PACS-1 defines a novel gene family of cytosolic sorting proteins required for trans-Golgi network localization

A novel casein kinase 2 alpha-subunit regulates membrane protein traffic in the human hepatoma cell line HuH-7

A previously isolated endocytic trafficking mutant (TRF1) isolated from HuH-7 cells is defective in the distribution of subpopulations of cell-surface receptors for asialoorosomucoid (asialoglycoprotein receptor (ASGR)), transferrin, and mannose-terminating glycoproteins. The pleiotropic phenotype of TRF1 also includes an increased sensitivity to Pseudomonas toxin and deficient assembly and function of gap junctions. HuH-7xTRF1 hybrids exhibited a normal subcellular distribution of ASGR, consistent with the TRF1 mutation being recessive. A cDNA expression library derived from HuH-7 mRNA was transfected into TRF1 cells, which were subsequently selected for resistance to Pseudomonas toxin. Sequence analysis of a recovered cDNA revealed a unique isoform of casein kinase 2 (CK2), CK2alpha". Western blot analysis of TRF1 proteins revealed a 60% reduction in total CK2alpha expression. Consistent with this finding, the hybrids HuH-7xHuH-7 and HuH-7xTRF1 expressed equivalent amounts of total CK2alpha. Immunoblots using antibodies against peptides unique to the previously described CK2 isoforms CK2alpha and CK2alpha' and the novel CK2alpha" isoform showed that, although TRF1 and parental HuH-7 cells expressed comparable amounts of CK2alpha and CK2alpha', the mutant did not express CK2alpha". Based on the genomic DNA sequence, RNA transcripts encoding CK2alpha" apparently originate from alternative splicing of a primary transcript. Protein overexpression following transfection of TRF1 cells with cDNAs encoding either CK2alpha or the newly cloned CK2alpha" restored the parental HuH-7 phenotype, including Pseudomonas toxin resistance, cell-surface ASGR binding activity, phosphorylation, and the assembly of gap junctions. This study suggests that HuH-7 cells express at least three CK2alpha isoforms and that the pleiotropic TRF1 phenotype is a consequence of a reduction in total CK2 expression.

Protein phosphatase 2A binds to the cytoplasmic tail of carboxypeptidase D and regulates post-trans-Golgi network trafficking

Carboxypeptidase D (CPD) is a transmembrane protein that processes proteins in the trans-Golgi network (TGN). A 20-residue region within the cytoplasmic tail of CPD binds protein phosphatase 2A (PP2A). PP2A also binds to the cytoplasmic tails of other secretory pathway proteins: peptidylglycine-(amino)-amidating mono-oxygenase, the cation-independent mannose-6-phosphate receptor and TGN38. The CPD tail is phosphorylated on Thr residues in the AtT-20 cell line. The CPD tail can also be phosphorylated by purified protein kinase A, protein kinase C and casein kinase II. Both the in vitro and the in vivo phosphorylated CPD tail can be dephosphorylated by purified PP2A. The binding of CPD tail peptide to PP2A does not influence phosphatase activity. The rate of transport of CPD from the TGN to the cell surface of AtT-20 cells is decreased 45% by okadaic acid, a PP2A inhibitor. Microinjection of the CPD tail into AtT-20 cells inhibits the transition of CPD from endosomal compartments to the TGN. However, okadaic acid does not affect the rate of budding of CPD from the TGN into nascent vesicles or the rate of uptake from the cell surface into endosomal compartments. These results are consistent with the model that PP2A is involved in the trafficking of proteins between a TGN recycling loop and a cell-surface recycling loop, but is not involved in the individual recycling loops.

Sortilin/neurotensin receptor-3: a new tool to investigate neurotensin signaling and cellular trafficking?

The identification of gp95sortilin, a sorting protein, as being the 100 kDa neurotensin (NT) receptor, a non-G-protein coupled receptor, constitutes a new and interesting but intriguing step in the neuropeptide signaling as well as in cellular trafficking. The isolation of the same protein by three different experimental approaches sum up the complexity for researchers involved in the functional significance of the so-called sortilin/neurotensin receptor 3 (NTR3). This review will concentrate on the putative physiological and cellular roles of sortilin/NTR3 as most results so far have proposed hypothetical conclusions rather than concrete evidence.

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.

Overexpression of a novel sorting nexin, SNX15, affects endosome morphology and protein trafficking

Sorting nexin (SNX) 15 is a novel member of the SNX family of proteins. Although the functions of most SNXs have not yet been determined, several family members (e.g., SNX1, SNX2, SNX3, and SNX8) are orthologs of yeast proteins involved in protein trafficking. Overexpression of myc-tagged SNX15 in COS-7 cells altered the morphology of several endosomal compartments. In transient transfection experiments, myc-SNX15 was first seen in small punctate spots and small ring structures. Later, myc-SNX15 was found in larger rings. Finally, myc-SNX15 was observed in large, amorphous membrane-limited structures. These structures contained proteins from lysosomes, late endosomes, early endosomes, and the trans-Golgi network. However, the morphology of the endoplasmic reticulum and Golgi was not affected by overexpression of myc-SNX15. In myc-SNX15-overexpressing cells, the endocytosis of transferrin was severely inhibited and endocytosis of tac-trans-Golgi network (TGN) 38 and tac-furin was slowed. In addition, the recycling of internalized tac-TGN38 and tac-furin was also inhibited. Both the morphological and biochemical data indicate that SNX15 plays a crucial role in trafficking through the endocytic pathway. This is the first demonstration that a mammalian SNX protein is involved in protein trafficking.

The dendritic cell receptor for endocytosis, DEC-205, can recycle and enhance antigen presentation via major histocompatibility complex class II-positive lysosomal compartments

Many receptors for endocytosis recycle into and out of cells through early endosomes. We now find in dendritic cells that the DEC-205 multilectin receptor targets late endosomes or lysosomes rich in major histocompatibility complex class II (MHC II) products, whereas the homologous macrophage mannose receptor (MMR), as expected, is found in more peripheral endosomes. To analyze this finding, the cytosolic tails of DEC-205 and MMR were fused to the external domain of the CD16 Fcgamma receptor and studied in stable L cell transfectants. The two cytosolic domains each mediated rapid uptake of human immunoglobulin (Ig)G followed by recycling of intact CD16 to the cell surface. However, the DEC-205 tail recycled the CD16 through MHC II-positive late endosomal/lysosomal vacuoles and also mediated a 100-fold increase in antigen presentation. The mechanism of late endosomal targeting, which occurred in the absence of human IgG, involved two functional regions: a membrane-proximal region with a coated pit sequence for uptake, and a distal region with an EDE triad for the unusual deeper targeting. Therefore, the DEC-205 cytosolic domain mediates a new pathway of receptor-mediated endocytosis that entails efficient recycling through late endosomes and a greatly enhanced efficiency of antigen presentation to CD4(+) T cells.

Endocytosis of insulin-like growth factor II by a mini-receptor based on repeat 11 of the mannose 6-phosphate/insulin-like growth factor II receptor

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) plays an important role in controlling the extracellular level of the insulin-like growth factor II (IGF-II) by mediating its binding at the cell surface and delivery to lysosomes. Loss of the receptor is associated with an accumulation of IGF-II, which can cause perinatal lethality if it is systemic, or local proliferation and tumorgenesis if it is spatially restricted. The extracytoplasmic domain of the receptor consists of 15 homologous repeats, of which repeat 11 carries the IGF-II-binding site of the multifunctional receptor. To investigate whether repeat 11 is sufficient to mediate binding and internalization of IGF-II, a construct consisting of repeat 11 fused to the transmembrane and cytoplasmic domain of the M6P/IGF-II receptor was transfected into mouse embryonic fibroblasts. The construct was expressed as a stable membrane protein which binds IGF-II with a 10-fold lower affinity as observed for the M6P/IGF-II receptor and is found at the cell surface and in endosomes. It mediates the internalization of IGF-II and its delivery to lysosomes, suggesting that it can function as a IGF-II mini-receptor controlling the extracellular IGF-II level.

Furin-mediated processing in the early secretory pathway: sequential cleavage and degradation of misfolded insulin receptors

Improperly folded membrane proteins are retained in the endoplasmic reticulum and then diverted to a degradative pathway by a network of molecular chaperones and intracellular proteases. Here we report that mutant insulin proreceptors (Pro(62)) retained in the early secretory pathway undergo proteolytic cleavage at a tetrabasic concensus site for the subtilisin-like protease furin (SPC 1), generating two unstable proteolytic intermediates of 80/120 kDa corresponding to alpha (135 kDa) and beta (90 kDa) subunits. These are degraded more rapidly than the uncleaved proreceptor protein. Site-directed mutagenesis of the normal RKRR processing site prevented cleavage. Use of inhibitors and furin-deficient cell lines confirmed that furin is responsible for proreceptor cleavage; furin overexpression increased the degradation of mutant but not wild-type receptors. Together, these results suggest that processing and degradation occur sequentially for mutant proreceptors.

The use of yeast two-hybrid screens in studies of protein:protein interactions involved in trafficking

The yeast two-hybrid system has provided a convenient means to both screen for proteins that interact with a protein of interest and to characterise the known interaction between two proteins. Several groups with an interest in the molecular mechanisms that underlie discrete steps along trafficking pathways have exploited the yeast two-hybrid system. Here, we provide a brief background to the technology, attempt to point out some of the pitfalls and benefits of the different systems that can be employed, and mention some of the areas (within the trafficking field) where yeast two-hybrid interaction assays have been particularly informative.

Constitutive phosphorylation of the vesicular inhibitory amino acid transporter in rat central nervous system

gamma-Aminobutyric acid (GABA) and glycine are stored into synaptic vesicles by a recently identified vesicular inhibitory amino acid transporter [VIAAT, also called vesicular GABA transporter (VGAT)]. Immunoblotting analysis revealed that rat brain VIAAT migrated as a doublet during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with a predominant slower band in all areas examined except olfactory bulb and retina. The slower band corresponded to a phosphorylated form of VIAAT as it was converted to the faster one by treating brain homogenates with alkaline phosphatase or with an endogenous phosphatase identified as type 2A protein-serine/threonine phosphatase using okadaic acid. In contrast, the recombinant protein expressed in COS-7 or PC12 cells co-migrated with the faster band of the brain doublet and was insensitive to alkaline phosphatase. To investigate the influence of VIAAT phosphorylation on vesicular neurotransmitter loading, purified synaptic vesicles were treated with alkaline phosphatase and assayed for amino acid uptake. However, neither GABA nor glycine uptake was affected by VIAAT phosphorylation. These results indicate that VIAAT is constitutively phosphorylated on cytosolic serine or threonine residues in most, but not all, regions of the rat brain. This phosphorylation does not regulate the vesicular loading of GABA or glycine, suggesting that it is involved at other stages of the synaptic vesicle life cycle.

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.

Effects of insulin on intracellular GLUT4 vesicles in adipocytes: evidence for a secretory mode of regulation

The facilitative glucose transporter, GLUT4 undergoes insulin-dependent movement to the cell surface in adipocytes. The magnitude of the insulin effect is much greater for GLUT4 than other recycling proteins such as the CD-MPR. In the present study we have studied the colocalisation of these proteins in adipocytes in an effort to explain this selective insulin-dependent recruitment of GLUT4. Using immunofluorescence microscopy or immuno-EM on 3T3-L1 adipocytes we find that there is considerable colocalisation between these proteins particularly within the area of the TGN. However, the distribution of CD-MPR was not significantly effected by insulin. The insulin-dependent recruitment of GLUT4 was concomitant with a selective decrease in GLUT4 labelling of cytoplasmic vesicles whereas the amount of GLUT4 in the TGN region (approx. 50% of total GLUT4) was relatively unaffected. To explore the possibility that the cytoplasmic GLUT4(+) vesicles represent an intracellular insulin-responsive storage compartment we performed quantitative immuno-EM on whole mounts of intracellular vesicles isolated from basal and insulin-stimulated adipocytes. These studies revealed that: (1) GLUT4 and CD-MPR were concentrated in small (30-200 nm) vesicles at a labelling density of 1–20+ gold particles/vesicle; (2) there was significant overlap between both proteins in that 70% of the total GLUT4 pool colocalised with CD-MPR; (3) a significant amount of GLUT4 (approx. 50% of total) was found in a subpopulation of vesicles that contained as little as 5% of the total CD-MPR pool; (4) the GLUT4(+)/CD-MPR(-) vesicles were highly insulin-responsive, and (5) the total number of GLUT4(+) vesicles, but not CD-MPR(+) vesicles, decreased by approx. 30% in response to insulin treatment. These data are consistent with a model in which GLUT4 is selectively sorted into a vesicular compartment in adipocytes that is recruited to the plasma membrane by insulin stimulation.

Membrane targeting properties of a herpesvirus tegument protein-retrovirus Gag chimera

The retroviral Gag protein is capable of directing the production and release of virus-like particles in the absence of all other viral components. Budding normally occurs after Gag is transported to the plasma membrane by its membrane-targeting and -binding (M) domain. In the Rous sarcoma virus (RSV) Gag protein, the M domain is contained within the first 86 amino acids. When M is deleted, membrane association and budding fail to occur. Budding is restored when M is replaced with foreign membrane-binding sequences, such as that of the Src oncoprotein. Moreover, the RSV M domain is capable of targeting heterologous proteins to the plasma membrane. Although the solution structure of the RSV M domain has been determined, the mechanism by which M specifically targets Gag to the plasma membrane rather than to one or more of the large number of internal membrane surfaces (e.g., the Golgi apparatus, endoplasmic reticulum, and nuclear, mitochondrial, or lysosomal membranes) is unknown. To further investigate the requirements for targeting proteins to discrete cellular locations, we have replaced the M domain of RSV with the product of the unique long region 11 (U(L)11) gene of herpes simplex virus type 1. This 96-amino-acid myristylated protein is thought to be involved in virion transport and envelopment at internal membrane sites. When the first 100 amino acids of RSV Gag (including the M domain) were replaced by the entire UL11 sequence, the chimeric protein localized at and budded into the Golgi apparatus rather than being targeted to the plasma membrane. Myristate was found to be required for this specific targeting, as were the first 49 amino acids of UL11, which contain an acidic cluster motif. In addition to shedding new light on UL11, these experiments demonstrate that RSV Gag can be directed to internal cellular membranes and suggest that regions outside of the M domain do not contain a dominant plasma membrane-targeting motif.

Quantitative analysis of TIP47-receptor cytoplasmic domain interactions: implications for endosome-to-trans Golgi network trafficking

TIP47 (tail-interacting protein of 47 kDa) binds to the cytoplasmic domains of the cation-independent and cation-dependent mannose 6-phosphate receptors and is required for their transport from late endosomes to the trans Golgi network in vitro and in vivo. We report here a quantitative analysis of the interaction of recombinant TIP47 with mannose 6-phosphate receptor cytoplasmic domains. Recombinant TIP47 binds more tightly to the cation-independent mannose 6-phosphate receptor (K(D) = 1 microm) than to the cation-dependent mannose 6-phosphate receptor (K(D) = 3 microm). In addition, TIP47 fails to interact with the cytoplasmic domains of the hormone-processing enzymes, furin, phosphorylated furin, and metallocarboxypeptidase D, as well as the cytoplasmic domain of TGN38, proteins that are also transported from endosomes to the trans Golgi network. Although these proteins failed to bind TIP47, furin and TGN38 were readily recognized by the clathrin adaptor, AP-2. These data suggest that TIP47 recognizes a very select set of cargo molecules. Moreover, our data suggest unexpectedly that furin, TGN38, and carboxypeptidase D may use a distinct vesicular carrier and perhaps a distinct route for transport between endosomes and the trans Golgi network.

Phosphorylation of the vesicle-tethering protein p115 by a casein kinase II-like enzyme is required for Golgi reassembly from isolated mitotic fragments

Coat protein I (COPI) transport vesicles can be tethered to Golgi membranes by a complex of fibrous, coiled-coil proteins comprising p115, Giantin and GM130. p115 has been postulated to act as a bridge, linking Giantin on the vesicle to GM130 on the Golgi membrane. Here we show that the acidic COOH terminus of p115 mediates binding to both GM130 and Giantin as well as linking the two together. Phosphorylation of serine 941 within this acidic domain enhances the binding as well as the link between them. Phosphorylation is mediated by casein kinase II (CKII) or a CKII-like kinase. Surprisingly, the highly conserved NH(2)-terminal head domain of p115 is not required for the NSF (N-ethylmaleimide-sensitive fusion protein)-catalyzed reassembly of cisternae from mitotic Golgi fragments in a cell-free system. However, the ability of p115 to link GM130 to Giantin and the phosphorylation of p115 at serine 941 are required for NSF-catalyzed cisternal regrowth. p115 phosphorylation may be required for the transition from COPI vesicle tethering to COPI vesicle docking, an event that involves the formation of trans-SNARE [corrected] (trans-soluble NSF attachment protein [SNAP] receptor) complexes.

Sorting in the endosomal system in yeast and animal cells

The endosomal system is a major membrane-sorting apparatus. New evidence reveals that novel coat proteins assist specific sorting steps and docking factors ensure the vectorial nature of trafficking in the endosomal compartment. There is also good evidence for ubiquitin regulating passage of certain proteins into multivesicular late endosomes, which mature by accumulating invaginated membrane. Lipids play a central role in this involution process, as do the class E vacuolar protein-sorting proteins.

The AP-3 complex required for endosomal synaptic vesicle biogenesis is associated with a casein kinase Ialpha-like isoform

The formation of small vesicles is mediated by cytoplasmic coats the assembly of which is regulated by the activity of GTPases, kinases, and phosphatases. A heterotetrameric AP-3 adaptor complex has been implicated in the formation of synaptic vesicles from PC12 endosomes (). When the small GTPase ARF1 is prevented from hydrolyzing GTP, we can reconstitute AP-3 recruitment to synaptic vesicle membranes in an assembly reaction that requires temperatures above 15 degrees C and the presence of ATP suggesting that an enzymatic step is involved in the coat assembly. We have now found an enzymatic reaction, the phosphorylation of the AP-3 adaptor complex, that is linked with synaptic vesicle coating. Phosphorylation occurs in the beta3 subunit of the complex by a kinase similar to casein kinase 1alpha. The kinase copurifies with neuronal-specific AP-3. In vitro, purified casein kinase I selectively phosphorylates the beta3A and beta3B subunit at its hinge domain. Inhibiting the kinase hinders the recruitment of AP-3 to synaptic vesicles. The same inhibitors that prevent coat assembly in vitro also inhibit the formation of synaptic vesicles in PC12 cells. The data suggest, therefore, that the mechanism of AP-3-mediated vesiculation from neuroendocrine endosomes requires the phosphorylation of the adaptor complex at a step during or after AP-3 recruitment to membranes.

The dileucine motif within the tail of MPR46 is required for sorting of the receptor in endosomes

The cytoplasmic tail of MPR46 carries a leucine-based motif that is required for the sorting of lysosomal enzymes by the receptor. In addition, it is one of three independent, but functionally redundant, internalization signals present in the cytoplasmic tail of MPR46. We have analyzed a mutant of MPR46, in which the dileucine pair was replaced by alanines (MPR46 LL/AA) with respect to its intracellular distribution and trafficking. Ultrastructural analysis of cells expressing the MPR46 LL/AA mutant revealed that the substitution of the dileucine pair causes a shift of the receptor distribution from the TGN, where it is packaged into AP1-containing vesicles, to vesicular structures distributed throughout the cytoplasm. The vesicles could be identified as early endosomes with internalized BSA-gold and rab5 as markers. By analyzing the receptor trafficking biochemically, we found that return of the LL/AA mutant receptor from the plasma membrane/endosome pool back to the TGN was impaired, while recycling from endosomes to the plasma membrane was enhanced. In conclusion, our data indicate that the dileucine motif in the MPR46 tail is required for a sorting event in endosomes.

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 between the exocytic and endocytic pathways in polarized Madin-Darby canine kidney cells

The compartments involved in polarized exocytosis of membrane proteins are not well defined. In this study we hypothesized that newly synthesized polymeric immunoglobulin receptors are targeted from the trans-Golgi network to endosomes prior to their appearance on the basolateral cell surface of polarized Madin-Darby canine kidney cells. To examine this hypothesis, we have used an assay designed to measure the meeting of newly synthesized receptors with a selective population of apical or basolateral endosomes loaded with horseradish peroxidase. We found that in the course of basolateral exocytosis, the wild-type polymeric immunoglobulin receptor is targeted from the trans-Golgi network to apical and basolateral endosomes. Phosphorylation of a Ser residue in the cytoplasmic tail of the receptor is implicated in this process. The biosynthetic pathway of apically sorted polymeric immunoglobulin receptor mutants similarly traversed apical endosomes, raising the possibility that apical receptors are segregated from basolateral receptors in apical endosomes. The post-endocytic pathway of transcytosing and recycling receptors also passed through apical endosomes. Together, these observations are consistent with the possibility that the biosynthetic and endocytic routes merge into endosomes and justify a model suggesting that endosomal recycling processes govern polarized trafficking of proteins traveling in both pathways.

mu1A-adaptin-deficient mice: lethality, loss of AP-1 binding and rerouting of mannose 6-phosphate receptors

The heterotetrameric AP-1 complex is involved in the formation of clathrin-coated vesicles at the trans-Golgi network (TGN) and interacts with sorting signals in the cytoplasmic tails of cargo molecules. Targeted disruption of the mouse mu1A-adaptin gene causes embryonic lethality at day 13.5. In cells deficient in micro1A-adaptin the remaining AP-1 adaptins do not bind to the TGN. Polarized epithelial cells are the only cells of micro1A-adaptin-deficient embryos that show gamma-adaptin binding to membranes, indicating the formation of an epithelial specific AP-1B complex and demonstrating the absence of additional mu1A homologs. Mannose 6-phosphate receptors are cargo molecules that exit the TGN via AP-1-clathrin-coated vesicles. The steady-state distribution of the mannose 6-phosphate receptors MPR46 and MPR300 in mu1A-deficient cells is shifted to endosomes at the expense of the TGN. MPR46 fails to recycle back from the endosome to the TGN, indicating that AP-1 is required for retrograde endosome to TGN transport of the receptor.

Directional transneuronal infection by pseudorabies virus is dependent on an acidic internalization motif in the Us9 cytoplasmic tail

The Us9 gene is conserved among most alphaherpesviruses. In pseudorabies virus (PRV), the Us9 protein is a 98-amino-acid, type II membrane protein found in the virion envelope. It localizes to the trans-Golgi network (TGN) region in infected and transfected cells and is maintained in this compartment by endocytosis from the plasma membrane. Viruses with Us9 deleted have no observable defects in tissue culture yet have reduced virulence and restricted spread to retinorecipient neurons in the rodent brain. In this report, we demonstrate that Us9-promoted transneuronal spread in vivo is dependent on a conserved acidic motif previously shown to be essential for the maintenance of Us9 in the TGN region and recycling from the plasma membrane. Mutant viruses with the acidic motif deleted have an anterograde spread defect indistinguishable from that of Us9 null viruses. Transneuronal spread, however, is not dependent on a dileucine endocytosis motif in the Us9 cytoplasmic tail. Through alanine scanning mutagenesis of the acidic motif, we have identified two conserved tyrosine residues that are essential for Us9-mediated spread as well as two serine residues, comprising putative consensus casein kinase II sites, that modulate the rate of PRV transneuronal spread in vivo.

GRP94 hyperglycosylation and phosphorylation in Sf21 cells

GRP94 is an inducible resident endoplasmic reticulum/sarcoplasmic reticulum (ER/SR) glycoprotein that functions as a protein chaperone and Ca(2+) regulator. GRP94 has been reported to be a substrate for protein kinase CK2 in vitro, although its phosphorylation in intact cells remains unreported. In Sf21 insect cells, overexpression of canine GRP94 led to the appearance of a multiplet of three or more molecular-mass isoforms which was reduced to a single mobility form following treatment of cells with tunicamycin, suggesting stable accumulations of consecutively modified protein. Metabolic labeling of Sf21 cells with (32)P(i) led to a constitutive phosphorylation of GRP94 which, based upon phosphopeptide mapping, occurred specifically on CK2-sensitive sites. Among the GRP94 multiplet, however, only the lowest mobility form of GRP94 was phosphorylated, even though in vitro phosphorylation of GRP94 by CK2 led to phosphorylation of all glycosylated forms. The (32)P(i) incorporation into GRP94 indicated a slow turnover of phosphate incorporation that was unaffected by inhibition of biosynthesis, resulting in a steady-state level of phospho-GRP94 on CK2 sites. These data support a role for protein kinase CK2 in the cell biology for GRP94 and other resident ER/SR proteins that may occur in ER compartments.

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.

Human cytomegalovirus pp28 (UL99) localizes to a cytoplasmic compartment which overlaps the endoplasmic reticulum-golgi-intermediate compartment

Although the assembly of herpesviruses has remained an active area of investigation, considerable controversy continues to surround the cellular location of tegument and envelope acquisition. This controversy is particularly evident when the proposed pathways for alpha- and beta-herpesvirus assembly are compared. We have approached this aspect of human cytomegalovirus (HCMV) assembly, specifically, envelopment, by investigating the intracellular trafficking of viral tegument proteins which localize in the cytoplasms of infected cells. In this study we have demonstrated that the virion tegument protein pp28 (UL99), a true late protein, was membrane associated as a result of myristoylation. A mutation in this protein which prevented incorporation of [(3)H]myristic acid also altered the detergent solubility and intracellular distribution of the protein when it was expressed in transfected cells. Using a panel of markers for intracellular compartments, we could localize the expression of wild-type pp28 to an intracellular compartment which colocalized with the endoplasmic reticulum-Golgi-intermediate compartment (ERGIC), a dynamic compartment of the secretory pathway which interfaces with both the ER and Golgi apparatus. The localization of this viral tegument protein within an early secretory compartment of the cell provided further evidence that the assembly of the HCMV tegument likely includes a cytoplasmic phase. Because pp28 has been shown to be localized to a cytoplasmic assembly compartment in HCMV-infected cells, our findings also suggested that viral tegument protein interactions within the secretory pathway may have an important role in the assembly of the virion.

The PC6B cytoplasmic domain contains two acidic clusters that direct sorting to distinct trans-Golgi network/endosomal compartments

The mammalian proprotein convertases (PCs) are a family of secretory pathway enzymes that catalyze the endoproteolytic maturation of peptide hormones and many bioactive proteins. Two PCs, furin and PC6B, are broadly expressed and share very similar cleavage site specificities, suggesting that they may be functionally redundant. However, germline knockout studies show that they are not. Here we report the distinct subcellular localization of PC6B and identify the sorting information within its cytoplasmic domain (cd). We show that in neuroendocrine cells, PC6B is localized to a paranuclear, brefeldin A-dispersible, BaCl(2)-responsive post-Golgi network (TGN) compartment distinct from furin and TGN38. The 88-amino acid PC6B-cd contains sorting information sufficient to direct reporter proteins to the same compartment as full-length PC6B. Mutational analysis indicates that endocytosis is predominantly directed by a canonical tyrosine-based motif (Tyr(1802)GluLysLeu). Truncation and sufficiency studies reveal that two clusters of acidic amino acids (ACs) within the PC6B-cd contain differential sorting information. The membrane-proximal AC (AC1) directs TGN localization and interacts with the TGN sorting protein PACS-1. The membrane-distal AC (AC2) promotes a localization characteristic of the full-length PC6B-cd. Our results demonstrate that AC motifs can target proteins to distinct TGN/endosomal compartments and indicate that the AC-mediated localization of PC6B and furin contribute to their distinct roles in vivo.

Targeting of an apical endosomal protein to endosomes in Madin-Darby canine kidney cells requires two sorting motifs

The efficient sorting and targeting of endocytosed macromolecules is critical for epithelial function. However, the distribution of endosomal compartments in these cells remains controversial. In this study, we show that polarized Madin-Darby canine kidney (MDCK) cells target the apical endosomal protein endotubin into an apical early endosomal compartment that is distinct from the apical recycling endosomes. Furthermore, through a panel of site-directed mutations we show that signals required for apical endosomal targeting of endotubin are composed of two distinct motifs on the cytoplasmic domain, a hydrophobic motif and a consensus casein kinase II site. Endotubin-positive endosomes in MDCK cells do not label with basolaterally internalized transferrin or ricin, do not contain the small guanosine triphosphate-binding protein rab11, and do not tubulate in response to low concentrations of brefeldin-A (BFA). Nevertheless, high concentrations of BFA reversibly inhibits the sorting of endotubin from transferrin and cause colocalization in tubular endosomes. These results indicate that, in polarized cells, endotubin targets into a distinct subset of apical endosomes, and the targeting information required both for polarity and endosomal targeting is provided by the cytoplasmic portion of the molecule.

HIV-1 Nef protein binds to the cellular protein PACS-1 to downregulate class I major histocompatibility complexes

Major-histocompatibility-complex (MHC) proteins are used to display, on the surface of a cell, peptides derived from foreign material - such as a virus - that is infecting that cell. Cytotoxic T lymphocytes then recognize and kill the infected cell. The HIV-1 Nef protein downregulates the cell-surface expression of class I MHC proteins, and probably thereby promotes immune evasion by HIV-1. In the presence of Nef, class I MHC molecules are relocalized from the cell surface to the trans-Golgi network (TGN) through as-yet-unknown mechanisms. Here we show that Nef-induced downregulation of MHC-I expression and MHC-I targeting to the TGN require the binding of Nef to PACS-1, a molecule that controls the TGN localization of the cellular protein furin. This interaction is dependent on Nef's cluster of acidic amino acids. A chimaeric integral membrane protein containing Nef as its cytoplasmic domain localizes to the TGN after internalization, in an acidic-cluster- and PACS-1-dependent manner. These results support a model in which Nef relocalizes MHC-I by acting as a connector between MHC-I's cytoplasmic tail and the PACS-1-dependent protein-sorting pathway.

Cell type-specific storage of dopamine beta-monooxygenase

Expression of dopamine beta-monooxygenase (DBM), the enzyme that converts dopamine into norepinephrine, is limited to adrenal chromaffin cells and a small population of neurons. We studied DBM trafficking to regulated granules by stably expressing rat DBM in AtT-20 corticotrope tumor cells, which contain regulated granules, and in Chinese hamster ovary (CHO) cells, which lack regulated granules. The behavior of exogenous DBM in both cell lines was compared with endogenous DBM in adrenal chromaffin cells. CHO cells secreted active DBM, indicating that production of active enzyme does not require features unique to neuroendocrine cells. Pulse-chase experiments indicated that early steps in DBM maturation followed a similar time course in AtT-20, CHO, and adrenal chromaffin cells. Use of a conformation-sensitive DBM antiserum indicated that acquisition of a folded structure occurred with a similar time course in all three cell types. Cell type-specific differences in DBM trafficking became apparent only when storage in granules was examined. As expected, DBM was stored in secretory granules in chromaffin cells; CHO cells failed to store DBM. Despite the fact that AtT-20 cells have regulated granules, exogenous DBM was not stored in these granules. Thus storage of DBM in secretory granules requires cell type specific factors.

Accumulation of virion tegument and envelope proteins in a stable cytoplasmic compartment during human cytomegalovirus replication: characterization of a potential site of virus assembly

The assembly of human cytomegalovirus (HCMV) is thought to be similar to that which has been proposed for alphaherpesviruses and involve envelopment of tegumented subviral particles at the nuclear membrane followed by export from the cell by a poorly defined pathway. However, several studies have shown that at least two tegument virion proteins remain in the cytoplasm during the HCMV replicative cycle, thereby suggesting that HCMV cannot acquire its final envelope at the nuclear envelope. We investigated the assembly of HCMV by determining the intracellular trafficking of the abundant tegument protein pp150 (UL32) in productively infected human fibroblasts. Our results indicated that pp150 remained within the cytoplasm throughout the replicative cycle of HCMV and accumulated in a stable, juxtanuclear structure late in infection. Image analysis using a variety of cell protein-specific antibodies indicated that the pp150-containing structure was not a component of the endoplasmic reticulum, (ER), ER-Golgi intermediate compartment, cis or medial Golgi, or lysosomes. Partial colocalization of the structure was noted with the trans-Golgi network, and it appeared to lie in close proximity to the microtubule organizing center. Two additional tegument proteins (pp28 and pp65) and three envelope glycoproteins (gB, gH, and gp65) localized in this same structure late infection. This compartment appeared to be relatively stable since pp150, pp65, and the processed form of gB could be coisolated following cell fractionation. Our findings indicated that pp150 was expressed exclusively within the cytoplasm throughout the infectious cycle of HCMV and that the accumulation of the pp150 in this cytoplasmic structure was accompanied by at least five other virion proteins. These results suggested the possibility that this virus-induced structure represented a cytoplasmic site of virus assembly.

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 novel kinase peptidylglycine alpha-amidating monooxygenase cytosolic interactor protein 2 interacts with the cytosolic routing determinants of the peptide processing enzyme peptidylglycine alpha-amidating monooxygenase

The cytosolic domain of the peptide-processing integral membrane protein peptidylglycine alpha-amidating monooxygenase (PAM; EC 1.14. 17.3) contains multiple signals determining its subcellular localization. Three PAM cytosolic interactor proteins (P-CIPs) were identified using the yeast two hybrid system (Alam, M. R., Caldwel, B. D., Johnson, R. C., Darlington, D. N., Mains, R. E., and Eipper, B. A. (1996) J. Biol. Chem. 271, 28636-28640); the partial amino acid sequence of P-CIP2 suggested that it was a protein kinase. In situ hybridization and immunocytochemistry show that P-CIP2 is expressed widely throughout the brain; PAM and P-CIP2 are expressed in the same neurons. Based on subcellular fractionation, the 47-kDa P-CIP2 protein is mostly cytosolic. P-CIP2 is a highly selective kinase, phosphorylating the cytosolic domain of PAM, but not the corresponding region of furin or carboxypeptidase D. Although P-CIP2 interacts with stathmin, it does not phosphorylate stathmin. Site-directed mutagenesis, phosphoamino acid analysis, and use of synthetic peptides demonstrate that PAM-Ser(949) is the major site phosphorylated by P-CIP2. Based on both in vitro binding experiments and co-immunoprecipitation from cell extracts, P-CIP2 interacts with PAM proteins containing the wild type cytosolic domain, but not with mutant forms of PAM whose trafficking is disrupted. P-CIP2, through its highly selective phosphorylation of a key site in the cytosolic domain of PAM, appears to play a critical role in the trafficking of this protein.

Differential distribution of mannose-6-phosphate receptors and furin in immature secretory granules

In neuroendocrine cells sorting of proteins from immature secretory granules (ISGs) occurs during maturation and is achieved by clathrin-coated vesicles containing the adaptor protein (AP)-1. We have investigated the role of the mannose-6-phosphate receptors (M6PRs) in the recruitment of AP-1 to ISGs. M6PRs were detected in ISGs isolated from PC12 cells by subcellular fractionation, and by immuno-EM labelling on cryosections. In light of our previous results, where greater than 80% of the ISGs were found to contain furin, we examined the relationship between furin and M6PR on ISGs. By immunoisolation techniques we find that 50% at most of the ISGs contain the cation-independent (CI)-M6PR. Using sequential immunoisolation we could demonstrate that there are two populations of ISGs: those that have both M6PR and furin, and those which contain only furin. Furthermore, using immobilized GST-fusion proteins containing the cytoplasmic domain of the CI-M6PR we have shown binding of AP-1 requires casein kinase II phosphorylation of the CI-M6PR fusion protein, and in particular phosphorylation of Ser(2474). Addition of these phosphorylated GST-CI-M6PR fusion proteins to a cell-free assay reconstituting AP-1 binding to ISGs inhibits AP-1 recruitment to ISGs.

Direct interaction of the trans-Golgi network membrane protein, TGN38, with the F-actin binding protein, neurabin

TGN38 is a type I integral membrane protein that constitutively cycles between the trans-Golgi network (TGN) and plasma membrane. The cytosolic domain of TGN38 interacts with AP2 clathrin adaptor complexes via the tyrosine-containing motif (-SDYQRL-) to direct internalization from the plasma membrane. This motif has previously been shown to direct both internalization and subsequent TGN targeting of TGN38. We have used the cytosolic domain of TGN38 in a two-hybrid screen, and we have identified the brain-specific F-actin binding protein neurabin-I as a TGN38-binding protein. We demonstrate a direct interaction between TGN38 and the ubiquitous homologue of neurabin-I, neurabin-II (also called spinophilin). We have used a combination of yeast two-hybrid and in vitro protein interaction assays to show that this interaction is dependent on the serine (but not tyrosine) residue of the known TGN38 trafficking motif. We show that TGN38 interacts with the coiled coil region of neurabin in vitro and binds preferentially with the dimeric form of neurabin. TGN38 and neurabin also interact in vivo as demonstrated by coimmunoprecipitation from stably transfected PC12 cells. These data suggest that neurabin provides a direct physical link between TGN38-containing membranes and the actin cytoskeleton.

Identification and characterization of polycystin-2, the PKD2 gene product

PKD2, the second gene for the autosomal dominant polycystic kidney disease (ADPKD), encodes a protein, polycystin-2, with predicted structural similarity to cation channel subunits. However, the function of polycystin-2 remains unknown. We used polyclonal antisera specific for the intracellular NH(2) and COOH termini to identify polycystin-2 as an approximately 110-kDa integral membrane glycoprotein. Polycystin-2 from both native tissues and cells in culture is sensitive to Endo H suggesting the continued presence of high-mannose oligosaccharides typical of pre-middle Golgi proteins. Immunofluorescent cell staining of polycystin-2 shows a pattern consistent with localization in the endoplasmic reticulum. This finding is confirmed by co-localization with protein-disulfide isomerase as determined by double indirect immunofluorescence and co-distribution with calnexin in subcellular fractionation studies. Polycystin-2 translation products truncated at or after Gly(821) retain their exclusive endoplasmic reticulum localization while products truncated at or before Glu(787) additionally traffic to the plasma membrane. Truncation mutants that traffic to the plasma membrane acquire Endo H resistance and can be biotinylated on the cell surface in intact cells. The 34-amino acid region Glu(787)-Ser(820), containing two putative phosphorylation sites, is responsible for the exclusive endoplasmic reticulum localization of polycystin-2 and is the site of specific interaction with an as yet unidentified protein binding partner for polycystin-2. The localization of full-length polycystin-2 to intracellular membranes raises the possibility that the PKD2 gene product is a subunit of intracellular channel complexes.

An acidic cluster in the cytosolic domain of human cytomegalovirus glycoprotein B is a signal for endocytosis from the plasma membrane

We previously reported that human cytomegalovirus (CMV) glycoprotein B (gB) is transported to apical membranes in CMV-infected polarized retinal pigment epithelial (ARPE-19) cells and in Madin-Darby canine kidney (MDCK) epithelial cells constitutively expressing gB. The cytosolic domain of gB contains a cluster of acidic amino acids, a motif that plays a pivotal role in vectorial trafficking in polarized epithelial cells and may also function as a signal for entry into the endocytic pathway. Here we compared gB internalization and recycling to the plasma membrane in CMV-infected human fibroblasts (HF) and ARPE-19 cells by using antibody-internalization experiments. Immunofluorescence and quantitative assays showed that gB was internalized from the cell surface into clathrin-coated transport vesicles and then recycled to the plasma membrane. gB colocalized with clathrin-coated vesicles containing the transferrin receptor in the early endocytic/recycling pathway, indicating that gB traffics in this pathway. The specific role of the acidic cluster in regulating the sorting of gB-containing vesicles in the early endocytic/recycling pathway was examined in MDCK cells expressing mutated gB derivatives. Immunofluorescence assays showed that derivatives lacking the acidic cluster were impaired in internalization and failed to recycle. These findings, together with our earlier observation that the acidic cluster is a key determinant for targeting gB molecules to apical membranes in epithelial cells, establish that this signal is recognized by cellular proteins that participate in polarized sorting and transport in the early endocytic/recycling pathway.

Membrane trafficking of neurotransmitter transporters in the regulation of synaptic transmission

Many psychoactive drugs influence the transport of neurotransmitters across biological membranes, suggesting that the physiological regulation of neurotransmitter transport might contribute to normal and perhaps abnormal behaviour. Over the past few years, molecular characterization of the neurotransmitter transporters has enabled investigation of their subcellular location and regulation. The analysis of location suggests that membrane trafficking has an important role in the normal function of these proteins. One of the major regulatory mechanisms also involves changes in localization that might contribute to synaptic plasticity. This article discusses recent work on the membrane trafficking of neurotransmitter transporters and its role in regulating their activity.

Disease models provide insights into post-golgi protein trafficking, localization and processing

Over the past two years, the use of in vitro systems and the identification of autoantibodies to Golgi proteins have provided important new tools for analyzing vesicle and cargo trafficking in the distal secretory pathway. In addition, the phenotypic characterization of mice with knockouts of various prohormone convertases has led to significant progress in understanding the biological relevance of prohormone processing in post-Golgi compartments.

Chimeric forms of furin and TGN38 are transported with the plasma membrane in the trans-Golgi network via distinct endosomal pathways

Furin and TGN38 are menbrane proteins that cycle between the plasma membrane and the trans-Golgi network (TGN), each maintaining a predominant distribution in the TGN. We have used chimeric proteins with an extracellular Tac domain and the cytoplasmic domain of TGN38 or furin to study the trafficking of these proteins in endosomes. Previously, we demonstrated that the postendocytic trafficking of Tac-TGN38 to the TGN is via the endocytic recycling pathway (Ghosh, R.N.,W.G. Mallet,T.T. Soe,T.E.McGraw, and F.R. Maxfield.1998.J. Cell Biol.142:923-936). Here we show that internalized Tac-furin is delivered to the TGN through late endosomes, bypassing the endocytic recycling compartment. The transport of Tac-furin from late endosomes to the TGN appears to proceed via an efficient, single-pass mechanism. Delivery of Tac-furin but not Tac-TGN38 to the TGN is blocked by nocodazole, and the two pathways are also differentially affected by wortmannin. These studies demonstrate the existence of two independentpathways for endosomal transport of proteins to the TGN from the plasma membrane.

Phosphorylation of cytosolic domain Ser(937) affects both biosynthetic and endocytic trafficking of peptidylglycine alpha-amidating monooxygenase

Peptidylglycine alpha-amidating monooxygenase (PAM), a bifunctional enzyme, catalyzes the COOH-terminal amidation of bioactive peptides. In test tube assays, PAM is phosphorylated by protein kinase C at Ser(937). The roles of phosphorylation and dephosphorylation of Ser(937) in the biosynthetic and endocytic trafficking of integral membrane PAM were examined using an antiserum specific for the phosphorylation of Ser(937) and using AtT-20 cells expressing membrane PAM in which Ser(937) was mutated to Ala or Asp. Although phosphorylation at Ser(937) can occur while PAM is in the endoplasmic reticulum, early steps in the biosynthetic trafficking of membrane PAM were not affected by Ser(937) phosphorylation. The inability to phosphorylate PAM/S937A increased its intracellular degradation and decreased secretion of the soluble monooxygenase portion of PAM. In contrast, the biosynthetic trafficking of PAM/S937D was indistinguishable from wild-type PAM. Despite the fact that Ser(937) is adjacent to the only Tyr-based internalization motif in PAM, internalization and trafficking through early endosomes were unaffected by phosphorylation. However, PAM antibody internalized by wild-type PAM acquired a perinuclear localization, while antibody internalized by PAM/S937A was routed to lysosomes, and antibody bound to PAM/S937D maintained a dispersed, punctate pattern. In cells stimulated with phorbol ester, phosphorylation of Ser(937) increased and phosphorylated PAM accumulated in large vesicular structures. Therefore, phosphorylation of PAM-1 at Ser(937) directs newly synthesized and internalized protein away from lysosomes, while dephosphorylation is needed for a different step in the late endocytic pathway.

Recognition of sorting signals by clathrin adaptors

Sorting of membrane proteins is generally mediated by cytosolic coats, which create a scaffold to form coated buds and vesicles and to selectively concentrate cargo by interacting with cytosolic signals. The classical paradigm is the interaction between clathrin coats and associated adaptor proteins, which cluster receptors with characteristic tyrosine and dileucine motifs during endocytosis. Clathrin in association with different sets of adaptors is found in addition at the trans-Golgi network and endosomes. Sequences similar to internalization signals also direct lysosomal and basolateral sorting, which implicates related clathrinadaptor coats in the respective sorting pathways. This review concentrates on the recognition of sorting signals by clathrin-associated adaptor proteins, an area of significant recent progress due to new methodological and conceptual approaches.

The dileucine-based sorting motif in HIV-1 Nef is not required for down-regulation of class I MHC

A dileucine-based protein sorting motif has recently been identified within the C-terminal, solvent-exposed loop of HIV-1 Nef and has been shown to be required for Nef-mediated down-regulation of CD4 and for optimal viral infectivity. Here, we report that mutation of the dileucine motif has no effect on Nef-mediated down-regulation of class I MHC heavy chain. Instead, deletion of an acidic domain just N-terminal of the polyproline helix of the SH3-binding domain significantly impairs this function. These data indicate that down-regulation of class I MHC and CD4 are mechanistically distinct processes. The data also suggest that protein interactions mediated by the acidic domain, rather than by the dileucine motif, may contribute to this function of Nef.

Biosynthesis and packaging of carboxypeptidase D into nascent secretory vesicles in pituitary cell lines

Metallocarboxypeptidase D (CPD) is a membrane-bound trans-Golgi network (TGN) protein. In AtT-20 cells, CPD is initially produced as a 170-kDa endoglycosidase H-sensitive glycoprotein. Within 30 min of chase, the CPD increases to 180 kDa and is resistant to endoglycosidase H as a result of carbohydrate maturation. CPD also undergoes an activation step required for binding to a substrate affinity resin. Blocking the protein exit from the endoplasmic reticulum inhibits the increase in molecular mass but not the step required for affinity column binding, suggesting that enzyme activation precedes carbohydrate maturation and that these reactions occur in distinct intracellular compartments. Only the higher molecular weight mature CPD enters nascent secretory vesicles, which bud from the TGN of permeabilized AtT-20 and GH3 cells. The budding efficiency of CPD into vesicles is 2-3-fold lower than that of endogenous proopiomelanocortin in AtT-20 cells or prolactin in GH3 cells. In contrast, the packaging of a truncated form of CPD, which lacks the cytoplasmic tail and transmembrane domain, was similar to that of proopiomelanocortin. Taken together, the results support the proposal that CPD functions in the TGN in the processing of proteins that transit the secretory pathway and that the C-terminal region plays a major role in TGN retention.

Nef-induced CD4 degradation: a diacidic-based motif in Nef functions as a lysosomal targeting signal through the binding of beta-COP in endosomes

The Nef protein of primate lentiviruses downregulates the cell surface expression of CD4 through a two-step process. First, Nef connects the cytoplasmic tail of CD4 with adaptor protein complexes (AP), thereby inducing the formation of CD4-specific clathrin-coated pits that rapidly endocytose the viral receptor. Second, Nef targets internalized CD4 molecules for degradation. Here we show that Nef accomplishes this second task by acting as a connector between CD4 and the beta subunit of COPI coatomers in endosomes. A sequence encompassing a critical acidic dipeptide, located nearby but distinct from the AP-binding determinant of HIV-1 Nef, is responsible for beta-COP recruitment and for routing to lysosomes. A novel class of endosomal sorting motif, based on acidic residues, is thus revealed, and beta-COP is identified as its downstream partner.

Distinct domains within Vps35p mediate the retrieval of two different cargo proteins from the yeast prevacuolar/endosomal compartment

Resident membrane proteins of the trans-Golgi network (TGN) of Saccharomyces cerevisiae are selectively retrieved from a prevacuolar/late endosomal compartment. Proper cycling of the carboxypeptidase Y receptor Vps10p between the TGN and prevacuolar compartment depends on Vps35p, a hydrophilic peripheral membrane protein. In this study we use a temperature-sensitive vps35 allele to show that loss of Vps35p function rapidly leads to mislocalization of A-ALP, a model TGN membrane protein, to the vacuole. Vps35p is required for the prevacuolar compartment-to-TGN transport of both A-ALP and Vps10p. This was demonstrated by phenotypic analysis of vps35 mutant strains expressing A-ALP mutants lacking either the retrieval or static retention signals and by an assay for prevacuolar compartment-to-TGN transport. A novel vps35 allele was identified that was defective for retrieval of A-ALP but functional for retrieval of Vps10p. Moreover, several other vps35 alleles were identified with the opposite characteristics: they were defective for Vps10p retrieval but near normal for A-ALP localization. These data suggest a model in which distinct structural features within Vps35p are required for associating with the cytosolic domains of each cargo protein during the retrieval process.

A role for a PDZ protein in the early secretory pathway for the targeting of proTGF-alpha to the cell surface

In general, plasma membrane integral proteins, such as the membrane-anchored growth factor proTGF-alpha, are assumed to be transported to the cell surface via a nonregulated, constitutive pathway. proTGF-alpha C-terminal mutants are retained in an early secretory compartment. Here, using a two-hybrid screen, we identify two TACIPs (proTGF-alpha cytoplasmic domain-interacting proteins) that contain PDZ domains and do not interact with proTGF-alpha C-terminal mutants. The binding specificity of one of them, TACIP18 (previously identified and named Syntenin or mda-9), coincides with that of the component that possibly mediates the normal trafficking of proTGF-alpha. TACIP18 colocalizes and interacts specifically with immature, intracellular forms of proTGF-alpha. Therefore, it appears that the interaction of TACIP18 with proTGF-alpha in the early secretory pathway is necessary for the targeting of the latter to the cell surface.