endocytic machinery, resulting in the down-regulation of cluster of differentiation antigen 4 (CD4) and major histocompatibility complex class I (MHCI) molecules on the surface of infected cells. Mutations in the C-terminal flexible loop of Nef result in a lower rate of internalization by this viral protein. However, no loop-dependent binding of Nef to adaptor protein-2 (AP-2), which is the adaptor protein complex that is required for the internalization of proteins from the plasma membrane, could be demonstrated. In this study we investigated the relevance of different motifs in Nef from SIV(mac239) for its internalization, CD4 down-regulation, binding to components of the trafficking machinery, and viral infectivity. Our data suggest that the binding of Nef to the catalytic subunit H of the vacuolar membrane ATPase (V-ATPase) facilitates its internalization. This binding depends on the integrity of the whole flexible loop. Subsequent studies on Nef mutant viruses revealed that the flexible loop is essential for optimal viral infectivity. Therefore, our data demonstrate how Nef contacts the endocytic machinery in the absence of its direct binding to AP-2 and suggest an important role for subunit H of the V-ATPase in viral infectivity.

A di-leucine sequence and a cluster of acidic amino acids are required for dynamic retention in the endosomal recycling compartment of fibroblasts

Insulin-regulated aminopeptidase (IRAP), a transmembrane aminopeptidase, is dynamically retained within the endosomal compartment of fibroblasts. The characteristics of this dynamic retention are rapid internalization from the plasma membrane and slow recycling back to the cell surface. These specialized trafficking kinetics result in <15% of IRAP on the cell surface at steady state, compared with 35% of the transferrin receptor, another transmembrane protein that traffics between endosomes and the cell surface. Here we demonstrate that a 29-amino acid region of IRAP's cytoplasmic domain (residues 56--84) is necessary and sufficient to promote trafficking characteristic of IRAP. A di-leucine sequence and a cluster of acidic amino acids within this region are essential elements of the motif that slows IRAP recycling. Rapid internalization requires any two of three distinct motifs: M(15,16), DED(64--66), and LL(76,77). The DED and LL sequences are part of the motif that regulates recycling, demonstrating that this motif is bifunctional. In this study we used horseradish peroxidase quenching of fluorescence to demonstrate that IRAP is dynamically retained within the transferrin receptor-containing general endosomal recycling compartment. Therefore, our data demonstrate that motifs similar to those that determine targeting among distinct membrane compartments can also regulate the rate of transport of proteins from endosomal compartments. We propose a model for dynamic retention in which IRAP is transported from the general endosomal recycling compartment in specialized, slowly budding recycling vesicles that are distinct from those that mediate rapid recycling back to the surface (e.g., transferrin receptor-containing transport vesicles). It is likely that the dynamic retention of IRAP is an example of a general mechanism for regulating the distribution of proteins between the surface and interior of cells.

Lysosomal membrane proteins

The lysosomal system is the main intracellular mechanism for the turnover of endogenous and exogenous macromolecules. This catabolism is brought about in the lumen of lysosomes by a cocktail of predominantly hydrolytic enzymes with characteristic acidic pH-optima. The lysosomal membrane, which has a typical single phospholipid bilayer, controls the passage of material into and out of lysosomes, by its permeability and ability to fuse with digestive vacuoles or engulf cytosolic material. About 20 systems for transporting small molecules across the lysosomal membrane have been characterized but only two proteins, cystinosin and sialin, involved in the transport of cystine and sialic acid, respectively, have been cloned. A distinct, vacuolar proton pump (V-type H+ ATPase), which maintains the low luminal pH, has been characterized. Ubiquitous, highly glycosylated, integral membrane proteins of largely unknown function, called lysosome-associated membrane proteins (LAMPS) or lysosomal integral membrane proteins (LIMPS), account for about 50% of the protein in the lysosomal membrane. They have a short cytosolic domain of 10-20 amino acids containing single tyrosine or di-leucine motifs, which interact with adaptor complexes (APS) for sorting at the trans-Golgi network and targeting to lysosomes. A deficiency of LAMP-2 is the primary defect in Danon disease. Other proteins associate with the membrane transiently or cell-specifically. The structure, function and intracellular transport of these different classes of lysosomal membrane proteins will be reviewed.

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

Leucine-based motifs mediate the sorting of membrane proteins at such cellular sites as the trans-Golgi network, endosomes, and plasma membrane. A Leu paired with a second Leu, Ile, or Met, while itself lacking the ability to mediate transport, is the key structural feature in these motifs. Here we have studied the structural differences between the leucine-based motifs contained in the COOH tails of LIMPII and GLUT4, two membrane proteins that are transported through the secretory pathway and are targeted to lysosomes () and to a perinuclear compartment adjacent to the Golgi complex (), respectively. LIMPII and GLUT4 display negatively (Asp(470)/Glu(471)) and positively (Arg(484)/Arg(485)) charged residues, respectively, at positions -4 and -5 upstream from the critical Leu residue. The change in the charge sign of residues -4 and -5 results in missorting of LIMPII and GLUT4. We note that the acidic Glu residue at position -4 is critical for efficient intracellular sorting of LIMPII to lysosomes, but is dispensable for its surface internalization by endocytosis. Efficient intracellular sorting and endocytosis of GLUT4 require an Arg pair between positions -4 and -7. These results are consistent with the existence of distinct leucine-based motifs and provide evidence of their different readings at different cellular sites.

Two independent regions of HIV-1 Nef are required for connection with the endocytic pathway through binding to the mu 1 chain of AP1 complex

The Nef protein from the human immunodeficiency virus (HIV) induces down-regulation of the CD4 and major histocompatibility complex class I molecules from the cell surface by interfering with the endocytic machinery. This work focuses on the interaction of HIV-1 Nef with the mu 1 chain of adaptor protein type 1 (AP1) complex and its contribution to the Nef-induced alterations of membrane trafficking. Two independent regions surrounding a disordered loop located in the C-terminal part of Nef are involved in mu 1 binding. Each region can separately interact with mu 1, and simultaneous point mutations within both regions are needed to abolish binding. We used CD8 chimeras in which the cytoplasmic tail was replaced by Nef mutants to show that these mu 1-binding sites contain determinants required to induce CD4 down-regulation and to target the chimera to the endocytic pathway by promoting AP1 complex recruitment. Ultrastructural analysis revealed that the CD8-Nef chimera provokes morphological alterations of the endosomal compartments and co-localizes with AP1 complexes. These data indicate that the recruitment by Nef of AP1 via binding to mu 1 participates in the connection of Nef with the endocytic pathway.

PTH-Induced downregulation of the type IIa Na/P(i)-cotransporter is independent of known endocytic motifs

Parathyroid hormone (PTH)-induced inhibition of renal proximal tubular Na/P(i) cotransport involves two consecutive steps: endocytosis followed by lysosomal degradation of the type IIa Na/P(i) cotransporter. Tyrosine-, dileucine-, and diacidic-based motifs are suggested to be involved in endocytosis and/or lysosomal targeting of different plasma membrane proteins. The rat type IIa cotransporter (NaPi2) contains two cytoplasmic tyrosine residues (Y) within sequences highly homologous to tyrosine-based motifs (GY(402)FAM and Y(509)RWF), three cytoplasmic dileucine (LL(101), LL(374), and LI(591)) and two cytoplasmic diacidic motifs (EE(81) and EE(616)). We studied the role of these motifs on the PTH-induced retrieval and lysosomal degradation of the NaPi2 cotransporter. To follow its trafficking in vivo, the NaPi2 protein was fused to the carboxyl-terminal end of the enhanced green fluorescence protein. This fusion did not impair the apical targeting or the PTH-induced endocytosis of the wild-type cotransporter when transfected in opossum kidney cells. Single and multiple Y and LL mutants retained the apical targeting and the PTH-induced degradation. Mutations of the diacidic motifs were also without effect. These data suggest that the above three motifs are not required for the PTH-induced internalization and/or degradation of the cotransporter.

Polar transmembrane domains target proteins to the interior of the yeast vacuole

Membrane proteins transported to the yeast vacuole can have two fates. Some reach the outer vacuolar membrane, whereas others enter internal vesicles, which form in late endosomes, and are ultimately degraded. The vacuolar SNAREs Nyv1p and Vam3p avoid this fate by using the AP-3-dependent pathway, which bypasses late endosomes, but the endosomal SNARE Pep12p must avoid it more directly. Deletion analysis revealed no cytoplasmic sequences necessary to prevent the internalization of Pep12p in endosomes. However, introduction of acidic residues into the cytoplasmic half of the transmembrane domain created a dominant internalization signal. In other contexts, this same feature diverted proteins from the Golgi to endosomes and slowed their exit from the endoplasmic reticulum. The more modestly polar transmembrane domains of Sec12p and Ufe1p, which normally serve to hold these proteins in the endoplasmic reticulum, also cause Pep12p to be internalized, as does that of the vacuolar protein Cps1p. It seems that quality control mechanisms recognize polar transmembrane domains at multiple points in the secretory and endocytic pathways and in endosomes sort proteins for subsequent destruction in the vacuole. These mechanisms may minimize the damaging effects of abnormally exposed polar residues while being exploited for the localization of some normal proteins.

The cytoplasmic domain of the interleukin-6 receptor gp80 mediates its basolateral sorting in polarized madin-darby canine kidney cells

The IL-6 receptor complex is expressed in different polarized epithelial cells such as liver hepatocytes and intestinal cells. It consists of two subunits: gp80, which binds the ligand, and gp130, which is responsible for signal transduction. In stably transfected Madin-Darby canine kidney (MDCK) cells we have studied the localization of the human IL-6 receptor subunits and found that gp80 and gp130 are predominantly expressed at the basolateral membrane. Analysis of MDCK cells expressing truncated forms of gp80 or gp130 showed that loss of the cytoplasmic domains results in apical delivery. Expression of deletion mutants of gp80 in MDCK cells led to the identification of two discontinous motifs responsible for basolateral sorting: a membrane-proximal tyrosine-based motif (YSLG) and a more membrane-distal dileucine-type motif (LI). Activation of signal transducer and activator of transcription-3 (STAT-3) only occurred via basolaterally located gp80, suggesting that endogenous gp130 is also constrained to the basolateral plasma membrane. Our identification of a basolateral sorting signal within the cytoplasmic region of gp80 for the first time attributes a function to this domain.

Identification of lysosomal and Golgi localization signals in GAP and ARF domains of ARF domain protein 1

ADP ribosylation factors (ARFs) are approximately 20-kDa guanine nucleotide-binding proteins that activate cholera toxin and phospholipase D and are critical components of vesicular trafficking pathways. ARF domain protein 1 (ARD1), a member of the ARF superfamily, contains a 46-kDa amino-terminal extension, which acts as a GTPase-activating protein (GAP) with activity towards its ARF domain. When overexpressed, ARD1 was associated with lysosomes and the Golgi apparatus. In agreement with this finding, lysosomal and Golgi membranes isolated from human liver by immunoaffinity contained native ARD1. ARD1, expressed as a green fluorescent fusion protein, was initially associated with the Golgi network and subsequently appeared on lysosomes, suggesting that ARD1 might undergo vectorial transport between the two organelles. Here we show by microscopic colocalization that GAP and ARF domains determine lysosomal and Golgi localization, respectively, consistent with the presence of more than one signal motif. Using truncated ARD1 molecules, expressed as green fluorescent fusion proteins, it was found that the signal for lysosomal localization was present in residues 301 to 402 of the GAP domain. Site-specific mutagenesis demonstrated that the sequence (369)KXXXQ(373) in the GAP domain was responsible for lysosomal localization. Association of ARD1 with the Golgi apparatus required tyrosine-based motifs. A green fluorescent fusion protein containing the QKQQQQF motif was partially associated with lysosomes, suggesting that this motif contains the information sufficient for lysosomal targeting. These results suggest that ARD1 is a multidomain protein with ARF and GAP regions, which contain Golgi and lysosomal localization signals, respectively, that could function in vesicular trafficking.

E3-13.7 integral membrane proteins encoded by human adenoviruses alter epidermal growth factor receptor trafficking by interacting directly with receptors in early endosomes

Animal cell viruses provide valuable model systems for studying many normal cellular processes, including membrane protein sorting. The focus of this study is an integral membrane protein encoded by the E3 transcription region of human adenoviruses called E3-13.7, which diverts recycling EGF receptors to lysosomes without increasing the rate of receptor internalization or intrinsic receptor tyrosine kinase activity. Although E3-13.7 can be found on the plasma membrane when it is overexpressed, its effect on EGF receptor trafficking suggests that the plasma membrane is not its primary site of action. Using cell fractionation and immunocytochemical experimental approaches, we now report that the viral protein is located predominantly in early endosomes and limiting membranes of endosome-to-lysosome transport intermediates called multivesicular endosomes. We also demonstrate that E3-13.7 physically associates with EGF receptors undergoing E3-13.7-mediated down-regulation in early endosomes. Receptor-viral protein complexes then dissociate, and EGF receptors proceed to lysosomes, where they are degraded, while E3-13.7 is retained in endosomes. We conclude that E3-13.7 is a resident early endocytic protein independent of EGF receptor expression, because it has identical intracellular localization in mouse cells lacking endogenous receptors and cells expressing a human cytomegalovirus-driven receptor cDNA. Finally, we demonstrate that EGF receptor residues 675-697 are required for E3-13.7-mediated down-regulation. Interestingly, this sequence includes a known EGF receptor leucine-based lysosomal sorting signal used during ligand-induced trafficking, which is also conserved in the viral protein. E3-13.7, therefore, provides a novel model system for determining the molecular basis of selective membrane protein transport in the endocytic pathway. Our studies also suggest new paradigms for understanding EGF receptor sorting in endosomes and adenovirus pathogenesis.

EGF receptor residues leu(679), leu(680) mediate selective sorting of ligand-receptor complexes in early endosomal compartments

Dileucine-based motifs have been shown to regulate endosomal sorting of a number of membrane proteins. Previously, we have shown that the dileucine motif Leu(679), Leu(680) in the juxtamembrane domain of the human epidermal growth factor receptor is involved in the endosome-to-lysosome transport of ligand-receptor complexes. Substitution of alanine residues for Leu(679), Leu(680) led to a reduction in ligand-induced receptor degradation without affecting internalization. In the current study, we have further characterized ligand-dependent intracellular sorting of EGF receptors containing a L679A, L680A. Immunocytochemical studies reveal that although mutant receptors redistribute from the cell surface to transferrin receptor-positive endocytic vesicles similar to wild-type following ligand stimulation, their accumulation in Lamp-1-positive late endosomes/lysosomes is retarded compared to wild-type. Kinetic analysis of (125)I-EGF trafficking shows that reduced accumulation of internalized mutant receptors in Lamp-1-positive vesicles is due to rapid recycling of ligand-receptor complexes from early endocytic compartments. In addition, the fraction of intracellular (125)I-EGF that is transported to late endocytic compartments in cells with mutant receptors is not as efficiently degraded as it is in cells with wild-type receptors. Furthermore, wild-type receptors in endocytic vesicles isolated by Percoll gradient fractionation are more resistant to in vitro digestion with proteinase K than mutant receptors. We propose that mutant receptors interact inefficiently with lysosomal sorting machinery, leading to their increased recycling. Our results are consistent with a model in which the Leu(679), Leu(680) signal facilitates sequestration of ligand-receptor complexes into internal vesicles of multivesicular endosome-to-lysosome transport intermediates.

Distinct trafficking pathways mediate Nef-induced and clathrin-dependent major histocompatibility complex class I down-regulation

The human immunodeficiency virus type 1 Nef protein alters the post-Golgi stages of major histocompatibility complex class I (MHC-I) biogenesis. Presumed mechanisms involve the disclosure of a cryptic tyrosine-based sorting signal (YSQA) located in the cytoplasmic tail of HLA-A and -B heavy chains. We changed this signal for a prototypic sorting motif (YSQI or YSQL). Modified HLA-A2 molecules, termed A2-endo, displayed constitutively low surface levels and accumulated in a region close to or within the Golgi apparatus, a behavior reminiscent of wild-type HLA-A2 in Nef-expressing cells. However, several lines of evidence indicate that the action of prototypic signals on MHC-I trafficking differs from that of Nef. Internalization of surface A2-endo was more rapid and was associated with efficient recycling to the surface. A transdominant-negative mutant of dynamin-1 inhibited A2-endo constitutive internalization and Nef-induced CD4 down-regulation, whereas it did not affect the activity of Nef on MHC-I. Moreover, trafficking of A2-endo was still affected by the viral protein, indicating additive effects of prototypic signals and Nef. Therefore, distinct trafficking pathways regulate clathrin-dependent and Nef-induced MHC-I modulation.

beta2-adaptin is constitutively de-phosphorylated by serine/threonine protein phosphatase PP2A and phosphorylated by a staurosporine-sensitive kinase

Clathrin-mediated endocytosis includes cycles of assembly and disassembly of the clathrin-coated vesicle constituents. How these cycles are regulated is still not fully known but previous studies have indicated that phosphorylation of coat subunits may play a role. Here we describe that beta2-adaptin undergoes cycles of phosphorylation/de-phosphorylation in intact cells. Thus, beta2-adaptin was constitutively de-phosphorylated by serine/threonine protein phosphatase 2A and phosphorylated by a staurosporine-sensitive kinase in vivo. Confocal laser scanning microscopy demonstrated that phosphorylated AP2 complexes were found more evenly distributed at the plasma membrane compared to non-phosphorylated AP2 complexes which were found in aggregates. Finally, we found that phosphorylation of beta2-adaptin correlated with inhibition of clathrin-mediated endocytosis. Our results support the hypothesis that phosphorylation/de-phosphorylation of coat proteins plays a regulatory role in the assembly/disassembly cycle of clathrin-coated vesicles.

The structure and function of the beta 2-adaptin appendage domain

The heterotetrameric AP2 adaptor (alpha, beta 2, mu 2 and sigma 2 subunits) plays a central role in clathrin-mediated endocytosis. We present the protein recruitment function and 1.7 A resolution structure of its beta 2-appendage domain to complement those previously determined for the mu 2 subunit and alpha appendage. Using structure-directed mutagenesis, we demonstrate the ability of the beta 2 appendage alone to bind directly to clathrin and the accessory proteins AP180, epsin and eps15 at the same site. Clathrin polymerization is promoted by binding of clathrin simultaneously to the beta 2-appendage site and to a second site on the adjacent beta 2 hinge. This results in the displacement of the other ligands from the beta 2 appendage. Thus clathrin binding to an AP2-accessory protein complex would cause the controlled release of accessory proteins at sites of vesicle formation.

The cytosolic C-terminus of the glucose transporter GLUT4 contains an acidic cluster endosomal targeting motif distal to the dileucine signal

The insulin-responsive glucose transporter GLUT4 is targeted to a post-endocytic compartment in adipocytes, from where it moves to the cell surface in response to insulin. Previous studies have identified two cytosolic targeting motifs that regulate the intracellular sequestration of this protein: FQQI(5-8) in the N-terminus and LL(489,490) (one-letter amino acid notation) in the C-terminus. In the present study we show that a GLUT4 chimaera in which the C-terminal 12 amino acids in GLUT4 have been replaced with the same region from human GLUT3 is constitutively targeted to the plasma membrane when expressed in 3T3-L1 adipocytes. To further dissect this domain it was divided into three regions, each of which was mutated en bloc to alanine residues. Analysis of these constructs revealed that the targeting information is contained within the residues TELEYLGP(498-505). Using the transferrin-horseradish peroxidase endosomal ablation technique in 3T3-L1 adipocytes, we show that mutants in which this C-terminal domain has been disrupted are more sensitive to chemical ablation than wild-type GLUT4. These data indicate that GLUT4 contains a targeting signal in its C-terminus, distal to the dileucine motif, that regulates its sorting into a post-endosomal compartment. Similar membrane-distal, acidic-cluster-based motifs are found in the cytosolic tails of the insulin-responsive aminopeptidase IRAP (insulin-regulated aminopeptidase) and the proprotein convertase PC6B, indicating that this type of motif may play an important role in the endosomal sequestration of a number of different proteins.

Biogenesis of insulin-responsive GLUT4 vesicles is independent of brefeldin A-sensitive trafficking

Insulin stimulates translocation of GLUT4 from an intracellular compartment to the plasma membrane in adipocytes. As a significant amount of GLUT4 is localised to the TGN, independently of the biosynthetic pathway, one possibility is that trafficking via the TGN is important in either intracellular sequestration or insulin-dependent movement to the cell surface. In this study we have used immuno-electron microscopy to show that GLUT4 is localised to AP-1 vesicles in the TGN region in 3T3-L1 adipocytes. To dissect the role of this trafficking pathway we used brefeldin A (BFA) to disrupt AP-1 association with membranes. Despite a reorganisation of GLUT4 compartments following BFA treatment, the intracellular sequestration of GLUT4, and its insulin-dependent movement to the cell surface, was unaffected. BFA increased the half time of reversal of insulin-stimulated glucose transport from 17 to 30 min but did not prevent complete reversal. Furthermore, following reversal restimulation of glucose transport activity by insulin was not compromised. We conclude that under basal conditions GLUT4 cycles between the TGN and endosomes via the AP-1 pathway. However, neither this pathway, nor any other BFA-sensitive pathway, appears to play a major role in insulin-dependent recruitment of GLUT4 to the cell surface.

Structural insights into clathrin-mediated endocytosis

The process of clathrin-mediated endocytosis from the plasma membrane has been the subject of many biological and biochemical investigations. Recent atomic resolution structures determined by X-ray crystallography now enable the molecular basis for the interactions of some components of the endocytic machinery to be understood in detail.

The interaction of beta-arrestin with the AP-2 adaptor is required for the clustering of beta 2-adrenergic receptor into clathrin-coated pits

Beta-arrestins are cytosolic proteins that regulate the signaling and the internalization of G protein-coupled receptors (GPCRs). Although termination of receptor coupling requires beta-arrestin binding to agonist-activated receptors, GPCR endocytosis involves the coordinate interactions between receptor-beta-arrestin complexes and other endocytic proteins such as adaptor protein 2 (AP-2) and clathrin. Clathrin interacts with a conserved motif in the beta-arrestin C-terminal tail; however, the specific molecular determinants in beta-arrestin that bind AP-2 have not been identified. Moreover, the respective contributions of the interactions of beta-arrestin with AP-2 and clathrin toward the targeting of GPCRs to clathrin-coated vesicles have not been established. Here, we identify specific arginine residues (Arg(394) and Arg(396)) in the beta-arrestin 2 C terminus that mediate beta-arrestin binding to AP-2 and show, in vitro, that these domains in beta-arrestin 1 and 2 interact equally well with AP-2 independently of clathrin binding. We demonstrate in HEK 293 cells by fluorescence microscopy that beta(2)-adrenergic receptor-beta-arrestin complexes lacking the beta-arrestin-clathrin binding motif are still targeted to clathrin-coated pits. In marked contrast, receptor-beta-arrestin complexes lacking the beta-arrestin/AP-2 interactions are not effectively compartmentalized in punctated areas of the plasma membrane. These results reveal that the binding of a receptor-beta-arrestin complex to AP-2, not to clathrin, is necessary for the initial targeting of beta(2)-adrenergic receptor to clathrin-coated pits.

Endocytosis in viral replication

Although initially recognised as essential for the entry of certain viruses, endocytosis is now known to also play important roles in the replication of, and adaptation to, the host cell of a number of viruses. Here we consider several aspects of this association and recent results that have emerged to support this view.

Bi-directional trafficking between the trans-Golgi network and the endosomal/lysosomal system

Protein transport in the secretory and endocytic pathways of eukaryotic cells is mediated by vesicular transport intermediates. Their formation is a tightly controlled multistep process in which coat components are recruited onto specific membranes, and cargo, as well as targeting molecules, become segregated into nascent vesicles. At the trans-Golgi network, two transport systems deliver cargo molecules to the endosomal system. They can be distinguished with regard to coat components that select cargo molecules. AP-1 assembly proteins mediate transport of MPRs and furin, whereas AP-3 adaptors mediate transport of lysosomal membrane glycoproteins to the endosomal/lysosomal system. The molecular basis for protein-specific sorting lies within sorting signals that are present in the cytoplasmic tails of cargo proteins and allow specific interactions with individual coat components. In order to maintain cellular homeostasis, some proteins are retrieved from endosomal compartments and transported back to the trans-Golgi network. Distinct points for protein retrieval exist within the endosomal system, retrieval occurring from either early or late endosomes. Whereas significant progress has been made in recent years in identifying anterograde and retrograde transport pathways, the molecular mechanisms underlying protein sorting and retrieval are only poorly defined. Recently, however, novel vesicle coats (e.g. AP-4) and proteins that might be involved in sorting (e.g. PACS-1 and TIP47) have been described, and the interactions between assembly proteins and sorting signals are becoming increasingly well defined.

Targeting of lysosomal proteins

Proper cell homeostasis requires the efficient transport of a large variety of soluble acid hydrolases and transmembrane proteins from the trans-Golgi network (TGN) to lysosomes. While most of these molecules reach this degradative compartment, some transmembrane proteins, in particular, the acid hydrolase receptors are retrieved to the TGN. This bidirectional transport process involves the formation of several vesicular transport intermediates in which cargo molecules are selectively packaged. This review summarizes our current understanding of the molecular mechanisms leading to the proper targeting of lysosomal proteins.

Interactions of HIV-1 nef with the mu subunits of adaptor protein complexes 1, 2, and 3: role of the dileucine-based sorting motif

HIV-1 Nef interacts with cellular adaptor protein (AP) complexes and their medium (mu) subunits. However, the role of the dileucine-based sorting motif within Nef in these interactions has been incompletely characterized. Here, yeast two-hybrid assays indicated that HIV-1 Nef interacted not only with the mu subunits of AP-1 and AP-2, but also with that of AP-3. The interactions with mu1 and mu3 were markedly stronger than the interaction with mu2. Leucine residues of the sorting motif were required for the interactions with mu3 and mu2 and contributed to the interaction with mu1. Confocal immunofluorescence microscopy indicated that Nef, AP-1, and AP-3 (but not AP-2) were concentrated in a juxtanuclear region near the cell center, potentially facilitating interaction between Nef and the mu1 and mu3 subunits. However, leucine residues of the sorting motif were not required for this subcellular localization of Nef. These data suggest that the dileucine motif, required for optimal viral replication, functions through interactions with a variety of AP complexes, including AP-3, potentially by recruiting adaptor complexes to subcellular locations specified by additional determinants in the Nef protein.

Intracellular targeting and retention of the glucose transporter GLUT4 by the perinuclear storage compartment involves distinct carboxyl-tail motifs

The mechanisms by which the insulin-sensitive glucose transporter, GLUT4, is targeted and retained in a storage compartment near to the Golgi complex are poorly understood. Here we report that removal of the carboxyl-terminal acidic Pro(505)AspGluAsnAsp(509) sequence prevents the storage of GLUT4 in the VAMP-2 positive compartment adjacent to the Golgi complex (GSC), and results in its targeting to GLUT4-positive vesicles and Rab7-positive late endosomes. Storage of the truncated GLUT4 in the GSC is restored by substitution of Phe for the Tyr(502) residue adjacent to Pro(505) or by treatment of cells with the tyrosine kinase inhibitor genistein. Ablation of the Leu(489)Leu(490)-based motif prevents the targeting of GLUT4delta5 to GLUT4-positive-vesicles and late endosomes as well as the retention of GLUT4delta5Phe(502)by the GSC. These results are consisting with a model of GLUT4 transport in which the targeting of the protein from the TGN to the GSC is mediated by the Leu(489)Leu(490)-based motif and its release from the GSC involves Tyr(502)and the adjacent carboxyl-terminal Pro(505)AspGluAsnAsp(509) sequence.

Selective inhibition of adaptor complex-mediated vesiculation

A short while ago, we could only inhibit post-Golgi membrane traffic with crude, unselective tools, such as low temperature or high extracellular sucrose. Molecular dissection of vesiculation steps has revealed unexpected complexity in the coating machinery that has initiated a search for more specific inhibitors. We have learned that membrane vesiculation is driven by a tightly regulated multicomponent, membrane-associated protein machine held together by carefully specified interaction domains. An experimental advantage of such complex interacting machinery is that it is very susceptible to disruption by dominant negative inhibitors or by overexpression. As a result, we now have much more specific inhibitors of post-Golgi membrane traffic. Some, such as dynamin K44A, may be general inhibitors, whereas others can distinguish classes of endocytotic events (10), binding events that require clathrin from those that do not (42), or specific steps of endocytosis (62). Ligand-mediated uptake of EGF and numerous, but not all, GPCRs can be inhibited by overexpression of an ARF GTPase-activating protein that has no effect on transferrin uptake (67). We can look forward to increasingly powerful and selective inhibitors that should help us to navigate successfully the complex routes of post-Golgi membrane traffic.

The class II phosphoinositide 3-kinase PI3K-C2alpha is concentrated in the trans-Golgi network and present in clathrin-coated vesicles

In recent years, a large family of phosphoinositide 3-kinase (PI3K) isozymes has been characterized and cloned. Several of these PI3K enzymes have overlapping tissue distributions and it remains unclear if and how their 3-phosphoinositide products elicit differential, intracellular effects. One possibility is that the PI3K enzymes display a restricted distribution within the cell to produce their 3-phospholipid products in specific, subcellular compartments. In the present study we characterize the subcellular distribution of the novel class II PI3K isozyme PI3K-C2alpha in several mammalian cell types. Differential centrifugation of COS-1 and U937 cells together with Western blot analysis demonstrated that PI3K-C2alpha is constitutively associated with phospholipid membranes. Centrifugation of rat brain homogenates and Western blotting revealed that in contrast to the class IA PI3K enzymes, PI3K-C2alpha could be co-purified with a population of clathrin-coated vesicles (CCVs). Furthermore, a PI3K activity refractory to wortmannin treatment was detected in CCV preparations consistent with the presence of the PI3K-C2alpha isozyme. These biochemical observations were supported by immunofluorescence analysis that revealed PI3K-C2alpha to have a punctate distribution and an enrichment of immunoreactivity within a perinuclear site consistent with its presence in the endoplasmic reticulum or Golgi apparatus. Dual label immunofluorescence demonstrated that in this region, the distribution of PI3K-C2alpha closely paralleled that of gamma-adaptin, a component of the AP-1 adaptor that is present in the trans-Golgi and the trans-Golgi network (TGN) resident protein TGN-46. Neither the phospholipid association nor the subcellular localization of PI3K-C2alpha was dependent upon either its COOH-terminal PX or C2 domains. Mutants lacking these domains demonstrated a similar distribution to the wild type enzyme when expressed as recombinant proteins. Treatment of cells with brefeldin A disrupted the perinuclear staining pattern of both PI3K-C2alpha and the AP-1 complex demonstrating that the localization of both molecules at the TGN is dependent upon ADP-ribosylation factor GTPase activity.

A phosphorylation site regulates sorting of the vesicular acetylcholine transporter to dense core vesicles

Vesicular transport proteins package classical neurotransmitters for regulated exocytotic release, and localize to at least two distinct types of secretory vesicles. In PC12 cells, the vesicular acetylcholine transporter (VAChT) localizes preferentially to synaptic-like microvesicles (SLMVs), whereas the closely related vesicular monoamine transporters (VMATs) localize preferentially to large dense core vesicles (LDCVs). VAChT and the VMATs contain COOH-terminal, cytoplasmic dileucine motifs required for internalization from the plasma membrane. We now show that VAChT undergoes regulated phosphorylation by protein kinase C on a serine (Ser-480) five residues upstream of the dileucine motif. Replacement of Ser-480 by glutamate, to mimic the phosphorylation event, increases the localization of VAChT to LDCVs. Conversely, the VMATs contain two glutamates upstream of their dileucine-like motif, and replacement of these residues by alanine conversely reduces sorting to LDCVs. The results provide some of the first information about sequences involved in sorting to LDCVs. Since the location of the transporters determines which vesicles store classical neurotransmitters, a change in VAChT trafficking due to phosphorylation may also influence the mode of transmitter release.

Clathrin coat construction in endocytosis

Electron cryomicroscopy of the clathrin coat and X-ray crystallography of parts of the clathrin heavy chain combine to give a detailed picture of the clathrin molecule, assembled as a cage. Recently determined domain structures of other components of the endocytic machinery, particularly the mu2 subunit and the alpha-appendage domain of the AP2 adaptor complex, provide important information on the sequence of recognition events involved in the dynamic process of clathrin coat assembly.

The cytoplasmic tail of CD1d contains two overlapping basolateral sorting signals

CD1d is a member of the CD1 polypeptide family that represents a new arm of host defense against invading pathogens. In our previous work (Rodionov, D. G., Nordeng, T. W., Pedersen, K., Balk, S. P., and Bakke, O. (1999) J. Immunol. 162, 1488-1495) we have shown that CD1d contained a classic tyrosine-based internalization signal (YQGV) in its short cytoplasmic tail. CD1d is expressed in polarized epithelial cells, and we found that the cytoplasmic tail of CD1d also contained information for basolateral sorting. Interestingly, a mutation of the critical tyrosine residue of the endosomal sorting signal did not result in the loss of basolateral targeting of the mutant CD1d. To search for a basolateral sorting signal we have constructed a full set of alanine mutants, but no single alanine substitution inactivated the signal. However, deletions or mutations of either the C-terminal valine/leucine pair or the critical tyrosine residue from the internalization signal and either residue from the C-terminal valine/leucine pair inactivated basolateral sorting. Our data thus suggest that the cytoplasmic tail contains two overlapping basolateral signals, one tyrosine- and the other leucine-based, each being sufficient to direct CD1d to the basolateral membrane of polarized Madin-Darby canine kidney cells.

Identification of a family of noncanonical ubiquitin-conjugating enzymes structurally related to yeast UBC6

Ubiquitin-conjugating enzymes (UBCs) selectively target proteins for proteasomal degradation by the covalent attachment of ubiquitin moieties. Yeast UBC6 is unusual in having an active site distinct from all other UBCs and in possessing a transmembrane domain that anchors it to the cytoplasmic surface of the endoplasmic reticulum. During a differential display analysis on chick growth plate chondrocytes we isolated a cDNA encoding a noncanonical ubiquitin-conjugating enzyme (NCUBE1) structurally similar to yeast UBC6. Chick NCUBE1 transcripts were detected in all tissues examined and decreased threefold during chondrocyte terminal differentiation. Database searches identified other related proteins; the human and mouse orthologues of NCUBE1, a second human homologue of yeast UBC6 (NCUBE2), and related proteins from S. pombe, C. elegans, and P. mariana. Together with yeast UBC6 these proteins constitute a distinct family of UBCs sharing a conserved noncanonical active site sequence and a C-terminal transmembrane domain. By analogy with yeast UBC6 they are likely to be localised to the endoplasmic reticulum where they may be involved in targeting retrotranslocated, ER-associated proteins for proteasomal degradation.

Mutations in sixth transmembrane domain of AQP2 inhibit its translocation induced by vasopression

Vasopression-induced phosphorylation of serine 256 of the aquaporin-2 (AQP2) water channel triggers translocation of the protein from cystolic reservoir vesicles to the apical membrane of collecting duct principal cells. Dileucine motifs are located in the sixth transmembrane domain (6TM) of AQP2 and are known as the signal sequence for internalization, sorting from the trans-Golgi network to endosomes/lysosomes, and basolateral sorting. In this study, involvement of 6TM in vasopressin-induced translocation of the protein was investigated. A series of mutations in 6TM of AQP2 was introduced to rat cDNA and expressed in LLC-PK(1) cells. Immunofluorescence microscopy indicated that the mutant AQP2 proteins were retained in the cytoplasm after vasopressin stimulation, which actually promoted the plasma membrane expression of wild-type protein. Immunoelectron microscopy showed that the mutant AQP2 proteins reached the endosomes but did not reach the plasma membrane. These results demonstrate that 6TM has essential domains for vasopressin-induced translocation from endosomes to the plasma membrane.

Asymmetrical targeting of type II Na-P(i) cotransporters in renal and intestinal epithelial cell lines

Targeting of newly synthesized transporters to either the apical or basolateral domains of polarized cells is crucial for the function of epithelia, such as in the renal proximal tubule or in the small intestine. Recently, different sodium-phosphate cotransporters have been identified. Type II cotransporters can be subdivided into two groups: type IIa and type IIb. Type IIa is predominantly expressed in renal proximal tubules, whereas type IIb is located on the intestinal and lung epithelia. To gain some insights into the polarized targeting of the type II cotransporters, we have transiently expressed type IIa and type IIb cotransporters in several epithelial cell lines: two lines derived from renal proximal cells (opossum kidney and LLC-PK(1)), one from renal distal cells (Madin-Darby canine kidney), and one from colonic epithelium (CaCo-2). We studied the expression of the transporters fused to the enhanced green fluorescent protein. Our data indicate that the polarized targeting is dependent on molecular determinants most probably located at the COOH terminus of the cotransporters as well as on the cellular context.

A screen of random sequences for those that alter the trafficking of the influenza virus hemagglutinin in vivo

In order to determine if the sequence patterns known to specify internalization represent the majority of possible internalization signals, we identified random sequences capable of causing a reporter protein to be internalized at least several-fold faster than the rate of non-selective internalization of membrane by clathrin-coated pits. A library of influenza hemagglutinin (HA) proteins, bearing short random sequences in place of the wild-type cytoplasmic domain, was prepared in recombinant SV40 virus. The library was expressed and screened for HAs that could internalize anti-HA antibody from the medium. The cytoplasmic sequences of the selected proteins were determined. From a small sample of sequences we detected several that did not resemble those previously identified. The known internalization signals must represent only a subset of the sequences that can serve as internalization signals.

Adaptors for clathrin-mediated traffic

Clathrin-based systems are responsible for a large portion of vesicular traffic originating from the plasma membrane and the trans-Golgi network that reaches the endosomal compartment. The assembly of cytosolic clathrin forms the scaffold required for the local deformation of the membrane and for the formation of coated pits and vesicles. In this process, clathrin interacts in a coordinated fashion with a large number of protein partners. A subset designated clathrin adaptors links integral membrane proteins to the clathrin coat, a process that results in the recruitment of specific cargo proteins to the budding vesicle. This review focuses on the most recent advances dealing with the molecular basis for sorting by clathrin adaptors.

Recycling of furin from the plasma membrane. Functional importance of the cytoplasmic tail sorting signals and interaction with the AP-2 adaptor medium chain subunit

The predominant intracellular localization of the eukaryotic subtilisin-like endoprotease furin is the trans-Golgi network (TGN), but a small fraction is also found on the cell surface. Furin on the cell surface is internalized and delivered to the TGN. The identification of three endocytosis motifs, a tyrosine (YKGL(765)) motif, a leucine-isoleucine (LI(760)) motif, and a phenylalanine (Phe(790)) signal, in the furin cytoplasmic domain suggested that endocytosis of furin occurs via an AP-2/clathrin-dependent pathway. Since little is known about proteins containing multiple sorting components in their cytoplasmic domain, the combination of diverse internalization signals in the furin tail raised the question of their individual role. Here we present data showing that the furin tail interacts with the medium (micro2) subunit of the AP-2 plasma membrane-specific adaptor complex in vitro and that this interaction primarily depends on recognition of the tyrosine-based sorting signal and to less extent on the leucine-isoleucine motif. We further provide evidence that the three endocytosis signals are of different functional importance for furin internalization and retrieval to the TGN in vivo, with the tyrosine-based motif being the major determinant, followed by the phenylalanine signal, whereas the leucine-isoleucine motif is only a minor component. Finally, we report that phosphorylation of the furin tail by casein kinase II is not only important for efficient interaction with micro2 and internalization from the plasma membrane but also determines fast retrieval of the protein from the plasma membrane to the TGN.

The leucine-based sorting motifs in the cytoplasmic domain of the invariant chain are recognized by the clathrin adaptors AP1 and AP2 and their medium chains

Recognition of sorting signals within the cytoplasmic tail of membrane proteins by adaptor protein complexes is a crucial step in membrane protein sorting. The three known adaptor complexes, AP1, AP2, and AP3, have all been shown to recognize tyrosine- and leucine-based sorting signals, which are the most common sorting signals within membrane protein cytoplasmic tails. Although tyrosine-based signals are recognized by the micro-chains of adaptor complexes, the subunit recognizing leucine-based sorting signals is less clear. In this report we show by surface plasmon resonance that the two leucine-based sorting signals within the cytoplasmic tail of the invariant chain bind independently from each other to AP1 and AP2 but not to AP3. We also show that both motifs can be recognized by the micro-chains of AP1 and AP2. Moreover, by using monomeric as well as trimeric invariant chain constructs, we show that adaptor binding does not require trimerization of the invariant chain.

Association of AP1 adaptor complexes with GLUT4 vesicles

Nycodenz gradients have been used to examine the in vitro effects of GTP-(gamma)-S on adaptor complex association with GLUT4 vesicles. On addition of GTP-(gamma)-S, GLUT4 fractionates as a heavier population of vesicles, which we suggest is due to a budding or coating reaction. Under these conditions there is an increase in co-sedimentation of GLUT4 with AP1, but not with AP3. Western blotting of proteins associated with isolated GLUT4 vesicles shows the presence of high levels of AP1 and some AP3 but very little AP2 adaptor complexes. Cell free, in vitro association of the AP1 complex with GLUT4 vesicles is increased approximately 4-fold by the addition of GTP-(gamma)-S and an ATP regenerating system. Following GTP-(gamma)-S treatment in vitro, ARF is also recruited to GLUT4 vesicles, and the temperature dependence of ARF recruitment closely parallels that of AP1. The recruitment of both AP1 and ARF are partially blocked by brefeldin A. These data demonstrate that the coating of GLUT4 vesicles can be studied in isolated cell-free fractions. Furthermore, at least two distinct adaptor complexes can associate with the GLUT4 vesicles and it is likely that these adaptors are involved in mediating distinct intracellular sorting events at the level of TGN and endosomes.