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

Mass spectrometry of cardiac calsequestrin characterizes microheterogeneity unique to heart and indicative of complex intracellular transit

Cardiac calsequestrin concentrates in junctional sarcoplasmic reticulum in heart and skeletal muscle cells by an undefined mechanism. During transit through the secretory pathway, it undergoes an as yet uncharacterized glycosylation and acquires phosphate on CK2-sensitive sites. In this study, we have shown that active calsequestrin phosphorylation occurred in nonmuscle cells as well as muscle cells, reflecting a widespread cellular process. To characterize this post-translational modification and resolve individual molecular mass species, we subjected purified calsequestrin to mass spectrometry using electrospray ionization. Mass spectra showed that calsequestrin glycan structure in nonmuscle cells was that expected for an endoplasmic reticulum-localized glycoprotein and showed that each glycoform existed as four mass peaks representing molecules that also had 0-3 phosphorylation sites occupied. In heart, mass peaks indicated carbohydrate modifications characteristic of transit through Golgi compartments. Phosphorylation did not occur on every glycoform present, suggesting a far more complex movement of calsequestrin molecules in heart cells. Significant amounts of calsequestrin contained glycan with only a single mannose residue, indicative of a novel post-endoplasmic reticulum mannosidase activity. In conclusion, glyco- and phosphoforms of calsequestrin chart a complex cellular transport in heart, with calsequestrin following trafficking pathways not present or not accessible to the same molecules in nonmuscle.

Phosphorylation-dependent interaction of the asialoglycoprotein receptor with molecular chaperones

A membrane protein trafficking mutant (Trf1) of HuH-7 alters the asialoglycoprotein (ASGPR) and transferrin receptor subcellular distribution. Expression cloning of a cDNA complementing the trf1 mutation led to the discovery of a novel casein Kinase 2 catalytic subunit (CK2alpha"). To purify potential CK2alpha" phosphorylation-dependent sorting proteins from cytosol, the ASGPR cytoplasmic domain was expressed as a GST fusion protein and immobilized on glutathione-agarose. In the absence of phosphorylation, only trace amounts of cytosol protein were bound and eluted. When the fusion protein was phosphorylated, a heterocomplex of potential sorting proteins was recovered. Mass spectrometer and immunoblot analysis identified five of these proteins as gp96, HSP70, HSP90, cyclophilin-A, and FKBP18. Treatment of HuH-7 with rapamycin to disrupt the heterocomplex reduced surface ASGPR binding activity by 65 +/- 5.7%. In Trf1 cells, surface-binding activity was 48 +/- 7% of that in HuH-7 and was not further reduced by rapamycin treatment. Immunoanalysis showed significantly fewer surface receptors on rapamycin-treated HuH7 cells than on nontreated cells, with no affect on the level of surface receptors in Trf1 cells. The data presented provide evidence that phosphorylation of the ASGPR cytoplasmic domain is required for the binding of specific molecular chaperones with the potential to regulate receptor trafficking.

Furin at the cutting edge: from protein traffic to embryogenesis and disease

Furin catalyses a simple biochemical reaction--the proteolytic maturation of proprotein substrates in the secretory pathway. But the simplicity of this reaction belies furin's broad and important roles in homeostasis, as well as in diseases ranging from Alzheimer's disease and cancer to anthrax and Ebola fever. This review summarizes various features of furin--its structural and enzymatic properties, intracellular localization, trafficking, substrates, and roles in vivo.

Cooperation of GGAs and AP-1 in packaging MPRs at the trans-Golgi network

The Golgi-localized, gamma-ear-containing, adenosine diphosphate ribosylation factor-binding proteins (GGAs) are multidomain proteins that bind mannose 6-phosphate receptors (MPRs) in the Golgi and have an essential role in lysosomal enzyme sorting. Here the GGAs and the coat protein adaptor protein-1 (AP-1) were shown to colocalize in clathrin-coated buds of the trans-Golgi networks of mouse L cells and human HeLa cells. Binding studies revealed a direct interaction between the hinge domains of the GGAs and the gamma-ear domain of AP-1. Further, AP-1 contained bound casein kinase-2 that phosphorylated GGA1 and GGA3, thereby causing autoinhibition. This could induce the directed transfer of the MPRs from GGAs to AP-1. MPRs that are defective in binding to GGAs are poorly incorporated into AP-1-containing clathrin-coated vesicles. Thus, the GGAs and AP-1 interact to package MPRs into AP-1-containing coated vesicles.

CD95 (Fas) expression is regulated by sequestration in the Golgi complex in B-cell lymphoma

The CD95 (Fas) molecule transmits apoptotic signals important in B-cell development and the genesis of B-cell lymphoma. We have investigated the surface and intracellular expression of CD95 in Burkitt's lymphoma (BL) cells, an important non-Hodgkin's lymphoma of B-cell origin. Group I BL cells did not express CD95 at the cell surface, but contained high levels of this receptor in the cytoplasm. In contrast, group III BL cells expressed CD95 intracellularly and at the cell surface. In group I and group III BL cells, cytoplasmic CD95 was localized to the Golgi complex, as assessed by confocal immunofluorescence microscopy and subcellular fractionation followed by immunoblotting. Trafficking through the Golgi complex is regulated by elements within the target protein and cellular sorting mechanisms. CD95 contains candidate signals for interaction with trafficking machinery. Group I BL cells can be induced to upregulate surface expression of CD95 following CD40 ligation and certain group I BL cell lines drift invitro to a group III phenotype, with consequent surface expression of CD95. Taken together, these observations show that CD95 can either be retained in the Golgi complex or exported to the cell surface, and suggest that membrane trafficking has an important and previously unrecognized role in regulating CD95 expression in B lymphocytes.

Autoinhibition of the ligand-binding site of GGA1/3 VHS domains by an internal acidic cluster-dileucine motif

The GGAs (Golgi-localizing, gamma-adaptin ear homology domain, ARF-binding proteins) are a family of proteins implicated in protein trafficking from the Golgi to endosomes/lysosomes. These proteins have modular structures with an N-terminal VHS (VPS-27, Hrs, and STAM) domain followed by a GAT (GGA and TOM1) domain, a connecting hinge segment, and a C-terminal GAE (gamma-adaptin ear) domain. Isolated VHS domains have been shown to bind specifically to acidic cluster (AC)-dileucine motifs present in the cytoplasmic tails of the mannose 6-phosphate receptors. Here we report that full-length cytoplasmic GGA1 and GGA3 but not GGA2 bind the cation-independent mannose 6-phosphate receptor very poorly because of autoinhibition. This inhibition is caused by the binding of an AC-LL sequence present in the hinge segment to the ligand-binding site in the VHS domain. The inhibition depends on the phosphorylation of a serine located three residues upstream of the AC-LL motif. The serine is phosphorylated by casein kinase 2 in in vitro assays. Substitution of the GGA1 inhibitory sequence into the analogous location in GGA2, which lacks the AC-LL motif, results in autoinhibition of the latter protein. These data indicate that the activity of GGA1 and GGA3 is regulated by cycles of phosphorylation/dephosphorylation.

Dominant-negative mutant of BIG2, an ARF-guanine nucleotide exchange factor, specifically affects membrane trafficking from the trans-Golgi network through inhibiting membrane association of AP-1 and GGA coat proteins

BIG2 is one of the guanine nucleotide exchange factors (GEFs) for the ADP-ribosylation factor (ARF) family of small GTPases, which regulate membrane association of COPI and AP-1 coat protein complexes and GGA proteins. Brefeldin A (BFA), an ARF-GEF inhibitor, causes redistribution of the coat proteins from membranes to the cytoplasm and membrane tubulation of the Golgi complex and the trans-Golgi network (TGN). We have recently shown that BIG2 overexpression blocks BFA-induced redistribution of the AP-1 complex but not TGN membrane tubulation. In the present study, we constructed a dominant-negative BIG2 mutant and found that when expressed in cells it induced redistribution of AP-1 and GGA1 and membrane tubulation of the TGN. By contrast, the mutant did not induce COPI redistribution or Golgi membrane tubulation. These observations indicate that BIG2 is involved in trafficking from the TGN by regulating membrane association of AP-1 and GGA through activating ARF.

Specific modulation of Kex2/furin family proteases by potassium

Kex2 protease is the prototype for a family of proteases responsible for endoproteolytic cleavage at multi-basic motifs in the eukaryotic secretory pathway. Here we demonstrate that potassium ion can act as a modulator of Kex2 activity with an apparent affinity of approximately 20 mm. Other monovalent cations (Li(+), Na(+), etc.) display similar effects, but affinities are all over 20-fold lower. Potassium ion binding stimulates turnover at physiologically relevant Lys-Arg cleavage sites but reduces turnover with at least one incorrect sequence. Furthermore, the mammalian Kex2 homolog furin displays similar effects. In contrast, the neuroendocrine homolog PC2 is inhibited by potassium ion with all substrates examined. The pre-steady-state behavior of Kex2 is also altered upon binding of potassium ion, with opposite effects on acylation and deacylation rates. These biochemical data indicate that potassium ion concentration may function as a regulator of processing protease specificity and activity in the eukaryotic secretory pathway, with such enzymes potentially encountering compartments high in potassium ion caused by the action of antiporters such as yeast NHX1 (VPS44) or the mammalian NHE7.

Retrieval of human cytomegalovirus glycoprotein B from cell surface is not required for virus envelopment in astrocytoma cells

Human cytomegalovirus (HCMV) is a prototypic member of the betaherpesvirus family. The HCMV virion is composed of a large DNA genome encapsidated within a nucleocapsid, which is wrapped within an inner proteinaceous tegument and an outer lipid envelope containing viral glycoproteins. Although genome encapsidation clearly occurs in the nucleus, the subsequent steps in the virion assembly process are unclear. HCMV glycoprotein B (gB) is a major component of the virion envelope that plays a critical role in virus entry and is essential for the production of infectious virus progeny. The aim of our present study was to identify the secretory compartment to which HCMV gB was localized and to investigate the role of endocytosis in mediating gB localization and HCMV biogenesis. We show that HCMV gB is localized to the trans-Golgi network (TGN) in HCMV-infected cells and that gB contains all of the trafficking information necessary for TGN localization. Endocytosis of gB was shown to play a role in mediating TGN localization of gB and in targeting of the protein to the site of virus envelopment. However, inhibition of endocytosis with a dominant-negative dynamin I molecule did not affect the production of infectious virus. These observations indicate that, although endocytosis is involved in the trafficking of gB to the site of glycoprotein accumulation in the TGN, endocytosis of gB is not required for the production of infectious HCMV.

The ordered and compartment-specfific autoproteolytic removal of the furin intramolecular chaperone is required for enzyme activation

The propeptide of furin has multiple roles in guiding the activation of the endoprotease in vivo. The 83-residue N-terminal propeptide is autoproteolytically excised in the endoplasmic reticulum (ER) at the consensus furin site, -Arg(104)-Thr-Lys-Arg(107)-, but remains bound to furin as a potent autoinhibitor. Furin lacking the propeptide is ER-retained and proteolytically inactive. Co-expression with the propeptide, however, restores trans-Golgi network (TGN) localization and enzyme activity, indicating that the furin propeptide is an intramolecular chaperone. Blocking this step results in localization to the ER-Golgi intermediate compartment (ERGIC)/cis-Golgi network (CGN), suggesting the ER and ERGIC/CGN recognize distinct furin folding intermediates. Following transport to the acidified TGN/endosomal compartments, furin cleaves the bound propeptide at a second, internal P1/P6 Arg site (-Arg-Gly-Val(72)-Thr-Lys-Arg(75)-) resulting in propeptide dissociation and enzyme activation. Cleavage at Arg(75), however, is not required for proper furin trafficking. Kinetic analyses of peptide substrates indicate that the sequential pH-modulated propeptide cleavages result from the differential recognition of these sites by furin. Altering this preference by converting the internal site to a canonical P1/P4 Arg motif (Val(72) --> Arg) caused ER retention and blocked activation of furin, demonstrating that the structure of the furin propeptide mediates folding of the enzyme and directs its pH-regulated, compartment-specific activation in vivo.

The yeast Vps10p cytoplasmic tail mediates lysosomal sorting in mammalian cells and interacts with human GGAs

Yeast Vps10p is a receptor for transport of the soluble vacuolar hydrolase carboxypeptidase Y to the lysosome-like vacuole. Its functional equivalents in mammalian cells are the mannose 6-phosphate receptors that mediate sorting to lysosomes of mannose 6-phosphate-containing lysosomal proteins. A chimeric receptor was constructed by substituting the cytoplasmic domain of M(r) 300,000 mannose 6-phosphate receptor with the Vps10p cytoplasmic tail. Expression of the chimera in cells lacking endogenous mannose 6-phosphate receptors resulted in a subcellular receptor distribution and an efficiency in sorting of lysosomal enzymes similar to that of the wild type M(r) 300,000 mannose 6-phosphate receptor. Moreover, the cytoplasmic tail of the Vps10p was found to interact with GGA1 and GGA2, two mammalian members of a recently discovered family of clathrin-binding cytosolic proteins that participate in trans-Golgi network-endosome trafficking in both mammals and yeast. Our findings suggest a conserved machinery for Golgi-endosome/vacuole sorting and may serve as a model for future studies of yeast proteins.

Regulated transport of the glucose transporter GLUT4

In muscle and fat cells, insulin stimulates the delivery of the glucose transporter GLUT4 from an intracellular location to the cell surface, where it facilitates the reduction of plasma glucose levels. Understanding the molecular mechanisms that mediate this translocation event involves integrating our knowledge of two fundamental processes--the signal transduction pathways that are triggered when insulin binds to its receptor and the membrane transport events that need to be modified to divert GLUT4 from intracellular storage to an active plasma membrane shuttle service.

The yeast clathrin adaptor protein complex 1 is required for the efficient retention of a subset of late Golgi membrane proteins

In yeast, certain resident trans-Golgi network (TGN) proteins achieve steady-state localization by cycling through late endosomes. Here, we show that chitin synthase III (Chs3p), an enzyme involved in the assembly of the cell wall at the mother-bud junction, populates an intracellular reservoir that is maintained by a cycle of transport between the TGN and early endosomes. Traffic of Chs3p from the TGN/early endosome to the cell surface requires CHS5 and CHS6, mutant alleles of which trap Chs3p in the TGN/early endosome. Disruption of the clathrin adaptor protein complex 1 (AP-1) restores Chs3p transport to the plasma membrane. Similarly, in AP-1 deficient cells, the resident TGN/early endosome syntaxin, Tlg1p, is missorted. We propose that clathrin and AP-1 act to recycle Chs3p and Tlg1p from the early endosome to the TGN.

Diverse trafficking patterns due to multiple traffic motifs in G protein-activated inwardly rectifying potassium channels from brain and heart

G protein-activated inwardly rectifying potassium channels (Kir3, GIRK) provide an important mechanism for neurotransmitter regulation of membrane excitability. GIRK channels are tetramers containing various combinations of Kir3 subunits (Kir3.1--Kir3.4). We find that different combinations of Kir3 subunits exhibit a surprisingly complex spectrum of trafficking phenotypes. Kir3.2 and Kir3.4, but not Kir3.1, contain ER export signals that are important for plasma membrane expression of Kir3.1/Kir3.2 and Kir3.1/Kir3.4 heterotetramers, the GIRK channels found in the brain and the heart, respectively. Additional motifs in Kir3.2 and Kir3.4 control the trafficking between endosome and plasma membrane. In contrast, the Kir3.3 subunit potently inhibits plasma membrane expression by diverting the heterotetrameric channels to lysosomes. Such rich trafficking behaviors provide a mechanism for dynamic regulation of GIRK channel density in the plasma membrane.

CLIPR-59, a new trans-Golgi/TGN cytoplasmic linker protein belonging to the CLIP-170 family

The microtubule cytoskeleton plays a fundamental role in cell organization and membrane traffic in higher eukaryotes. It is well established that molecular motors are involved in membrane-microtubule interactions, but it has also been proposed that nonmotor microtubule-binding (MTB) proteins known as CLIPs (cytoplasmic linker proteins) have basic roles in these processes. We report here the characterization of CLIPR-59, a CLIP-170-related protein localized to the trans-most part of the Golgi apparatus. CLIPR-59 contains an acidic region followed by three ankyrin-like repeats and two CLIP-170-related MTB motifs. We show that the 60-amino acid-long carboxy-terminal domain of CLIPR-59 is necessary and sufficient to achieve Golgi targeting, which represents the first identification of a membrane targeting domain in a CLIP-170-related protein. The MTB domain of CLIPR-59 is functional because it localizes to microtubules when expressed as a fragment in HeLa cells. However, our results suggest that this domain is normally inhibited by the presence of adjacent domains, because neither full-length CLIPR-59 nor a CLIPR-59 mutant missing its membrane-targeting region localize to microtubules. Consistent with this observation, overexpression of CLIPR-59 does not affect the microtubule network. However, CLIPR-59 overexpression strongly perturbs early/recycling endosome-TGN dynamics, implicating CLIPR-59 in the regulation of this pathway.

Routing of membrane proteins to large dense core vesicles in PC12 cells

Routing of membrane proteins to large dense core vesicles in neuroendocrine cells can depend on information in both the lumenal and cytoplasmic domains. This study in PC12 cells focuses on the routing, cleavage, and secretion of an integral membrane protein, peptidylglycine alpha-amidating monooxygenase (PAM), examining both endogenous and virally derived membrane PAM. The role of the lumenal catalytic domains in membrane PAM trafficking was examined by replacement with an epitope tag. Virally derived membrane PAM is localized to the perinuclear area and to slender processes where the large dense core vesicles are located. Expression of PAM along with a neuroendocrine-specific endoprotease liberates a soluble monooxygenase domain, yielding regulated secretion of both the monooxygenase and the prohormone convertase from large dense core vesicles. The subcellular distribution of the epitope-substituted version of PAM within the cells is similar to that of membrane PAM, and both proteins are internalized from the plasma membrane. When secretion is stimulated, Serine937 in the cytoplasmic domain of PAM is phosphorylated to a similar extent in endogenous membrane PAM, virally encoded membrane PAM, and epitope-substituted PAM. Thus, the lumenal PAM catalytic domains are not required for routing or phosphorylation of PAM in PC12 cells.

Identification of glycoprotein gpTRL10 as a structural component of human cytomegalovirus

Human cytomegalovirus (HCMV) has a coding capacity for glycoproteins which far exceeds that of other herpesviruses. Few of these proteins have been characterized. We have investigated the gene product(s) of reading frame 10, which is present in both the internal and terminal repeat regions of HCMV strain AD169 and only once in clinical isolates. The putative protein product is a 171-amino-acid glycoprotein with a theoretical mass of 20.5 kDa. We characterized the protein encoded by this reading frame in the laboratory strain AD169 and a recent isolate, TB40E. The results from both strains were comparable. Northern blot analyses showed that the gene was transcribed with early/late kinetics. Two proteins of 22 and 23.5-kDa were detected in virus-infected cells and in cells transiently expressing recombinant TRL10. Both forms contained only high-mannose-linked carbohydrate modifications. In addition, virus-infected cells expressed small amounts of the protein modified with complex N-linked sugars. Image analysis localized transiently expressed TRL10 to the endoplasmic reticulum. Immunoblot analyses as well as immunoelectron microscopy of purified virions demonstrated that TRL10 represents a structural component of the virus particle. Immunoblot analysis in the absence of reducing agents indicated that TRL10, like the other HCMV envelope glycoproteins, is present in a disulfide-linked complex. Sequence analysis of the TRL10 coding region in nine low-passage clinical isolates revealed strain-specific variation. In summary, the protein product of the TRL10 open reading frame represents a novel structural glycoprotein of HCMV and was termed gpTRL10.

Nef: agent of cell subversion

Primate lentiviruses encode a small protein designated Nef that has been shown to be a major determinant of virus pathogenicity. Nef regulates multiple host factors in order to optimize the cellular environment for virus replication. The mechanisms by which this small protein modulates distinct host cell properties provide intriguing insight into the intricate interaction between virus and host.

Biological basket weaving: formation and function of clathrin-coated vesicles

There has recently been considerable progress in understanding the regulation of clathrin-coated vesicle (CCV) formation and function. These advances are due to the determination of the structure of a number of CCV coat components at molecular resolution and the identification of novel regulatory proteins that control CCV formation in the cell. In addition, pathways of (a) phosphorylation, (b) receptor signaling, and (c) lipid modification that influence CCV formation, as well as the interaction between the cytoskeleton and CCV transport pathways are becoming better defined. It is evident that although clathrin coat assembly drives CCV formation, this fundamental reaction is modified by different regulatory proteins, depending on where CCVs are forming in the cell. This regulatory difference likely reflects the distinct biological roles of CCVs at the plasma membrane and trans-Golgi network, as well as the distinct properties of these membranes themselves. Tissue-specific functions of CCVs require even more-specialized regulation and defects in these pathways can now be correlated with human diseases.

The tyrosine motifs of Lamp 1 and LAP determine their direct and indirect targetting to lysosomes

Lamp 1 and lysosomal acid phosphatase (LAP) are lysosomal membrane proteins that harbour a tyrosine-based sorting motif within their short cytoplasmic tails. Lamp 1 is delivered from the trans-Golgi network (TGN) via endosomes directly to lysosomes bypassing the plasma membrane, whereas LAP is indirectly transported to lysosomes and recycles between endosomes and the plasma membrane before being delivered to lysosomes. By analysing truncated forms of LAP and chimeras in which the cytoplasmic tail or part of the cytoplasmic tails of LAP and Lamp 1 were exchanged, we were able to show that the YRHV tyrosine motif of LAP is necessary and sufficient to mediate recycling between endosomes and the plasma membrane. When peptides corresponding to the cytoplasmic tails of LAP and Lamp 1 and chimeric or mutant forms of these tails were assayed for in vitro binding of AP1 and AP2, we found that AP2 bound to LAP- and Lamp-1-derived peptides, whereas AP1 bound only to peptides containing the YQTI tyrosine motif of Lamp 1. Residues +2 and +3 of the tyrosine motif were critical for the differential binding of adaptors. LAP in which these residues (-HV) were substituted for those of Lamp 1 (-TI) was transported directly to lysosomes, whereas a chimera carrying the Lamp 1 tail in which residues +2 and +3 were substituted for those of LAP (-HV) gained the ability to recycle. In conclusion, the residues +2 and +3 of the tyrosine motifs determine the sorting of Lamp 1 and LAP in endosomes, mediating either the direct or the indirect pathway to lysosomes.

The Di-leucine motif of vesicle-associated membrane protein 4 is required for its localization and AP-1 binding

Heterotetrameric adaptor complexes and SNAREs play key roles in the specificity of membrane budding and fusion. Here we test the hypothesis that vesicle budding and membrane fusion are coupled by the interaction of these molecules. We investigate the role of the di-leucine motif of vesicle-associated membrane protein 4 (VAMP4) in adaptor binding and localization of VAMP4. Mutation of the di-leucine motif inhibits AP-1 binding in vitro and affects the steady state distribution of VAMP4 in vivo.

Polarized trafficking and surface expression of the AQP4 water channel are coordinated by serial and regulated interactions with different clathrin-adaptor complexes

Aquaporin 4 (AQP4) is the predominant water channel in the brain. It is targeted to specific membrane domains of astrocytes and plays a crucial role in cerebral water balance in response to brain edema formation. AQP4 is also specifically expressed in the basolateral membranes of epithelial cells. However, the molecular mechanisms involved in its polarized targeting and membrane trafficking remain largely unknown. Here, we show that two independent C-terminal signals determine AQP4 basolateral membrane targeting in epithelial MDCK cells. One signal involves a tyrosine-based motif; the other is encoded by a di-leucine-like motif. We found that the tyrosine-based basolateral sorting signal also determines AQP4 clathrin-dependent endocytosis through direct interaction with the mu subunit of AP2 adaptor complex. Once endocytosed, a regulated switch in mu subunit interaction changes AP2 adaptor association to AP3. We found that the stress-induced kinase casein kinase (CK)II phosphorylates the Ser276 immediately preceding the tyrosine motif, increasing AQP4-mu 3A interaction and enhancing AQP4-lysosomal targeting and degradation. AQP4 phosphorylation by CKII may thus provide a mechanism that regulates AQP4 cell surface expression.

Intracellular trafficking of the UL11 tegument protein of herpes simplex virus type 1

Growing evidence indicates that herpes simplex virus type 1 (HSV-1) acquires its final envelope in the trans-Golgi network (TGN). During the envelopment process, the viral nucleocapsid as well as the envelope and tegument proteins must arrive at this site in order to be incorporated into assembling virions. To gain a better understanding of how these proteins associate with cellular membranes and target to the correct compartment, we have been studying the intracellular trafficking properties of the small tegument protein encoded by the U(L)11 gene of HSV-1. This 96-amino-acid, myristylated protein accumulates on the cytoplasmic face of internal membranes, where it is thought to play a role in nucleocapsid envelopment and egress. When expressed in the absence of other HSV-1 proteins, the UL11 protein localizes to the Golgi apparatus, and previous deletion analyses have revealed that the membrane-trafficking information is contained within the first 49 amino acids. The goal of this study was to map the functional domains required for proper Golgi membrane localization. In addition to N-terminal myristylation, which allows for weak membrane binding, UL11 appears to be palmitylated on one or more of three consecutive N-terminal cysteines. Using membrane-pelleting experiments and confocal microscopy, we show that palmitylation of UL11 is required for both Golgi targeting specificity and strong membrane binding. Furthermore, we found that a conserved acidic cluster within the first half of UL11 is required for the recycling of this tegument protein from the plasma membrane to the Golgi apparatus. Taken together, our results demonstrate that UL11 has highly dynamic membrane-trafficking properties, which suggests that it may play multiple roles on the plasma membrane as well as on the nuclear and TGN membranes.

Access of a membrane protein to secretory granules is facilitated by phosphorylation

Peptidylglycine alpha-amidating monooxygenase (PAM), an integral membrane protein essential for the biosynthesis of amidated peptides, was used to assess the role of cytosolic acidic clusters in trafficking to regulated secretory granules. Casein kinase II phosphorylates Ser(949) and Thr(946) of PAM, generating a short, cytosolic acidic cluster. P-CIP2, a protein kinase identified by its ability to interact with several juxtamembrane determinants in the PAM cytosolic domain, also phosphorylates Ser(949). Antibody specific for phospho-Ser(949)-PAM-CD demonstrates that a small fraction of the PAM-1 localized to the perinuclear region bears this modification. Pituitary cell lines expressing PAM-1 mutants that mimic (TS/DD) or prevent (TS/AA) phosphorylation at these sites were studied. PAM-1 TS/AA yields a lumenal monooxygenase domain that enters secretory granules inefficiently and is rapidly degraded. In contrast, PAM-1 TS/DD is routed to regulated secretory granules more efficiently than wild-type PAM-1 and monooxygenase release is more responsive to secretagogue. Furthermore, this acidic cluster affects exit of internalized PAM-antibody complexes from late endosomes; internalized PAM-1 TS/DD accumulates in a late endocytic compartment instead of the trans-Golgi network. The increased ability of solubilized PAM-1 TS/DD to aggregate at neutral pH may play an important role in its altered trafficking.

Differential utilization of enzyme-substrate interactions for acylation but not deacylation during the catalytic cycle of Kex2 protease

Kex2 protease from Saccharomyces cerevisiae is the prototype for a family of eukaryotic proprotein processing proteases belonging to the subtilase superfamily of serine proteases. Kex2 can be distinguished from degradative subtilisins on the basis of stringent substrate specificity and distinct pre-steady-state behavior. To better understand these mechanistic differences, we have examined the effects of substrate residues at P(1) and P(4) on individual steps in the Kex2 catalytic cycle with a systematic series of isosteric peptidyl amide and ester substrates. The results demonstrate that substrates based on known, physiological cleavage sites exhibit high acylation rates (> or =550 s(-1)) with Kex2. Substitution of Lys for the physiologically correct Arg at P(1) resulted in a > or =200-fold drop in acylation rate with almost no apparent effect on binding or deacylation. In contrast, substitution of the physiologically incorrect Ala for Nle at P(4) resulted in a much smaller defect in acylation and a modest but significant effect on binding with Lys at P(1). This substitution also had no effect on deacylation. These results demonstrate that Kex2 utilizes enzyme-substrate interactions in different ways at different steps in the catalytic cycle, with the S(1)-P(1) contact providing a key specificity determinant at the acylation step.

Vps26p, a component of retromer, directs the interactions of Vps35p in endosome-to-Golgi retrieval

Endosome-to-Golgi retrieval of the carboxypeptidase Y receptor Vps10p is mediated by a recently discovered membrane coat complex termed retromer. Retromer comprises five conserved proteins: Vps35p, Vps29p, Vps5p, Vps17p, and Vps26p. Vps35p recognizes cargo molecules such as Vps10p and interacts strongly with Vps29p. Vps5p forms a subcomplex with Vps17p and has been proposed to play a structural role by self-assembling into large multimeric structures. The function of Vps26p is currently unknown. We have investigated the role that Vps26p plays in retromer-mediated endosome-to-Golgi transport by analyzing dominant negative alleles of Vps26p. These mutants have identified a crucial region of Vps26p that plays an important role in its function. Functional domains of Vps26p have been investigated by the creation of yeast-mouse hybrid molecules in which domains of Vps26p have been replaced by the similar domain in the protein encoded by the mouse VPS26 gene, Hbeta58. These domain swap experiments have shown that Vps26p promotes the interactions between the cargo-selective component Vps35p and the structural components Vps5p/Vps17p.

Basolateral membrane expression of a K+ channel, Kir 2.3, is directed by a cytoplasmic COOH-terminal domain

The inwardly rectifying potassium channel Kir 2.3 is specifically targeted and expressed on the basolateral membrane of certain renal epithelial cells. In the present study, the structural basis for polarized targeting was elucidated. Deletion of a unique COOH-terminal domain produced channels that were mistargeted to the apical membrane, consistent with the removal of a basolateral membrane-sorting signal. By characterizing a series of progressively smaller truncation mutants, an essential targeting signal was defined (residues 431-442) within a domain that juxtaposes or overlaps with a type I PDZ binding motif (442). Fusion of the COOH-terminal structure onto CD4 was sufficient to change a random membrane-trafficking and expression pattern into a basolateral membrane one. Using metabolic labeling and pulse-chase and surface immunoprecipitation, we found that CD4-Kir2.3 COOH-terminal chimeras were rapidly and directly targeted to the basolateral membrane, consistent with a sorting signal that is processed in the biosynthetic pathway. Collectively, the data indicate that the basolateral sorting determinant in Kir 2.3 is composed of a unique arrangement of trafficking motifs, containing tandem, conceivably overlapping, biosynthetic targeting and PDZ-based signals. The previously unrecognized domain corresponds to a highly degenerate structure within the Kir channel family, raising the possibility that the extreme COOH terminus of Kir channels may differentially coordinate membrane targeting of different channel isoforms.

Golgi-localizing, gamma-adaptin ear homology domain, ADP-ribosylation factor-binding (GGA) proteins interact with acidic dileucine sequences within the cytoplasmic domains of sorting receptors through their Vps27p/Hrs/STAM (VHS) domains

GGA (Golgi-localizing, gamma-adaptin ear homology domain, ARF-binding) proteins are potential effectors of ADP-ribosylation factors, are associated with the trans-Golgi network (TGN), and are involved in protein transport from this compartment. By yeast two-hybrid screening and subsequent two-hybrid and pull-down analyses, we have shown that GGA proteins, through their VHS (Vps27p/Hrs/STAM) domains, interact with acidic dileucine sequences found in the cytoplasmic domains of TGN-localized sorting receptors such as sortilin and mannose 6-phosphate receptor. A mutational analysis has revealed that a leucine pair and a cluster of acidic residues adjacent to the pair are mainly responsible for the interaction. A chimeric receptor with the sortilin cytoplasmic domain localizes to the TGN, whereas the chimeric receptor with a mutation at the leucine pair or the acidic cluster is mislocalized to punctate structures reminiscent of early endosomes. These results indicate that GGA proteins regulate the localization to or exit from the TGN of the sorting receptors.

CHMP1 functions as a member of a newly defined family of vesicle trafficking proteins

A multivesicular body is a vesicle-filled endosome that targets proteins to the interior of lysosomes. We have identified a conserved eukaryotic protein, human CHMP1, which is strongly implicated in multivesicular body formation. Immunocytochemistry and biochemical fractionation localize CHMP1 to early endosomes and CHMP1 physically interacts with SKD1/VPS4, a highly conserved protein directly linked to multivesicular body sorting in yeast. Similar to the action of a mutant SKD1 protein, overexpression of a fusion derivative of human CHMP1 dilates endosomal compartments and disrupts the normal distribution of several endosomal markers. Genetic studies in Saccharomyces cerevisiae further support a conserved role of CHMP1 in vesicle trafficking. Deletion of CHM1, the budding yeast homolog of CHMP1, results in defective sorting of carboxypeptidases S and Y and produces abnormal, multi-lamellar prevacuolar compartments. This phenotype classifies CHM1 as a member of the class E vacuolar protein sorting genes. Yeast Chm1p belongs to a structurally-related, but rather divergent family of proteins, including Vps24p and Snf7p and three novel proteins, Chm2p, Chm5p and Chm6p, which are all essential for multivesicular body sorting. These observations identify the conserved CHMP/Chmp family as a set of proteins fundamental to understanding multivesicular body sorting in eukaryotic organisms.

Trans-Golgi network sorting

The trans-Golgi network (TGN) is a major secretory pathway sorting station that directs newly synthesized proteins to different subcellular destinations. The TGN also receives extracellular materials and recycled molecules from endocytic compartments. In this review, we summarize recent progress on understanding TGN structure and the dynamics of trafficking to and from this compartment. Protein sorting into different transport vesicles requires specific interactions between sorting motifs on the cargo molecules and vesicle coat components that recognize these motifs. Current understanding of the various targeting signals and vesicle coat components that are involved in TGN sorting are discussed, as well as the molecules that participate in retrieval to this compartment in both yeast and mammalian cells. Besides proteins, lipids and lipid-modifying enzymes also participate actively in the formation of secretory vesicles. The possible mechanisms of action of these lipid hydrolases and lipid kinases are discussed. Finally, we summarize the fundamentally different apical and basolateral cell surface delivery mechanisms and the current facts and hypotheses on protein sorting from the TGN into the regulated secretory pathway in neuroendocrine cells.

VZV gB endocytosis and Golgi localization are mediated by YXXphi motifs in its cytoplasmic domain

The cytoplasmic domains of many membrane proteins contain sorting signals that mediate their endocytosis from the plasma membrane. VZV gB contains three consensus internalization motifs within its cytoplasmic domain: YMTL (aa 818-821), YSRV (aa 857-860), and LL (aa 841-842). To determine whether VZV gB is internalized from the plasma membrane, and whether these motifs are required for its endocytosis, we compared the internalization of native gB to that of gB containing mutations in each of the predicted internalization motifs. VZV gB present on the surface of transfected cells associated with clathrin and was efficiently internalized to the Golgi apparatus within 60 min at 37 degrees C. VZV gB containing the mutation Y857 failed to be internalized, while gB-Y818A was internalized but did not accumulate in the Golgi. These data indicate that the internalization of VZV gB, and its subsequent localization to the Golgi, is mediated by two tyrosine-based sequence motifs in its cytoplasmic domain.

A novel cellular phenotype for familial hypercholesterolemia due to a defect in polarized targeting of LDL receptor

Basolateral targeting of membrane proteins in polarized epithelial cells typically requires cytoplasmic domain sorting signals. In the familial hypercholesterolemia (FH)-Turku LDL receptor allele, a mutation of glycine 823 residue affects the signal required for basolateral targeting in MDCK cells. We show that the mutant receptor is mistargeted to the apical surface in both MDCK and hepatic epithelial cells, resulting in reduced endocytosis of LDL from the basolateral/sinusoidal surface. Consequently, virally encoded mutant receptor fails to mediate cholesterol clearance in LDL receptor-deficient mice, suggesting that a defect in polarized LDL receptor expression in hepatocytes underlies the hypercholesterolemia in patients harboring this allele. This evidence directly links the pathogenesis of a human disease to defects in basolateral targeting signals, providing a genetic confirmation of these signals in maintaining epithelial cell polarity.

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.

Stem cell factor presentation to c-Kit. Identification of a basolateral targeting domain

Stem cell factor (also known as mast cell growth factor and kit-ligand) is a transmembrane growth factor with a highly conserved cytoplasmic domain. Basolateral membrane expression in epithelia and persistent cell surface exposure of stem cell factor are required for complete biological activity in pigmentation, fertility, learning, and hematopoiesis. Here we show by site-directed mutagenesis that the cytoplasmic domain of stem cell factor contains a monomeric leucine-dependent basolateral targeting signal. N-terminal to this motif, a cluster of acidic amino acids serves to increase the efficiency of basolateral sorting mediated by the leucine residue. Hence, basolateral targeting of stem cell factor requires a mono-leucine determinant assisted by a cluster of acidic amino acids. This mono-leucine determinant is functionally conserved in colony-stimulating factor-1, a transmembrane growth factor related to stem cell factor. Furthermore, this leucine motif is not capable of inducing endocytosis, allowing for persistent cell surface expression of stem cell factor. In contrast, the mutated cytoplasmic tail found in the stem cell factor mutant Mgf(Sl17H) induces constitutive endocytosis by a motif that is related to signals for endocytosis and lysosomal targeting. Our findings therefore present mono-leucines as a novel type of protein sorting motif for transmembrane growth factors.

HIV-1 Nef blocks transport of MHC class I molecules to the cell surface via a PI 3-kinase-dependent pathway

HIV causes a chronic infection by evading immune eradication. A key element of HIV immune escape is the HIV-1 Nef protein. Nef causes a reduction in the level of cell surface major histocompatibility complex class I (MHC-I) protein expression, thus protecting HIV-infected cells from anti-HIV cytotoxic T lymphocyte (CTL) recognition and killing. Nef also reduces cell surface levels of the HIV receptor, CD4, by accelerating endocytosis. We show here that endocytosis is not required for Nef-mediated downmodulation of MHC-I molecules. The main effect of Nef is to block transport of MHC-I molecules to the cell surface, leading to accumulation in intracellular organelles. Furthermore, the effect of Nef on MHC-I molecules (but not on CD4) requires phosphoinositide 3-kinase (PI 3-kinase) activity. We propose that Nef diverts MHC-1 proteins into a PI 3-kinase-dependent transport pathway that prevents expression on the cell surface.

The molecular machinery for lysosome biogenesis

The lysosome serves as a site for delivery of materials targeted for removal from the eukaryotic cell. The mechanisms underlying the biogenesis of this organelle are currently the subject of renewed interest due to advances in our understanding of the protein sorting machinery. Genetic model systems such as yeast and Drosophila have been instrumental in identifying both protein and lipid components of this machinery. Importantly, many of these components, as well as the processes in which they are involved, are proving conserved in mammals. Other recently identified components, however, appear to be unique to higher eukaryotes. BioEssays 23:333-343, 2001. Published 2001 John Wiley & Sons, Inc.

Cloning of ACP33 as a novel intracellular ligand of CD4

CD4 recruitment to T cell receptor (TCR)-peptide-major histocompatibility class II complexes is required for stabilization of low affinity antigen recognition by T lymphocytes. The cytoplasmic portion of CD4 is thought to amplify TCR-initiated signal transduction via its association with the protein tyrosine kinase p56(lck). Here we describe a novel functional determinant in the cytosolic tail of CD4 that inhibits TCR-induced T cell activation. Deletion of two conserved hydrophobic amino acids from the CD4 carboxyl terminus resulted in a pronounced enhancement of CD4-mediated T cell costimulation. This effect was observed in the presence or absence of p56(lck), implying involvement of alternative cytosolic ligands of CD4. A two-hybrid screen with the intracellular portion of CD4 identified a previously unknown 33-kDa protein, ACP33 (acidic cluster protein 33), as a novel intracellular binding partner of CD4. Since interaction with ACP33 is abolished by deletion of the hydrophobic CD4 C-terminal amino acids mediating repression of T cell activation, we propose that ACP33 modulates the stimulatory activity of CD4. Furthermore, we demonstrate that interaction with CD4 is mediated by the noncatalytic alpha/beta hydrolase fold domain of ACP33. This suggests a previously unrecognized function for alpha/beta hydrolase fold domains as a peptide binding module mediating protein-protein interactions.

An acidic motif retains vesicular monoamine transporter 2 on large dense core vesicles

The release of biogenic amines from large dense core vesicles (LDCVs) depends on localization of the vesicular monoamine transporter VMAT2 to LDCVs. We now find that a cluster of acidic residues including two serines phosphorylated by casein kinase 2 is required for the localization of VMAT2 to LDCVs. Deletion of the acidic cluster promotes the removal of VMAT2 from LDCVs during their maturation. The motif thus acts as a signal for retention on LDCVs. In addition, replacement of the serines by glutamate to mimic phosphorylation promotes the removal of VMAT2 from LDCVs, whereas replacement by alanine to prevent phosphorylation decreases removal. Phosphorylation of the acidic cluster thus appears to reduce the localization of VMAT2 to LDCVs by inactivating a retention mechanism.

'Shed' furin: mapping of the cleavage determinants and identification of its C-terminus

The human endoprotease furin is involved in the proteolytic maturation of the precursor molecules of a wide variety of bioactive proteins. Despite its localization in the membranes of the trans-Golgi system by means of a transmembrane domain, it has repeatedly been reported to form a C-terminally truncated, naturally secreted form referred to as 'shed' furin. In order to identify the cleavage site, internal deletion mutants of increasing size, N-terminal to Leu(708), and subsequently individual amino acid substitutions were introduced, and Arg(683) was identified as the prime determinant for shedding. MS analysis determined Ser(682) as the C-terminus of shed furin, suggesting that monobasic cleavage may occur N-terminal to Arg(683). Alteration of Arg(683) directs the shedding mechanism to alternative cleaving sites previously unused.

Signaling mediated by the cytosolic domain of peptidylglycine alpha-amidating monooxygenase

The luminal domains of membrane peptidylglycine alpha-amidating monooxygenase (PAM) are essential for peptide alpha-amidation, and the cytosolic domain (CD) is essential for trafficking. Overexpression of membrane PAM in corticotrope tumor cells reorganizes the actin cytoskeleton, shifts endogenous adrenocorticotropic hormone (ACTH) from mature granules localized at the tips of processes to the TGN region, and blocks regulated secretion. PAM-CD interactor proteins include a protein kinase that phosphorylates PAM (P-CIP2) and Kalirin, a Rho family GDP/GTP exchange factor. We engineered a PAM protein unable to interact with either P-CIP2 or Kalirin (PAM-1/K919R), along with PAM proteins able to interact with Kalirin but not with P-CIP2. AtT-20 cells expressing PAM-1/K919R produce fully active membrane enzyme but still exhibit regulated secretion, with ACTH-containing granules localized to process tips. Immunoelectron microscopy demonstrates accumulation of PAM and ACTH in tubular structures at the trans side of the Golgi in AtT-20 cells expressing PAM-1 but not in AtT-20 cells expressing PAM-1/K919R. The ability of PAM to interact with P-CIP2 is critical to its ability to block exit from the Golgi and affect regulated secretion. Consistent with this, mutation of its P-CIP2 phosphorylation site alters the ability of PAM to affect regulated secretion.

Biogenesis of regulated exocytotic carriers in neuroendocrine cells

Ca(2+)-triggered exocytosis is a hallmark of neurosecretory granules, but the cellular pathway leading to the assembly of these regulated exocytotic carriers is poorly understood. Here we used the pituitary AtT-20 cell line to study the biogenesis of regulated exocytotic carriers involved in peptide hormone secretion. We show that immature secretory granules (ISGs) freshly budded from the trans-Golgi network (TGN) exhibit characteristics of unregulated exocytotic carriers. During a subsequent maturation period they undergo an important switch to become regulated exocytotic carriers. We have identified a novel sorting pathway responsible for this transition. The SNARE proteins, VAMP4 and synaptotagmin IV (Syt IV), enter ISGs initially but are sorted away during maturation. Sorting is achieved by vesicle budding from the ISGs, because it can be inhibited by brefeldin A (BFA). Inhibition of this sorting pathway with BFA arrested the maturing granules in a state that responded poorly to stimuli, suggesting that the transition to regulated exocytotic carriers requires the removal of a putative inhibitor. In support of this, we found that overexpression of Syt IV reduced the stimulus-responsiveness of maturing granules. We conclude that secretory granules undergo a switch from unregulated to regulated secretory carriers during biogenesis. The existence of such a switch may provide a mechanism for cells to modulate their secretory activities under different physiological conditions.

Identification and characterization of SNX15, a novel sorting nexin involved in protein trafficking

Sorting nexins are a family of phox homology domain containing proteins that are homologous to yeast proteins involved in protein trafficking. We have identified a novel 342-amino acid residue sorting nexin, SNX15, and a 252-amino acid splice variant, SNX15A. Unlike many sorting nexins, a SNX15 ortholog has not been identified in yeast or Caenorhabditis elegans. By Northern blot analysis, SNX15 mRNA is widely expressed. Although predicted to be a soluble protein, both endogenous and overexpressed SNX15 are found on membranes and in the cytosol. The phox homology domain of SNX15 is required for its membrane association and for association with the platelet-derived growth factor receptor. We did not detect association of SNX15 with receptors for epidermal growth factor or insulin. However, overexpression of SNX15 led to a decrease in the processing of insulin and hepatocyte growth factor receptors to their mature subunits. Immunofluorescence studies showed that SNX15 overexpression resulted in mislocalization of furin, the endoprotease responsible for cleavage of insulin and hepatocyte growth factor receptors. Based on our data and the existing findings with yeast orthologs of other sorting nexins, we propose that overexpression of SNX15 disrupts the normal trafficking of proteins from the plasma membrane to recycling endosomes or the trans-Golgi network.

Distribution and function of AP-1 clathrin adaptor complexes in polarized epithelial cells

Expression of the epithelial cell-specific heterotetrameric adaptor complex AP-1B is required for the polarized distribution of many membrane proteins to the basolateral surface of LLC-PK1 kidney cells. AP-1B is distinguished from the ubiquitously expressed AP-1A by exchange of its single 50-kD mu subunit, mu1A, being replaced by the closely related mu1B. Here we show that this substitution is sufficient to couple basolateral plasma membrane proteins, such as a low-density lipoprotein receptor (LDLR), to the AP-1B complex and to clathrin. The interaction between LDLR and AP-1B is likely to occur in the trans-Golgi network (TGN), as was suggested by the localization of functional, epitope-tagged mu1 by immunofluorescence and immunoelectron microscopy. Tagged AP-1A and AP-1B complexes were found in the perinuclear region close to the Golgi complex and recycling endosomes, often in clathrin-coated buds and vesicles. Yet, AP-1A and AP-1B localized to different subdomains of the TGN, with only AP-1A colocalizing with furin, a membrane protein that uses AP-1 to recycle between the TGN and endosomes. We conclude that AP-1B functions by interacting with its cargo molecules and clathrin in the TGN, where it acts to sort basolateral proteins from proteins destined for the apical surface and from those selected by AP-1A for transport to endosomes and lysosomes.

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.

Cytoplasmic domain of herpes simplex virus gE causes accumulation in the trans-Golgi network, a site of virus envelopment and sorting of virions to cell junctions

Alphaherpesviruses express a heterodimeric glycoprotein, gE/gI, that facilitates cell-to-cell spread between epithelial cells and neurons. Herpes simplex virus (HSV) gE/gI accumulates at junctions formed between polarized epithelial cells at late times of infection. However, at earlier times after HSV infection, or when gE/gI is expressed using virus vectors, the glycoprotein localizes to the trans-Golgi network (TGN). The cytoplasmic (CT) domains of gE and gI contain numerous TGN and endosomal sorting motifs and are essential for epithelial cell-to-cell spread. Here, we swapped the CT domains of HSV gE and gI onto another HSV glycoprotein, gD. When the gD-gI(CT) chimeric protein was expressed using a replication-defective adenovirus (Ad) vector, the protein was found on both the apical and basolateral surfaces of epithelial cells, as was gD. By contrast, the gD-gE(CT) chimeric protein, gE/gI, and gE, when expressed by using Ad vectors, localized exclusively to the TGN. However, gD-gE(CT), gE/gI, and TGN46, a cellular TGN protein, became redistributed largely to lateral surfaces and cell junctions during intermediate to late stages of HSV infection. Strikingly, gE and TGN46 remained sequestered in the TGN when cells were infected with a gI(-)HSV mutant. The redistribution of gE/gI to lateral cell surfaces did not involve widespread HSV inhibition of endocytosis because the transferrin receptor and gE were both internalized from the cell surface. Thus, gE/gI accumulates in the TGN in early phases of HSV infection then moves to lateral surfaces, to cell junctions, at late stages of infection, coincident with the redistribution of a TGN marker. These results are related to recent observations that gE/gI participates in the envelopment of nucleocapsids into cytoplasmic vesicles (A. R. Brack, B. G. Klupp, H. Granzow, R. Tirabassi, L. W. Enquist, and T. C. Mettenleiter, J. Virol. 74:4004-4016, 2000) and that gE/gI can sort nascent virions from cytoplasmic vesicles specifically to the lateral surfaces of epithelial cells (D. C. Johnson, M. Webb, T. W. Wisner, and C. Brunetti, J. Virol. 75:821-833, 2000). Therefore, gE/gI localizes to the TGN, through interactions between the CT domain of gE and cellular sorting machinery, and then participates in envelopment of cytosolic nucleocapsids there. Nascent virions are then sorted from the TGN to cell junctions.

Living in oblivion: HIV immune evasion

The human and simian immunodeficiency viruses (HIV and SIV, respectively) are members of the lentiviridae subgroup of retroviruses that cause a progressive failure of the host immunological functions culminating in the clinical collapse known as AIDS, or acquired immunodeficiency syndrome. In the absence of antiviral therapy, this course is inexorable in spite of an initially vigorous immune response. Two fundamental characteristics of the biology of primate lentiviruses explain this apparent paradox. First, HIV and SIV infect CD4(+)targets such as helper T lymphocytes and macrophages, that is, cells that normally play an essential role in the emergence and maintenance of an effective antiviral response. Second, these viruses have evolved a number of strategies to evade control by the immune system. These include mutational escape, latency, masking of antibody-binding sites on the viral envelope, downmodulation of the class I major histocompatibility complex (MHC-I), and upregulation of the Fas ligand on the surface of infected cells. Examining the mechanisms of these phenomena not only helps to understand how HIV wins its war against the immune system, but it also suggests as yet unexploited avenues to combat the virus through therapies and to develop a vaccine.