The human and simian immunodeficiency virus envelope glycoprotein transmembrane subunits are palmitoylated

Envelope glycoprotein cytoplasmic domains from diverse lentiviruses interact with the prenylated Rab acceptor

Lentivirus envelope glycoproteins have unusually long cytoplasmic domains compared to those of other retroviruses. To identify cellular binding partners of the simian immunodeficiency virus (SIV) envelope transmembrane protein (gp41) cytoplasmic domain (CD), we performed a yeast two-hybrid screen of a phytohemagglutinin-activated human T-cell cDNA library with the SIV gp41 CD. The majority of positive clones (50 of 54) encoded the prenylated Rab acceptor (PRA1). PRA1 is a 21-kDa protein associated with Golgi membranes that binds to prenylated Rab proteins in their GTP-bound state. While the cellular function of PRA1 is presently unknown, this protein appears to participate in intracellular vesicular trafficking, based on its cellular localization and ability to bind multiple members of the Rab protein family. Mammalian two-hybrid assays confirmed the interaction between the SIV gp41 CD and PRA1. Furthermore, gp41 sequences important for PRA1 binding were mapped to a central leucine-rich, amphipathic alpha-helix in the SIV gp41 cytoplasmic tail. Although the human immunodeficiency virus (HIV-1) gp41 CD failed to interact with PRA1 in the yeast two-hybrid system, its interaction with PRA1 was significantly better than that of the SIV gp41 CD in mammalian two-hybrid assays. Interestingly, PRA1 also interacted with the Env CDs of HIV-2, bovine immunodeficiency virus, equine infectious anemia virus, and feline immunodeficiency virus. Thus, PRA1 associates with envelope glycoproteins from widely divergent lentiviruses.

Efficient assembly of an HIV-1/MLV Gag-chimeric virus in murine cells

In human cells infected by HIV type 1 (HIV-1), the viral Gag protein directs the assembly of nascent viral particles at the plasma membrane. In murine cells, HIV-1 Gag fails to reach the plasma membrane and instead forms nonfunctional intracellular aggregates. The viral determinants of this species incompatibility are previously undefined. To address this problem, we replaced a region of HIV-1 Gag known to direct its localization, the matrix (MA) domain, with functionally homologous regions from Moloney murine leukemia virus (MLV), a murine retrovirus. An HIV-1 clone carrying such a chimeric Gag protein, designated murine HIV (MHIV), assembled more efficiently than nonchimeric HIV-1 and restored plasma membrane localization of Gag in murine cells. Increased efficiency of viral assembly in murine cells was observed from MHIV constructs carrying MLV MA in place of HIV-1 MA. Efficient processing of the HIV-1 capsid protein from the chimeric Gag polyprotein and subsequent infectivity of MHIV required the presence of MLV p12 in addition to MLV MA. These findings strongly suggest that the HIV-1 MA domain of HIV-1 Gag is responsible for the assembly defect in mouse cells. Although these MHIV do not recruit native HIV-1 Env efficiently, they are capable of single-round infection when produced by high-efficiency transfection of human 293 cells and provided with an HIV-1 Env lacking its cytoplasmic tail. With further adaptation, this chimeric MHIV approach may provide the basis for creating an infectious mouse model for HIV/AIDS.

Palmitoylation of the Rous sarcoma virus transmembrane glycoprotein is required for protein stability and virus infectivity

The Rous sarcoma virus (RSV) transmembrane (TM) glycoprotein is modified by the addition of palmitic acid. To identify whether conserved cysteines within the hydrophobic anchor region are the site(s) of palmitoylation, and to determine the role of acylation in glycoprotein function, cysteines at residues 164 and 167 of the TM protein were mutated to glycine (C164G, C167G, and C164G/C167G). In CV-1 cells, palmitate was added to env gene products containing single mutations but was absent in the double-mutant Env. Although mutant Pr95 Env precursors were synthesized with wild-type kinetics, the phenotypes of the mutants differed markedly. Env-C164G had properties similar to those of the wild type, while Env-C167G was degraded faster, and Env containing the double mutant C164G/C167G was very rapidly degraded. Degradation occurred after transient plasma membrane expression. The decrease in steady-state surface expression and increased rate of internalization into endosomes and lysosomes paralleled the decrease in palmitoylation observed for the mutants. The phenotypes of mutant viruses were assessed in avian cells in the context of the pATV8R proviral genome. Virus containing the C164G mutation replicated with wild-type kinetics but exhibited reduced peak reverse transcriptase levels. In contrast, viruses containing either the C167G or the C164G/C167G mutation were poorly infectious or noninfectious, respectively. These phenotypes correlated with different degrees of glycoprotein incorporation into virions. Infectious revertants of the double mutant demonstrated the importance of cysteine-167 for efficient plasma membrane expression and Env incorporation. The observation that both cysteines within the membrane-spanning domain are accessible for acylation has implications for the topology of this region, and a model is proposed.

Cellular membrane-binding ability of the C-terminal cytoplasmic domain of human immunodeficiency virus type 1 envelope transmembrane protein gp41

The amphipathic alpha-helices located in the cytoplasmic tail of the envelope (Env) transmembrane glycoprotein gp41 of human immunodeficiency virus type 1 have been implicated in membrane association and cytopathicity. Deletion of the last 12 amino acids in the C terminus of this domain severely impairs infectivity. However, the nature of the involvement of the cytoplasmic tail in Env-membrane interactions in cells and the molecular basis for the defect in infectivity of this mutant virus are still poorly understood. In this study we examined the interaction of the cytoplasmic tail with membranes in living mammalian cells by expressing a recombinant cytoplasmic tail fragment and an Escherichia coli beta-galactosidase/cytoplasmic tail fusion protein, both of them lacking gp120, the gp41 ectodomain, and the transmembrane region. We found through cell fractionation, in vivo membrane flotation, and confocal immunofluorescence studies that the cytoplasmic tail contained determinants to be routed to a perinuclear membrane region in cells. Further mapping showed that each of the three lentivirus lytic peptide (LLP-1, LLP-2, and LLP-3) sequences conferred this cellular membrane-targeting ability. Deletion of the last 12 amino acids from the C terminus abolished the ability of the LLP-1 motif to bind to membranes. High salt extraction, in vitro transcription and translation, and posttranslational membrane binding analyses indicated that the beta-galactosidase/LLP fusion proteins were inserted into membranes via the LLP sequences. Subcellular fractionation and confocal microscopy studies revealed that each of the LLP motifs, acting in a position-independent manner, targeted non-endoplasmic reticulum (ER)-associated beta-galactosidase and enhanced green fluorescence protein to the ER. Our study provides a basis for the involvement of the gp41 cytoplasmic tail during Env maturation and also supports the notion that the membrane apposition of the C-terminal cytoplasmic tail plays a crucial role in virus-host interaction.

Multimerization of human immunodeficiency virus type 1 Gag promotes its localization to barges, raft-like membrane microdomains

The Gag polyprotein of human immunodeficiency virus type 1 (HIV-1) organizes the assembly of nascent virions at the plasma membrane of infected cells. Here we demonstrate that a population of Gag is present in distinct raft-like membrane microdomains that we have termed "barges." Barges have a higher density than standard rafts, most likely due to the presence of oligomeric Gag-Gag assembly complexes. The regions of the Gag protein responsible for barge targeting were mapped by examining the flotation behavior of wild-type and mutant proteins on Optiprep density gradients. N-myristoylation of Gag was necessary for association with barges. Removal of the NC and p6 domains shifted much of the Gag from barges into typical raft fractions. These data are consistent with a model in which multimerization of myristoylated Gag proteins drives association of Gag oligomers into raft-like barges. The functional significance of barge association was revealed by several lines of evidence. First, Gag isolated from virus-like particles was almost entirely localized in barges. Moreover, a comparison of wild-type Gag with Fyn(10)Gag, a chimeric protein containing the N-terminal sequence of Fyn, revealed that Fyn(10)Gag exhibited increased affinity for barges and a two- to fourfold increase in particle production. These results imply that association of Gag with raft-like barge membrane microdomains plays an important role in the HIV-1 assembly process.

Lipid rafts and pseudotyping

Specific interactions between envelope and core proteins govern the membrane assembly of most enveloped viruses. Despite this, mixed infections lead to pseudotyping, the association of the viral cores of one virus with the envelopes of another. How does this occur? We show here that the detergent-insoluble lipid rafts of the plasma membrane function as a natural meeting point for the transmembrane and core components of a phylogenetically diverse collection of enveloped viruses. As a result, viral particles preferentially incorporate both the envelope components of other viruses as well as the extra- and intracellular constituents of host cell lipid rafts, including gangliosides, glycosyl phosphatidylinositol-anchored surface proteins, and intracellular signal transduction molecules. Pharmacological disruption of lipid rafts interferes with virus production.

Deacylation of the transmembrane domains of Sindbis virus envelope glycoproteins E1 and E2 does not affect low-pH-induced viral membrane fusion activity

The envelope glycoproteins E1 and E2 of Sindbis virus are palmitoylated at cysteine residues within their transmembrane domains (E1 at position 430, and E2 at positions 388 and 390). Here, we investigated the in vitro membrane fusion activity of Sindbis virus variants (derived from the Toto 1101 infectious clone), in which the E1 C430 and/or E2 C388/390 residues had been substituted for alanines. Both the E1 and E2 mutant viruses, as well as a triple mutant virus, fused with liposomes in a strictly low-pH-dependent manner, the fusion characteristics being indistinguishable from those of the parent Toto 1101 virus. These results demonstrate that acylation of the transmembrane domain of Sindbis virus E1 and E2 is not required for expression of viral membrane fusion activity.

Mode of action of an antiviral peptide from HIV-1. Inhibition at a post-lipid mixing stage

DP178, a synthetic peptide corresponding to a segment of the transmembrane envelope glycoprotein (gp41) of human immunodeficiency virus, type 1 (HIV-1), is a potent inhibitor of viral infection and virus-mediated cell-cell fusion. Nevertheless, DP178 does not contain gp41 coiled-coil cavity binding residues postulated to be essential for inhibiting HIV-1 entry. We find that DP178 inhibits phospholipid redistribution mediated by the HIV-1 envelope glycoprotein at a concentration 8 times greater than that of solute redistribution (the IC(50) values are 43 and 335 nm, respectively). In contrast, C34, a synthetic peptide which overlaps with DP178 but contains the cavity binding residues, did not show this phenomenon (11 and 25 nm, respectively). The ability of DP178 to inhibit membrane fusion at a post-lipid mixing stage correlates with its ability to bind and oligomerize on the surface of membranes. Furthermore, our results are consistent with a model in which DP178 inhibits the formation of gp41 viral hairpin structure at low affinity, whereas C34 inhibits its formation at high affinity: the failure to form the viral hairpin prevents both lipid and solute from redistributing between cells. However, our data also suggest an additional membrane-bound inhibitory site for DP178 in the ectodomain of gp41 within a region immediately adjacent to the membrane-spanning domain. By binding to this higher affinity site, DP178 inhibits the recruitment of several gp41-membrane complexes, thus inhibiting fusion pore formation.

Organization of immature human immunodeficiency virus type 1

Immature retrovirus particles contain radially arranged Gag polyproteins in which the N termini lie at the membrane and the C termini extend toward the particle's center. We related image features to the polyprotein domain structure by combining mutagenesis with cryoelectron microscopy and image analysis. The matrix (MA) domain appears as a thin layer tightly associated with the inner face of the viral membrane, separated from the capsid (CA) layer by a low-density region corresponding to its C terminus. Deletion of the entire p6 domain has no effect on the width or spacing of the density layers, suggesting that p6 is not ordered in immature human immunodeficiency virus type 1 (HIV-1). In vitro assembly of a recombinant Gag polyprotein containing only capsid (CA) and nucleocapsid (NC) domains results in the formation of nonenveloped spherical particles which display two layers with density matching that of the CA-NC portion of immature HIV-1 Gag particles. Authentic, immature HIV-1 displays additional surface features and an increased density between the lipid bilayers which reflect the presence of gp41. The other internal features match those of virus-like particles.

Palmitoylation of the HIV-1 envelope glycoprotein is critical for viral infectivity

Recent studies suggest that HIV-1 budding occurs selectively from detergent-insoluble membrane domains, referred to as lipid rafts. Palmitoylation is thought to be one of the factors responsible for targeting membrane proteins to lipid rafts. The cytoplasmic domain of the HIV-1 envelope glycoprotein (gp160) contains two palmitoylated cysteine residues. In this work, we studied the solubility of gp160 after detergent extraction. We show that wild-type gp160 is mostly insoluble after ice-cold Triton X-100 extraction, but that it becomes almost completely soluble at 37 degrees C. In contrast, we find that a mutant gp160, in which the two palmitoylated cysteine residues are replaced by serine, is Triton X-100 soluble even under ice-cold extraction. These findings are consistent with the properties of proteins that localize to lipid rafts and strongly suggest that gp160 is associated with lipid rafts. Further, removal of both palmitoylation sites results in the formation of virus with low levels of gp160 incorporation as well as a decrease in viral infectivity by 60-fold. Our results strongly support the suggestion that HIV-1 buds from lipid rafts and point to a role for rafts as a viral assembly hub.

Genetic evidence for an interaction between human immunodeficiency virus type 1 matrix and alpha-helix 2 of the gp41 cytoplasmic tail

The incorporation of envelope (Env) glycoproteins into virions is an essential step in the retroviral replication cycle. Lentiviruses, including human immunodeficiency virus type 1 (HIV-1), encode Env glycoproteins with unusually long cytoplasmic tails, the functions of which have not been fully elucidated. In this study, we examine the effects on virus replication of a number of mutations in a helical motif (alpha-helix 2) located near the center of the HIV-1 gp41 cytoplasmic tail. We find that, in T-cell lines, small deletions in this domain disrupt the incorporation of Env glycoproteins into virions and markedly impair virus infectivity. Through the analysis of viral revertants, we demonstrate that a single amino acid change (34VI) in the matrix domain of Gag reverses the Env incorporation and infectivity defect imposed by a small deletion near the C terminus of alpha-helix 2. These results provide genetic evidence, in the context of infected T cells, for an interaction between HIV-1 matrix and the gp41 cytoplasmic tail and identify domains of both proteins involved in this putative interaction.

Evidence for budding of human immunodeficiency virus type 1 selectively from glycolipid-enriched membrane lipid rafts

A number of recent studies have demonstrated the significance of detergent-insoluble, glycolipid-enriched membrane domains or lipid rafts, especially in regard to activation and signaling in T lymphocytes. These domains can be viewed as floating rafts composed of sphingolipids and cholesterol which sequester glycosylphosphatidylinositol (GPI)-linked proteins, such as Thy-1 and CD59. CD45, a 200-kDa transmembrane phosphatase protein, is excluded from these domains. We have found that human immunodeficiency virus type 1 (HIV-1) particles produced by infected T-cell lines acquire the GPI-linked proteins Thy-1 and CD59, as well as the ganglioside GM1, which is known to partition preferentially into lipid rafts. In contrast, despite its high expression on the cell surface, CD45 was poorly incorporated into virus particles. Confocal fluorescence microscopy revealed that HIV-1 proteins colocalized with Thy-1, CD59, GM1, and a lipid raft-specific fluorescent lipid, DiIC(16)(3), in uropods of infected Jurkat cells. CD45 did not colocalize with HIV-1 proteins and was excluded from uropods. Dot immunoassay of Triton X-100-extracted membrane fractions revealed that HIV-1 p17 matrix protein and gp41 were present in the detergent-resistant fractions and that [(3)H]myristic acid-labeled HIV Gag showed a nine-to-one enrichment in lipid rafts. We propose a model for the budding of HIV virions through lipid rafts whereby host cell cholesterol, sphingolipids, and GPI-linked proteins within these domains are incorporated into the viral envelope, perhaps as a result of preferential sorting of HIV Gag to lipid rafts.

A new class of fusion-associated small transmembrane (FAST) proteins encoded by the non-enveloped fusogenic reoviruses

The non-enveloped fusogenic avian and Nelson Bay reoviruses encode homologous 10 kDa non-structural transmembrane proteins. The p10 proteins localize to the cell surface of transfected cells in a type I orientation and induce efficient cell-cell fusion. Mutagenic studies revealed the importance of conserved sequence-predicted structural motifs in the membrane association and fusogenic properties of p10. These motifs included a centrally located transmembrane domain, a conserved cytoplasmic basic region, a small hydrophobic motif in the N-terminal domain and four conserved cysteine residues. Functional analysis indicated that the extreme C-terminus of p10 functions in a sequence-independent manner to effect p10 membrane localization, while the N-terminal domain displays a sequence-dependent effect on the fusogenic property of p10. The small size, unusual arrangement of structural motifs and lack of any homologues in previously described membrane fusion proteins suggest that the fusion-associated small transmembrane (FAST) proteins of reovirus represent a new class of membrane fusion proteins.

Expanded host cell tropism and cytopathic properties of feline immunodeficiency virus strain PPR subsequent to passage through interleukin-2-independent T cells

A cytopathic variant of feline immunodeficiency virus (FIV) strain PPR emerged after passage of wild-type virus on an interleukin-2-independent cell line. The virus, termed FIV-PPRglial, displayed a phenotype markedly different from the parental virus, including the ability to productively infect previously refractory cell lines, induction of large syncytia, and accelerated kinetic properties. A chimeric molecular clone, FIV-PPRchim42, containing the FIV-PPRglial envelope within the backbone of FIV-PPR, exhibited all the characteristics of the FIV-PPRglial phenotype, demonstrating that the viral envelope was responsible for the acquired traits. Subsequent molecular characterization revealed that the FIV-PPRglial envelope contained five amino acid substitutions relative to wild-type FIV-PPR. Mutagenic analyses further demonstrated that the acquired phenotype was minimally attributable to a combination of three mutations, specifically, a glutamine-to-proline change within the second constant domain of the surface protein (SU); a threonine-to-proline change within the V4 loop, also in the SU; and a premature stop codon in the cytoplasmic tail of the transmembrane protein. All three changes were required to produce the FIV-PPRglial phenotype. Cotransfection studies with mutant viruses in combination with each other and with FIV-PPR indicated that the truncated cytoplasmic tail was responsible for the induction of syncytium formation. Receptor usage analyses were pursued, and distinctions were observed between FIV-PPR and FIV-PPRglial. In vitro infections with FIV-PPR, FIV-PPRglial, and FIV-34TF10 on two adherent cell lines were ablated in the presence of SDF1alpha, the natural ligand for CXCR4. In contrast, viral infection of T cells was not limited to CXCR4 usage, and inhibition studies indicate the potential involvement of a CC chemokine receptor.

Alkylation of cysteine-containing peptides to mimic palmitoylation

Numerous proteins that are involved in cell signaling and viral replication require post-translational modification by palmitoylation to function properly. The molecular details by which this palmitoyl modification affects protein function remain poorly understood. To facilitate in vitro biochemical and structural studies of the role of palmitoylation on protein function, a method was developed for alkylating peptides with saturated C16 groups at cysteine residues and demonstrated using peptides derived from the palmitoylated region of Sindbis virus E2 glycoprotein. The synthetic approach takes advantage of disulfide chemistry to specifically modify only the cysteine residues within peptides and covalently links C16 groups via disulfide bridges using a new thioalkylating reagent, hexyldexyldithiopyridine. The chemistry presented here takes place in solution under mild conditions without the need for protection of the peptide functional groups. A method for purifying these modified peptides is also described. This protocol can be of general use to investigators studying the role of palmitoylation in biological systems.

Palmitoylation of the intracytoplasmic R peptide of the transmembrane envelope protein in Moloney murine leukemia virus

Previously it was reported that the 16-amino-acid (aa) C-terminal cytoplasmic tail of Moloney murine leukemia virus (MoMLV) transmembrane protein Pr15E is cleaved off during virus synthesis, yielding the mature, fusion active transmembrane protein p15E and the 16-aa peptide (R peptide or p2E). It remains to be elucidated how the R peptide impairs fusion activity of the uncleaved Pr15E. The R peptide from MoMLV was analyzed by Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunostained with antiserum against the synthetic 16-aa R peptide. The R peptide resolved with an apparent molecular mass of 7 kDa and not the 4 kDa seen with the corresponding synthetic peptide. The 7-kDa R peptide was found to be membrane bound in MoMLV-infected NIH 3T3 cells, showing that cleavage of the 7-kDa R-peptide tail must occur before or during budding of progeny virions, in which only small amounts of the 7-kDa R peptide were found. The 7-kDa R peptide was palmitoylated since it could be labeled with [(3)H]palmitic acid, which explains its membrane association, slower migration on gels, and high sensitivity in immunoblotting. The present results are in contrast to previous findings showing equimolar amounts of R peptide and p15E in virions. The discrepancy, however, can be explained by the presence of nonpalmitoylated R peptide in virions, which were poorly detected by immunoblotting. A mechanistic model is proposed. The uncleaved R peptide can, due to its lipid modification, control the conformation of the ectodomain of the transmembrane protein and thereby govern membrane fusion.

Palmitoylation of the three isoforms of human endothelin-converting enzyme-1

Endothelin-converting enzyme-1 (ECE-1) is a membrane-bound metalloprotease that catalyses the conversion of inactive big endothelins into active endothelins. Here we have examined whether the three isoforms of human ECE-1 (ECE-1a, ECE-1b and ECE-1c) are modified by the covalent attachment of the fatty acid palmitate and have evaluated a potential functional role of this modification. To do this, wild-type and mutant enzymes were expressed and analysed by metabolic labelling with [3H]palmitate, immunoprecipitation and SDS/PAGE. All three ECE-1 isoforms were found to be palmitoylated via hydroxylamine-sensitive thioester bonds. In addition, the isoforms showed similar levels of acylation. Cys46 in ECE-1a, Cys58 in ECE-1b and Cys42 in ECE-1c were identified as sites of palmitoylation and each of these cysteines accounted for all the palmitoylation that occured in the corresponding isoform. Immunofluorescence analysis demonstrated further that palmitoylated and non-palmitoylated ECE-1 isoforms had the same subcellular localizations. Moreover, complete solubility of the three isoforms in Triton X-100 revealed that palmitoylation does not target ECE-1 to cholesterol and sphingolipid-rich membrane domains or caveolae. The enzymic activities of ECE-1a, ECE-1b and ECE-1c were also not significantly affected by the absence of palmitoylation.

Effects of site-directed mutations of transmembrane cysteines in sindbis virus E1 and E2 glycoproteins on palmitylation and virus replication

The two glycoproteins that form the external spikes of the alphaviruses are type 1 membrane proteins whose transmembrane domains of hydrophobic amino acids are close to the carboxyl termini of the polypeptides and anchor the proteins in the lipid bilayer. Most of the members of the alphavirus genus contain within this transmembrane sequence one or more highly conserved cysteines, which are positioned close to the cytoplasmic face of the lipid bilayer. Cysteines in the cytoplasmic domains of the alphavirus glycoproteins and other enveloped viruses have been shown to be modified by palmitylation. To determine whether the transmembranal cysteines in Sindbis virus also were palmitylated, we used site-directed mutation to change the single transmembranal cysteine in the E1 glycoprotein and two of the transmembranal cysteines in the E2 glycoprotein to alanines. Transfection of RNA transcribed from the differently mutated Sindbis virus cDNAs led to production of infectious virus. Cells infected with the mutant virions and labeled with [3H]-palmitic acid showed that the E1 mutant no longer contained fatty acid in the E1 glycoprotein and that the extent of palmitylation was reduced about twofold in the E2 glycoprotein of virions containing the E2 mutations. At early times postinfection, the mutants grew slightly slower than the wild type in cultures of chicken embryo fibroblasts and secreted about half the amount of virus particles as wild type, but little difference was found at later time points. A triple mutant containing both the E1 and E2 mutations formed virions deficient in palmitylation of both glycoproteins, and this mutant had growth properties that were similar to those of the independent E1 and E2 mutants. Virions with the mutated glycoproteins that were deficient in fatty acid were more susceptible than the wild-type virions to inactivation by the detergent Triton X-100.

Adenovirus death protein, a transmembrane protein encoded in the E3 region, is palmitoylated at the cytoplasmic tail

The 11.6-K protein of human adenovirus 2 (Ad2), which was recently renamed as adenovirus death protein (ADP), is a type III membrane glycoprotein that ultimately localizes to the nuclear membrane. ADP is encoded in the E3 transcription unit of Ad2 and migrates as a set of multiple bands in SDS-PAGE with three major forms. The corresponding gene product of adenovirus 5 (Ad5) has a slightly lower molecular weight and shows the same pattern in SDS-PAGE. We report here the covalent attachment of fatty acids to cysteine residues of ADP. In the case of Ad5-ADP all three major forms of this protein can be labeled by [3H]palmitic acid, but not by [3H]myristic acid, whereas only two [3H]palmitic acid-labeled Ad2-ADP species could be detected. The label is sensitive to treatment with 1 M hydroxylamine at pH 7 and with 20% beta-mercaptoethanol indicating that the fatty acids are linked via a thioester bond. By thin layer chromatography, the vast majority of the incorporated label was identified as palmitic acid. Two cysteine residues at the boundary between transmembrane domain and cytoplasmic tail which could serve as acceptor sites were mutated to alanine residues by site-directed mutagenesis of the cloned Ad5-ADP gene. Expression of wild-type Ad5-ADP and the resulting mutants was performed in HeLa cells using the vaccinia virus T7 expression system. As demonstrated by labeling with [3H]palmitic acid, only the mutants with one remaining cysteine residue in the cytoplasmic tail were able to incorporate [3H]palmitic acid, indicating that either could serve as acceptor site. In contrast the double cysteine mutant could not be labeled by [3H]palmitic acid, clearly demonstrating that cysteines 53 and 54 are required for palmitoylation and probably represent the palmitoylation sites in Ad5-ADP.

Function of the KKXX motif in endoplasmic reticulum retrieval of a transmembrane protein depends on the length and structure of the cytoplasmic domain

Transmembrane glycoproteins with type 1 topology can be retrieved to the endoplasmic reticulum (ER) by a retrieval signal containing a di-lysine (KK) motif near the C terminus. To investigate the structural requirements for ER retrieval, we have constructed mutants of the simian immunodeficiency virus (SIV) envelope (Env) protein with cytoplasmic tails of different lengths and containing a KK motif at the -3 and -4 positions. Such proteins were found to be retained intracellularly when the signal was located 18 amino acids or more away from the membrane spanning domain. The retrieval signal was found to be functional even when placed at the distal end of the wild-type SIV Env protein with 164 amino acids in the cytoplasmic tail, as shown by the lack of proteolytic processing and lack of cell surface expression of the mutant proteins. However, proteins with a cytoplasmic tail length of 13 amino acids or less having the di-lysine motif at the -3 and -4 positions were not retrieved to the ER since they were found to be processed and transported to the cell surface. The surface-expressed proteins were found to be functional in inducing cell fusion, whereas the proteins retained intracellularly were defective in fusion activity. We also found that the KK motif introduced near an amphipathic helical region in the cytoplasmic tail was not functional. These results demonstrate that the ability of the KK motif to cause protein retrieval and retention in the endoplasmic reticulum depends on the length and structure of the cytoplasmic domain. The ER retrieval of the mutant proteins was found to correlate with increased intracellular binding to beta COP proteins.

Budding of enveloped viruses from the plasma membrane

Many enveloped viruses are released from infected cells by maturing and budding at the plasma membrane. During this process, viral core components are incorporated into membrane vesicles that contain viral transmembrane proteins, termed 'spike' proteins. For many years these spike proteins, which are required for infectivity, were believed to be incorporated into virions via a direct interaction between their cytoplasmic domains and viral core components. More recent evidence shows that, while such direct interactions drive budding of alphaviruses, this may not be the case for negative strand RNA viruses and retroviruses. These viruses can bud particles in the absence of spike proteins, using only viral core components to drive the process. In some cases the spike proteins, without the viral core, can be released as virus-like particles. Optimal budding and release may, therefore, depend on a 'push-and-pull' concerted action of core and spike, where oligomerization of both components plays a crucial role.

HIV/SIV glycoproteins: structure-function relationships

The various functions of human (HIV) and simian (SIV) immunodeficiency virus glycoproteins are similar, so it may be assumed that the overall structure of the folded proteins will be maintained. To preserve structure there must be constraints on sequence variation. The majority of mutations tolerated will be involved in immune escape but changes at some positions are known to have direct effects on glycoprotein expression and function. This allows the virus to change its phenotype and escape immune pressure. These properties will influence the fitness of the virus to infect and replicate in potential hosts. A better understanding of the structure-function relationships of HIV/SIV glycoproteins will assist in the development of vaccines and antivirals. Here, we identify similarities and differences between HIV-1 subtypes and HIV/SIV types that may be relevant to the phenotypes of the various groups. The results are discussed in relation to what is known of domain-function associations for HIV/SIV glycoproteins.

The cytopathicity of a simian immunodeficiency virus Mne variant is determined by mutations in Gag and Env

Previous studies suggested that the rapidly replicating, highly cytopathic, syncytium-inducing (rapid-high/SI) phenotype of simian immunodeficiency virus Mne variants that evolved in macaques inoculated with a slowly replicating, minimally cytopathic, non-syncytium-inducing (slow-low/NSI) molecular clone was not solely the result of changes in the envelope surface protein (Env SU). To define the viral determinants responsible for the change in phenotype, we molecularly cloned a rapid-high/SI variant (designated SIVMne170) derived from the peripheral blood mononuclear cells (PBMCs) of a pig-tailed macaque that was inoculated with a slow-low/NSI molecular clone, SIVMneCL8. SIVMne170 was SI and replicated with faster kinetics and was more cytopathic than the parent SIVMneCL8 in CEMx174 cells. Additionally, SIVMne170 was more cytopathic for the CD4+ T-cell population than SIVMneCL8 in macaque PBMCs. An analysis of chimeric viruses constructed between the variant SIVMne170 and the parent virus SIVMneCL8 demonstrated that there are determinants encoded within both the 5' and 3' halves of SIVMne170 that independently contribute to its rapid-high/SI phenotype. As we previously observed with other SIVMne variants, the Env SU of SIVMne170 was important for syncytium induction but was not a key determinant of cytopathicity. By contrast, the intracellular domain of the envelope transmembrane protein (Env TM) contributed to both the SI and cytopathic properties of SIVMne170. We also found that the minimal determinant within the 5' half of SIVMne170 that conferred its rapid replication kinetics and cytopathicity mapped to the capsid- and nucleocapsid-encoding regions of gag. Together, these data demonstrate that mutations selected in Gag and Env TM intracytoplasmic tail influence the replication and cytopathicity of SIVMne variants that evolve in the host.

Characterization of posttranslational modifications of human A33 antigen, a novel palmitoylated surface glycoprotein of human gastrointestinal epithelium

Monoclonal antibody (mAb) A33 recognizes a differentiation antigen (A33) expressed in normal human gastrointestinal epithelium and in 95% of human colon cancers. Murine mAb A33 shows specific targeting of colon cancer in humans and a humanized A33 antibody is currently being evaluated in the clinic. The cDNA for the human A33 antigen has recently been cloned, and sequence comparison indicated that the A33 antigen is a novel human cell surface molecule of the immunoglobulin superfamily. Because mAb A33 recognizes a conformational epitope, only a partial characterization of the A33 antigen has been carried out to date. In this report we show that the A33 antigen is (I) N-glycosylated, containing approximately 8 K of N-linked carbohydrate and there is no evidence for O-glycosylation, sialylation or glycophosphatidylinositol, and (ii) S-acylated in vitro, incorporating [3H] palmitic acid linked through a hydroxylamine-sensitive thioester bond. The S-palmitoylation may be involved in regulating the internalization process initiated by binding of mAb A33 to cell surface A33 antigen.

Palmitylation of the vaccinia virus 37-kDa major envelope antigen. Identification of a conserved acceptor motif and biological relevance

Computer-assisted alignment of known palmitylproteins was used to identify a potential peptide motif, TMDX1-12AAC(C)A (TMD, transmembrane domain; X, any amino acid; C, cysteine acceptor residues; A, aliphatic residue) responsible for directing internal palmitylation of the vaccinia virus 37-kDa major envelope antigen, p37. Site-directed mutagenesis was used to confirm this motif as the site of modification and to produce a nonpalmitylated version of the p37 protein. Comparative phenotypic analysis of the wild-type and mutant p37 alleles confirmed that the p37 protein is involved in viral envelopment and egress, and suggested that attachment of the palmitate moiety was essential for correct intracellular targeting and protein function.

Amphipathic domains in the C terminus of the transmembrane protein (gp41) permeabilize HIV-1 virions: a molecular mechanism underlying natural endogenous reverse transcription

Reverse transcription of HIV-1, without detergent or amphipathic peptide-induced permeability of the viral envelope, has been demonstrated to occur in the intact HIV-1 virion. In this report, we demonstrate that the amphipathic domains in the C terminus of the transmembrane glycoprotein (gp41) account for the natural permeability of the HIV-1 envelope to deoxyribonucleoside triphosphates, the substrates for DNA polymerization. In addition, nonphysiological deoxyribonucleoside triphosphates, such as 3'-azido-3'-deoxythymidine 5'-triphosphate and 3'-deoxythymidine 5'-triphosphate, can also penetrate the viral envelope, incorporate into, and irreversibly terminate reverse transcripts. As a result, viral infectivity is potently inhibited. Since the lentiviral envelope with these newly demonstrated characteristics can serve as a delivery pathway for anti-reverse transcription agents, we propose a unique strategy to prevent HIV-1 interand, possibly, intrahost transmission.