Identification of cellular proteins that bind to the human immunodeficiency virus type 1 nef gene product in vitro: a role for myristylation

Probing the interaction between U24 and the SH3 domain of Fyn tyrosine kinase

The putative membrane protein U24 from HHV-6A shares a seven-residue sequence identity (which includes a PxxP motif) with myelin basic protein (MBP), a protein responsible for the compaction of the myelin sheath in the central nervous system. U24 from HHV-6A also shares a PPxY motif with U24 from the related virus HHV-7, allowing them both to block early endosomal recycling. Recently, MBP has been shown to have protein-protein interactions with a range of proteins, including proteins containing SH3 domains. Given that this interaction is mediated by the proline-rich segment in MBP, and that similar proline-rich segments are found in U24, we investigate here whether U24 also interacts with SH3 domain-containing proteins and what the nature of that interaction might be. The implications of a U24-Fyn tyrosine kinase SH3 domain interaction are discussed in terms of the hypothesis that U24 may function like MBP through molecular mimicry, potentially contributing to the disease state of multiple sclerosis or other demyelinating disorders.

Nef inhibits glucose uptake in adipocytes and contributes to insulin resistance in human immunodeficiency virus type I infection

Human immunodeficiency virus (HIV) infection is associated with insulin resistance. HIV type 1 Nef downregulates cell surface protein expression, alters signal transduction, and interacts with the cytoskeleton and proteins involved in actin polymerization. These functions are required for glucose uptake by insulin-stimulated adipocytes. We sought to determine whether Nef alters adipocyte glucose homeostasis. Using radiolabeled glucose, we found that adipocytes exposed to recombinant Nef took in 42% less glucose after insulin stimulation than did control cells. This reduction resulted from a Nef-dependent inhibition of glucose transporter 4 (GLUT4) trafficking, as assessed by means of immunofluorescence microscopy. Immunoblot analysis revealed a decrease in phosphorylation of signal transducing proteins after Nef treatment, and fluorescence microscopy showed a dramatic alteration in cortical actin organization. We conclude that Nef interferes with insulin-stimulated processes in adipocytes. We have identified HIV Nef, which is detectable and antigenic in serum samples from HIV-infected people, as a novel contributor to the development of insulin resistance.

Identification of a novel phosphorylation site in hepatitis C virus NS5A

Hepatitis C virus (HCV) NS5A protein is phosphorylated on multiple residues; however, despite extensive study, the precise identity of these sites has not been determined unambiguously. In this study, we have used a combination of immunoprecipitation and mass spectrometry to identify these phosphorylation sites. This analysis revealed the presence of a major phosphorylated residue within NS5A from the genotype 1b Con1 isolate - serine 249 (serine 2221 in polyprotein numbering). However, mutation of this residue (or the corresponding threonine in the JFH-1 isolate) to either a phosphomimetic (aspartate) or a phosphoablative (alanine) residue resulted in no phenotype. We conclude that phosphorylation of this residue, in the context of a highly culture-adapted HCV genome, does not play a role in either viral RNA replication or virus assembly. It is possible that it might be important in an aspect of virus biology that is not recapitulated faithfully in the Huh-7 cell-culture system.

Kaposi's sarcoma-associated herpesvirus ORF57 protein interacts with PYM to enhance translation of viral intronless mRNAs

Kaposi's sarcoma-associated herpesvirus (KSHV) expresses numerous intronless mRNAs that are unable to access splicing-dependent cellular mRNA nuclear export pathways. To circumvent this problem, KSHV encodes the open reading frame 57 (ORF57) protein, which orchestrates the formation of an export-competent virus ribonucleoprotein particle comprising the nuclear export complex hTREX, but not the exon-junction complex (EJC). Interestingly, EJCs stimulate mRNA translation, which raises the intriguing question of how intronless KSHV transcripts are efficiently translated. Herein, we show that ORF57 associates with components of the 48S pre-initiation complex and co-sediments with the 40S ribosomal subunits. Strikingly, we observed a direct interaction between ORF57 and PYM, a cellular protein that enhances translation by recruiting the 48S pre-initiation complex to newly exported mRNAs, through an interaction with the EJC. Moreover, detailed biochemical analysis suggests that ORF57 recruits PYM to intronless KSHV mRNA and PYM then facilitates the association of ORF57 and the cellular translation machinery. We, therefore, propose a model whereby ORF57 interacts directly with PYM to enhance translation of intronless KSHV transcripts.

A conserved proline between domains II and III of hepatitis C virus NS5A influences both RNA replication and virus assembly

We previously demonstrated that two closely spaced polyproline motifs, with the consensus sequence Pro-X-X-Pro-X-Lys/Arg, located between residues 343 to 356 of NS5A, mediated interactions with cellular SH3 domains. The N-terminal motif (termed PP2.1) is only conserved in genotype 1 isolates, whereas the C-terminal motif (PP2.2) is conserved throughout all hepatitis C virus (HCV) isolates, although this motif was shown to be dispensable for replication of the genotype 1b subgenomic replicon. In order to investigate the potential role of these motifs in the viral life cycle, we have undertaken a detailed mutagenic analysis of these proline residues in the context of both genotype 1b (FK5.1) or 2a subgenomic replicons and the genotype 2a infectious clone, JFH-1. We show that the PP2.2 motif is dispensable for RNA replication of all subgenomic replicons and, furthermore, is not required for virus production in JFH-1. In contrast, the PP2.1 motif is only required for genotype 1b RNA replication. Mutation of proline 346 within PP2.1 to alanine dramatically attenuated genotype 1b replicon replication in three distinct genetic backgrounds, but the corresponding proline 342 was not required for replication of the JFH-1 subgenomic replicon. However, the P342A mutation resulted in both a delay to virus release and a modest (up to 10-fold) reduction in virus production. These data point to critical roles for these proline residues at multiple stages in the HCV life cycle; however, they also caution against extrapolation of data from culture-adapted replicons to infectious virus.

Domain III of NS5A contributes to both RNA replication and assembly of hepatitis C virus particles

The hepatitis C virus (HCV) NS5A protein plays a critical role in viral RNA replication and has recently been shown to play a role in particle production in the infectious genotype 2a HCV clone (JFH-1). Here, we show that alanine substitutions of serines 2428/2430 within the C-terminal domain III of NS5A do not affect subgenomic replicon RNA replication but do reduce particle production. In contrast, substitution of serines 2390/2391 had no effect on either RNA replication or particle production. Relative to genotype 1, all genotype 2 HCV isolates contain a 19 residue insertion near the C terminus of domain III which, when deleted (Delta2408-2426), resulted in a delay to both RNA replication and particle production. None of these mutations affected the ratio of basal to hyperphosphorylated NS5A, suggesting that serines between residues 2390 and 2430 are not phosphorylated. We propose that although domain III is dispensable for RNA replication, it nevertheless influences this process.

Co-translational myristoylation alters the quaternary structure of HIV-1 Nef in solution

We have studied the solution properties of Nef, a 24-kDa cotranslationally myristoylated protein produced by HIV-1 and other primate lentiviruses. Nef is found in the cytosol and also in association with cytoplasmic membranes, the latter, mediated in part by the myristoyl group attached to the N-terminal glycine. Recombinant Nef was coexpressed in Escherichia coli in tandem with N-myristoyl-transferase and is fully myristoylated. Analysis by circular dichroism showed the myristoylated form to contain a greater alpha-helical content than the nonmyristoylated form. Analysis of modified and unmodified Nef in solution using small angle X-ray scattering, dynamic laser light scattering and analytical ultracentrifugation consistently showed differences in the oligomeric states of the two forms of Nef. Myristoylated Nef is predominantly monomeric and small oligomers which are also present, can be converted to the monomeric form under reducing conditions. By contrast, the nonmyristoylated form exists as a stable hexadecamer in solution which disassociates into tetramers upon addition of reducing agents. Shape reconstructions from small angle scattering curves of nonmyristoylated Nef are compatible with a large disc-like structure in the hexadecameric oligomer consisting of four Nef tetramers. From these findings, we hypothesize that Nef undergoes a substantial conformational change from an "open" into a "closed" form whereby the myristate group is sequestered in a hydrophobic pocket. The myristoylated protein can switch to the open conformation by association of the N-terminal region of molecule with membranes. These changes would allow Nef to carry out various functions depending on the conformational and oligomeric states.

Mapping the binding site of full length HIV-1 Nef on human Lck SH3 by NMR spectroscopy

The Nef protein of human immunodeficiency virus type 1 (HIV-1) is known to directly bind to the SH3 domain of human lymphocyte specific kinase (Lck) via a proline-rich region located in the amino terminal part of Nef. To address the question whether Nef binding to Lck SH3 involves residues outside the typical poly-proline peptide binding site and whether the Lck unique domain is involved in Nef-Lck interaction, we studied the direct interaction between both molecules using recombinant full-length HIV-1 Nef protein on one side and recombinantly expressed and uniformly 15N-isotope labeled Lck protein comprising unique and SH3 domains on the other side. Applying nuclear magnetic resonance spectroscopy we could show that only residues of Lck SH3, that are typically involved in binding poly-proline peptides, are affected by Nef binding. Further, for the first time we could rule out that residues of Lck unique domain are involved in binding to full length Nef protein. Thus, interactions of Lck unique domain to cellular partners e.g. CD4 or CD8, are not necessarily competitive with Lck binding to HIV-1 Nef.

The Hepatitis C virus NS5A protein activates a phosphoinositide 3-kinase-dependent survival signaling cascade

Hepatitis C virus (HCV) establishes a persistent infection, with up to 80% of infected individuals proceeding to chronic hepatitis, which in many cases may result in liver cirrhosis and hepatocellular carcinoma (HCC); indeed HCV infection is increasingly associated with the development of HCC. The long time period (up to 30 years) between primary infection and the onset of HCC implies that HCV is not directly oncogenic but in some way predisposes patients to develop tumors, though the mechanism for this is unclear as yet. We report here that NS5A binds directly to the Src homology 3 domain of the p85 regulatory subunit of phosphoinositide 3-kinase (PI3K), and this interaction is mediated by a novel (non-proline-rich) motif within NS5A. Coimmunoprecipitation analysis revealed that NS5A bound native heterodimeric PI3K and enhanced the phosphotransferase activity of the catalytic (p110) subunit both in vitro and in human cell lines harboring a subgenomic HCV replicon or expressing NS5A alone. NS5A-mediated activation of PI3K resulted in increased phosphorylation and activity of Akt/protein kinase B and concomitantly provided protection against the induction of apoptosis in both replicon-harboring cells and cells stably expressing NS5A alone. These data suggest that stimulation of PI3K by NS5A may represent an indirect mechanism for development of HCC in HCV-infected patients and further suggests potential therapeutic strategies to counteract the occurrence of HCV-related HCC.

The di-leucine motif in the cytoplasmic tail of CD4 is not required for binding to human immunodeficiency virus type 1 Nef, but is critical for CD4 down-modulation

The human immunodeficiency virus type 1 (HIV-1) nef gene encodes a 205 residue, myristoylated phosphoprotein that has been shown to play a critical role in the replication and pathogenesis of the virus. One of the most studied functions of the Nef protein is the down-modulation of cell surface CD4. Nef has been reported to interact with both the cytoplasmic tail of CD4 and proteins that are components of the endocytic machinery, thereby enhancing the endocytosis of CD4 through clathrin-coated pits. A di-leucine motif in the cytoplasmic tail of CD4 (residues 413/414) was reported to be essential both for Nef mediated down-modulation and for Nef binding. In order to further characterize the involvement of this di-leucine motif in CD4 down-modulation we generated a CD4 mutant in which the leucines were substituted by alanines, termed CD4(LL-AA). We demonstrate here that, contrary to previous data obtained with the cytoplasmic tail of CD4 alone, full-length CD4(LL-AA) bound to Nef both in vivo, in recombinant baculovirus-infected Sf9 cells, and in vitro. In contrast the di-leucine motif was required for both Nef-mediated and phorbol ester-induced CD4 down-modulation, suggesting that the essential requirement for the di-leucine motif in CD4 down-modulation reflects the fact that this motif is needed for the interactions of CD4 with the endocytic machinery, not for the interaction with Nef. We have also exploited the observation that CD4(LL-AA) is refractory to Nef-mediated down-modulation to provide the first experimental evidence for a physical interaction between Nef and CD4 in intact mammalian cells.

Presence of a helix in human CD4 cytoplasmic domain promotes binding to HIV-1 Nef protein

The Nef proteins of simian and human immunodeficiency viruses are known to directly bind and downregulate the CD4 receptor of infected cells. Recent results suggest that residues forming an alpha-helix N-cap in the CD4 cytoplasmic domain play a role in binding of CD4 to human immunodeficiency virus type 1 Nef protein. We determined the dissociation constants between Nef and several CD4 peptides that contain or do not contain the respective alpha-helix N-cap. Further, we compared helical secondary structure content of these CD4 peptide variants by circular dichroism spectroscopy. We conclude that presence of an alpha-helix in CD4 cytoplasmic domain increases CD4 affinity to Nef. In addition, the amino acid sequence of residues forming the helix N-cap influences CD4 affinity to Nef, too. Finally, the structural changes induced in Nef and CD4 upon binding to each other are investigated.

Direct in vitro binding of full-length human immunodeficiency virus type 1 Nef protein to CD4 cytoplasmic domain

The Nef protein of the simian and human immunodeficiency viruses is known to directly bind and downregulate the CD4 receptor. Although the molecular mechanism is well understood, direct binding of Nef and CD4 is difficult to demonstrate and is believed to be of low affinity. Applying nuclear magnetic resonance and fluorescence spectroscopy, we biophysically reevaluated the CD4-Nef complex and found the dissociation constant to be in the submicromolar range. We conclude that additional, so far disregarded residues in the N terminus of Nef are important for interaction with CD4.

Expression of enzymes of covalent protein modification during regulated and dysregulated proliferation of mammary epithelial cells: PKA, PKC and NMT

Three proteins are functionally interlinked in the targeting of protein phosphorylation catalyzed by the C-subunit of PKA: PKA itself, AKAPs and NMT. Furthermore, in a variety of biological contexts, mechanisms exist whereby PKA and PKC are able to modulate the activity of one another. We have investigated the expression and subcellular distribution of these proteins in two models of mammary cell proliferation and differentiation--the normal rat mammary gland during pregnancy and lactation and human breast tissue before and after malignant transformation. Modulation of PKA does not acutely affect activity or sub-cellular distribution of PKC in mammary acini, nor does modulation of PKC acutely affect PKA activity or subcellular distribution. Therefore, the co-ordinate expression of these two protein kinases in normal and cancerous mammary epithelial cells and the greater basal activation level of them both accompanying increased mitogenic activity, which we have reported, does not result from short-term cross-talk between them. Although basal and total levels of PKA diminish in rodent mammary epithelial cells during the transition from proliferative to secretory functional mode, the level of expression of AKAPs increases. The expression of two apparently mammary-specific and mostly membrane-associated AKAPs is tightly linked to the onset and maintenance of differentiated function in rat mammary tissue. Paradoxically, the probable analogues of these two AKAPs in human mammary tissue are hyperexpressed when normal epithelial cells transform to a cancer phenotype--conventionally regarded as a process involving a degree of dedifferentiation. Mammary AKAP hyperexpression in breast cancers is accompanied by increases in the levels of total and basal PKA. One mechanism whereby NMT is targeted to membranes, via interaction with ribosomal proteins, has recently been elucidated. Our data support the contention that the localization of NMT is an important variable in the regulation of cellular proliferation, but they do not characterize the mechanisms whereby the differential targeting of NMT is achieved. As yet we lack a full tool-kit with which to examine NMT either to draw firm conclusions regarding the identity of particular isoforms found in particular sub-cellular locations or to define the relationships between these different molecular variants. However, it is technically possible to transfect cells with inducible NMT expression constructs engineered in such a way that the recombinant, catalytically competent, NMT that they encode is targeted either to membranes or to cytosol: an exploration of the effects of such transfections on cellular proliferation would afford a critical test of the mechanistic involvement of NMT in the control of mitogenesis.

Monoclonal antibodies recognize at least five epitopes on the SIV Nef protein and identify an in vitro-induced mutation

Eleven monoclonal antibodies (MAbs) to SIV Nef were produced and characterized. Five antibody-binding sites on SIV Nef were identified on the basis of the reactivity of the antibodies with recombinant proteins. Two of the five epitopes were defined using overlapping peptides. A further three epitopes could not be defined with peptides but all antibodies reacted in Western blot, suggesting that the epitopes were at least partially conformation dependent. Antibodies in two of the five epitope groups were further differentiated by competition analysis. The panel of MAbs described is able to distinguish between a number of recombinant Nef proteins currently under investigation in vivo in macaques. Two of the MAbs described are able to distinguish between the Nef protein from pathogenic (J5) and attenuated (C8) strains of SIV, thus providing useful tools for studying the relevance of the Nef protein in the pathogenesis of SIV infection. In FACScan analysis two of the MAbs, KK70 and KK75, were used to identify an in vitro-induced mutation in J5 Nef grown in C8166 cells. Sequence analysis of the phenotypic variants identified a mutation of the tryptophan (TGG) at amino acid 214 to a stop codon (TGA), thus truncating the Nef protein. The functional significance of this observation remains unclear but highlights the need to interpret data with caution if virus has been cultured in vitro even for a short period of time.

Induction of activator protein 1 (AP-1) in macrophages by human immunodeficiency virus type-1 NEF is a cell-type-specific response that requires both hck and MAPK signaling events

Human immunodeficiency virus type 1 (HIV-1) Nef is important for viral infectivity and pathogenicity. HIV-1 infection is associated with inappropriate activation and defects in the function of monocytes/macrophages. We have studied the effects of HIV-1 Nef in the murine (RAW264.7) and human (THP-1) monocyte-macrophage cell lines. Investigation of the activator protein-1 (AP-1) transcription factor showed that Nef expression induced both its DNA binding and transcriptional activities. Increased AP-1 DNA binding activity in RAW264.7 cells was associated with raised levels of c-Fos expression and induction of mRNA for the AP-1 responsive tissue inhibitor of metalloproteinases-1 (TIMP-1) gene. Mutagenesis and kinase inhibition studies were employed to determine signaling pathways used by Nef to induce AP-1. Data from these studies indicated that induction of AP-1 by Nef is likely to be mediated through the MAPK (ERK1 and 2) signaling pathway and requires the proline-rich PxxP motif of Nef, suggesting the involvement of upstream protein kinases belonging to the Src family. Effects of Nef on AP-1 induction were cell lineage-specific, being stimulatory in macrophages, inhibitory in T cells and without effect in HeLa cells. These latter two observations led us to test the possibility that cell-specific interactions of Nef with Src family proteins may modulate AP-1 activity. To this end we demonstrated that a dominant-negative Hck mutant caused inhibition of Nef-mediated AP-1 DNA binding activity in RAW cells. In conclusion, induction of AP-1 by Nef is a specific feature of human and murine macrophage cell lines that requires signal transduction events involving Hck and MAPKs.

2H NMR studies of a myristoylated peptide in neutral and acidic phospholipid bicelles

Deuterium NMR spectra of Myr-d27-GNAAAAKKGSEQES (Cat14), the N-terminal 14-residue peptide from the catalytic subunit of cAMP-dependent protein kinase A (PKA), illustrate how magnetically aligned neutral and acidic phospholipid bicelles can be used to characterize the ordering and mode of binding of peptides to membranes. Since Cat14 is electrically neutral, the major interaction responsible for binding is the insertion of the myristoyl group into the hydrophobic core of the bilayer. The inclusion of 25% phosphatidylserine or phosphatidylglycerol into phosphatidylcholine bicelles results in a moderate increase in the ordering of the peptide relative to the bicelle normal, presumably because of favorable electrostatic interactions between the phospholipid headgroups and the two lysines in positions 7 and 8. Successful preparation of acidic bicelles was achieved by careful adjustment of lipid composition, pH and ionic strength.

Binding of c-Raf1 kinase to a conserved acidic sequence within the carboxyl-terminal region of the HIV-1 Nef protein

Nef is a membrane-associated cytoplasmic phosphoprotein that is well conserved among the different human (HIV-1 and HIV-2) and simian immunodeficiency viruses and has important roles in down-regulating the CD4 receptor and modulating T-cell signaling pathways. The ability to modulate T-cell signaling pathways suggests that Nef may physically interact with T-cell signaling proteins. In order to identify Nef binding proteins and map their site(s) of interaction, we targeted a highly conserved acidic sequence at the carboxyl-terminal region of Nef sharing striking similarity with an acidic sequence at the c-Raf1-binding site within the Ras effector region. Here, we used deletion and site-specific mutagenesis to generate mutant Nef proteins fused to bacterial glutathione S-transferase in in vitro precipitation assays and immunoblot analysis to map the specific interaction between the HIV-1LAI Nef and c-Raf1 to a conserved acidic sequence motif containing the core sequence Asp-Asp-X-X-X-Glu (position 174-179). Significantly, we demonstrate that substitution of the nonpolar glycine residue for either or both of the conserved negatively charged aspartic acid residues at positions 174 and 175 in the full-length recombinant Nef protein background completely abrogated binding of c-Raf1 in vitro. In addition, lysates from a permanent CEM T-cell line constitutively expressing the native HIV-1 Nef protein was used to coimmunoprecipitate a stable Nef-c-Raf1 complex, suggesting that molecular interactions between Nef and c-Raf1, an important downstream transducer of cell signaling through the c-Raf1-MAP kinase pathway, occur in vivo. This interaction may account for the Nef-induced perturbations of T-cell signaling and activation pathways in vitro and in vivo.

T cells chronically infected with HIV do not contain sufficient Nef to promote CD4 downmodulation in the absence of envelope-mediated effects

Among HIV viral proteins, envelope glycoproteins and Nef have been both suggested to participate in CD4 downregulation during the course of HIV infection. In a previous study, we provided evidence that a mutant form of CD4 that does not bind gp120 was never downregulated in chronically HIV-1- and HIV-2-infected CEM cells. To further investigate the relative effects of Nef or glycoproteins in CD4 downregulation, recombinant vaccinia virus (VV) vectors were used to express high levels of HIV-1 viral proteins in cells expressing both wild-type and mutant CD4. It was demonstrated that during HIV infection, overexpression of Nef, achieved through the VV expression system, was necessary to induce CD4 downregulation in the mutant CD4-expressing cell model. These results are consistent with the hypothesis that Nef-mediated CD4 downregulation depends on the cellular levels of Nef expression. We concluded that during the late stage of viral replication, CD4 downregulation is mostly due to gp120 and not to Nef because of a low level of Nef expression.

Association of human immunodeficiency virus Nef protein with actin is myristoylation dependent and influences its subcellular localization

Human immunodeficiency virus (HIV) Nef functions are thought to be mediated via interactions with cellular proteins. Utilizing zone velocity sedimentation in glycerol gradients we found that recombinant HIV-1 Nef non-covalently associates with actin forming a high-molecular-mass complex of 150-300 kDa. This Nef/actin complex was present in human B and T lymphocytes but not in insect cells and was dependent on the N-terminal myristoylation of Nef, whereas the SH3-binding proline motif of Nef was not involved. Despite being myristoylated, HIV-2 Nef did not associate with actin. This might reflect differences in the subcellular localization of Nef since cell-fractionation experiments revealed that HIV-1 Nef was virtually exclusively localized in the cytoskeletal (detergent-insoluble) fraction whereas HIV-2 Nef had significantly reduced affinity for the cytoskeleton. Colocalization experiments in HIV-1-infected CD4+ fibroblasts revealed that Nef/actin complexes may also exist in HIV-infected cells. This novel interaction of HIV-1 Nef with actin provides insight into the association of Nef with cellular structures and reveals general differences in the interactions of the Nef proteins from HIV-1 and HIV-2.

Protein kinase C-mediated phosphorylation of HIV-I nef in human cell lines

Stable human cell lines expressing the human immunodeficiency virus type I (HIV-I) Nef protein from inducible promoters were used to analyze the phosphorylation status of Nef in vivo. Nef phosphorylation in both HeLa and Jurkat cells was stimulated by phorbol ester treatment. Phosphoamino acid analysis revealed a predominance of phosphoserine with a small proportion of phosphothreonine. Treatment of cells with selective protein kinase inhibitors revealed that Nef phosphorylation was markedly reduced by bisindolylmaleimide, an inhibitor of protein kinase C, but was unaffected by inhibitors of mitogen-activated protein kinase kinase or cAMP-dependent kinase. These data implicate protein kinase C in Nef phosphorylation in vivo, and thus confirm and extend earlier in vitro data. Phosphorylation of a nonmyristoylated Nef mutant was impaired, suggesting that membrane targeting of Nef was required for phosphorylation. This was expected given that activated protein kinase C translocates from the cytosol to the plasma membrane. However, analysis of the subcellular localization of phosphorylated wild-type Nef revealed that both the cytosolic and membrane-associated pools of Nef were phosphorylated to an equivalent extent. Thus the significance of myristoylation for Nef function may be in influencing protein conformation, although these data could be explained by a transient and dynamic interaction between myristoylated Nef and the plasma membrane.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

Expression and purification of the seven nonstructural proteins of the flavivirus Kunjin in the E. coli and the baculovirus expression systems

All seven nonstructural (ns) proteins of the flavivirus Kunjin (KUN) ranging from NS1 to NS5 were expressed either alone or as fusion proteins with Glutathione-S-transferase (GST). High level expression of recombinant proteins was achieved in Spodoptera frugiperda (Sf9) cells using the baculovirus expression system in contrast to the low level of expression in E. coli. The order of the level of expression of the recombinant fusion proteins per 4 x 10(7) Sf9 cells was: GST-NS5 (yields approximately 4-5 mg) > GST-delta NS3 (approximately 1-2 mg) > GST-4A (approximately 1 mg) > GST-2B (approximately 0.5-1 mg) > GST-2A (approximately 0.5 mg) > GST-4B (approximately 0.1-0.2 mg). NS1 protein was expressed in a native form at the level of approximately 2-4 mg per 4 x 10(7) Sf9 cells. All the GST-fusion proteins were purified by adsorption on Glutathione Sepharose (GS) beads from solubilized lysates of Sf9 cells infected with the recombinant baculoviruses, or of E. coli cultures transformed with the expression plasmid and induced with IPTG. Only delta NS3 protein was recovered intact by removing GST from the fusion protein by digestion with Factor Xa protease. Attempts to cleave off the GST moiety from all the other purified recombinant proteins resulted either in inefficient cleavage or in degradation of the proteins. No GST-NS5 but from 20 to 50% of the purified GST-NS2A, GST-NS2B, GST-delta NS3, GST-NS4A, and GST-NS4B was eluted off the GS beads by adding glutathione. Thus, KUN purified recombinant proteins, either in eluted form or while immobilized on GS beads, could be used to raise monospecific antibodies, to perform functional assays or to participate in protein-protein or RNA-protein binding reactions.

Molecular biology of human immunodeficiency virus type-1

Tremendous progress has been made in our understanding of the multiplication and pathogenesis of the human immunodeficiency virus, the causative agent of acquired immunodeficiency syndrome (AIDS). To block virus multiplication several targets in the life cycle of the virus have already been identified for which antiviral drugs can be developed and gene therapy can be envisaged as a possible treatment or cure of AIDS. The combination of several therapies might be needed for effective treatment. Prevention of HIV infections through effective vaccines still awaits novel, unconventional strategies.

Human immunodeficiency virus type 1 Nef associates with a member of the p21-activated kinase family

Although human immunodeficiency virus (HIV) Nef is essential for the induction of AIDS, its biochemical function has remained an enigma. In this study, HIV Nef protein is shown to associate with a serine-threonine kinase that recognizes histone H4 as a substrate, is serologically related to rat p21-activated kinase (PAK), and is specifically activated by Rac and Cdc42. These characteristics define the Nef-associated kinase as belonging to the PAK family. PAKs initiate kinase cascades in response to environmental stimuli, and their identification as a target of Nef implicates these signaling molecules in HIV pathogenesis and provides a novel target for clinical intervention.

Evidence for intact CD28 signaling in T cell hyporesponsiveness induced by the HIV-1 nef gene

Infection by human immunodeficiency virus (HIV)-1 is associated with quantitative and qualitative T cell alterations that severely impair the host's immune defense system. The molecular basis for this immunosuppression remains unclear. Peripheral blood mononuclear cells (PBMC) isolated from patients show markedly decreased interleukin (IL)-2 secretion but unaffected or even increased T helper (Th)2 cytokine production. T cell functional defects were recently reported to correlate more with T cell receptor (TcR) signaling, whereas signals provided by ligation of co-receptors CD27 and CD28 appeared to be preserved. Among the various mechanisms proposed to be involved in HIV-1-induced T cell dysfunction, we and others have reported that the nef gene product exhibited significant immunosuppressive activity. By using an inducible stably integrated nef gene, we demonstrated that Nef specifically down-regulated IL-2 and interferon (IFN)-gama produced upon TcR triggering. Here, using the same experimental system, we extended our initial observations to additional mitogenic signals, and investigated the co-stimulatory function of CD28. Nef down-regulated IL-2, but not IL-4 produced upon induction by combinations of mitogens that mimicked TcR signals together with CD28 mAb or CD28's natural ligand (CD80 and CD86). However, the co-signals provided by CD28 to up-regulate IL-2 induction were unaffected by Nef, since IL-2 produced by nef-transfected cells was proportionally enhanced to the same extent as that of control cells, either upon stimulation by the CD28 mAb or CD80 and CD86. In addition, phosphatidylinositol-3 kinase recruitment induced upon CD28 triggering was also found to be unaltered by nef expression. Together with the observation that similar levels of the Nef protein were detected in nef-transfected cells and upon infection of PBMC, these data suggest a selective immunosuppression induced by nef in human T cells by altering TcR signaling without detectable impact on CD28 co-receptor function. These data agree with the T cell defects observed in PBMC isolated from HIV-infected individuals.

Physical and functional interaction of Nef with Lck. HIV-1 Nef-induced T-cell signaling defects

The nef gene is unique to the primate lentiviruses and encodes a cytoplasmic membrane-associated protein that affects T-cell signaling and is essential for both maintenance of a high virus load in vivo and for disease progression. Here we investigated the perturbation of cell signaling by Nef in T-cells and found that Nef interacts with the T-cell restricted Lek tyrosine kinase both in vitro and in vivo. The molecular basis for this interaction was analyzed. We show that cell-derived Nef is precipitated in a synergistic manner by the recombinant Src homology 2 (SH2) and SH3 domains from Lck. A functional proline-rich motif and the tyrosine phosphorylation of Nef were evidenced as likely participants in this interaction. The precipitation of Nef by the Lck recombinant proteins was specific, since neither Fyn, Csk, p85 phosphatidylinositol 3-kinase nor phospholipase Cgamma SH2 domains coprecipitated Nef from T-cells. Finally, depressed Lck kinase activity resulted from the presence of Nef, both in vitro and in intact cells, and nef expression resulted in impairment of both proximal and distal Lck-mediated signaling events. These results provide a molecular basis for the Nef-induced T-cell signaling defect and its role in AIDS pathogenesis.

HIV type 1 Nef perturbs eye lens development in transgenic mice

HIV transgenic mice often display lens cataracts as a consequence of viral-specific expression of HIV gene products in the developing lens. Cataractous mouse lines encoding either HIV-1 proviral DNA, HIV delta Gag/Pol] proviral DNA, or the HIV-1 nef gene alone were examined to ascertain the effect of Nef on murine lens development. Ocular disease was characterized by a progressive architectural disorganization within the lens fiber cell compartment developing in 100% of HIV-positive mice in five reported transgenic lines. Late embryonic stage transgenic lenses featured a mild microphthalmia, pyknotic nuclei within the lens fiber department, ballooning lens fiber cells, and elongated lens epithelial cells. Increased DNA fragmentation was evident in transgenic embryonic lenses, suggesting that cell death occurred by apoptosis. As studied in HIV delta Gag/Pol] transgenic mice, HIV transcription was developmentally linked to alpha A- and alpha B-crystallin gene expression, preceded disease development (in E14.5-E16.5 embryos), and persisted for weeks after birth. HIV-1 Nef was the predominant HIV gene product detected in the lens fiber cells of this line and was expressed almost to the exclusion of other HIV gene products. Nef was implicated as a major determinant of disease because (1) cataracts developed in mice transgenic for Nef alone and (2) the expression of other HIV gene products in wild-type HIV provirus transgenic mice occurred without a concomitant change in lens pathology.

Eukaryotic virus display: engineering the major surface glycoprotein of the Autographa californica nuclear polyhedrosis virus (AcNPV) for the presentation of foreign proteins on the virus surface

We describe the development of the baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV) as a vector for the display of distinct proteins on the viral surface in a manner that is analogous to the established bacterial "phage display" systems. As a model system, the marker gene encoding the 26kDa protein glutathione-S-transferase (GST) was used to construct several fusions with the major baculovirus glycoprotein gp64 gene. Following expression in Spodoptera frugiperda (Sf9) cells, the yield and cellular distribution of each GST-gp64 protein was assessed by Western blot of both cell and supernatant fractions. One fusion, in which GST was inserted between the leader peptide and the nature protein, was found to be efficiently secreted into the cell medium. In the context of expression of the full length gp64, the hybrid GST-gp64 was shown by immunogold labelling to be incorporated onto the virion surface. In addition, the affinity purification of the soluble transmembrane gp64-GST fusion protein resulted in the co-purification of wild type gp64 suggesting that co-oligomerization of the GST-tagged fusion and the wild type molecule was the basis for virion incorporation. The HIV major surface glycoprotein, gp120 was also efficiently displayed in functional form on the viral surface following fusion to the amino terminus of gp64. A general expression vector, pAcSurf-2, was constructed in which multiple cloning sites were positioned in-phase between the gp64 signal sequence and the sequence encoding the mature protein under the control of the polyhedrin promoter.

A conserved domain and membrane targeting of Nef from HIV and SIV are required for association with a cellular serine kinase activity

Among the primate lentiviruses (human immunodeficiency virus (HIV) -1, HIV-2, and simian immunodeficiency virus (SIV), the nef gene is highly conserved and encodes a myristylated protein of approximately 27 kDa (HIV-1) or approximately 34 kDa (HIV-2, SIV). Previously, we found Nef expressed either as a CD8-Nef fusion protein or as a native protein in virally infected T cell lines associates with a cellular serine kinase. This kinase activity phosphorylated two proteins of 62 and 72 kDa that coimmunoprecipitate with Nef in in vitro kinase assays. Using transient expression, various Nef alleles and mutants have been analyzed for association with the cellular kinase activity. The ability of Nef to associate with the kinase activity is conserved among several alleles of HIV-1 as well as SIVmac239 and is observed in non-lymphoid cell lines of simian and murine origins. Two separate regions of HIV-1SF2 Nef are critical for the associated kinase activity. One domain overlaps with a central highly conserved region found in all primate lentivirus nef genes and has been provisionally mapped to amino acids 45-127. Because membrane localization of Nef is important for the associated cellular kinase activity, the second domain represents a membrane targeting signal. Moreover, point mutations within the central region that abrogate the Nef-associated kinase activity in HIV-1SF2 Nef have the same effect when introduced into SIVmac239open Nef.

Human immunodeficiency virus type 1 Nef protein inhibits activation pathways in peripheral blood mononuclear cells and T-cell lines

Human immunodeficiency virus type 1 (HIV-1) Nef protein causes the loss of cell surface CD4 and interleukin-2 (IL-2) receptor (Tac) from peripheral blood mononuclear cells (PBMC) and CD4+ T-cell lines. As both CD4 and the IL-2 receptor play crucial roles in antigen-driven helper T-cell signalling and T-cell proliferation, respectively, the role of Nef in the viral life cycle may be to perturb signalling pathways emanating from these receptors. However, the intracellular targets for Nef that result in receptor down-regulation are unknown. Using a recombinant glutathione S-transferase-full-length 27 kDa Nef (Nef27) fusion protein, produced in Escherichia coli by translation from the first start codon of HIV-1 nef clone pNL4-3, as an affinity reagent to probe cytoplasmic extracts of MT-2 cells and PBMC, we have shown interaction with at least seven host cell protein species ranging from 24 to 75 kDa. Immunoblotting identified four of these proteins as p56lck, CD4, p53, and p44mapk/erk1, all of which are intimately involved in intracellular signalling. To assess the relevance of these interactions and further define the biochemical activity of Nef in signal transduction pathways, highly purified Nef27 protein was introduced directly into PBMC by electroporation. Nef27-treated PBMC showed reduced proliferative responsiveness to exogenous recombinant IL-2. Normally, stimulation of T-cells by IL-2 or phorbol 12-myristate 13-acetate provokes both augmentation of p56lck activity and corresponding posttranslational modification of p56lck. These changes were also inhibited by treatment of PBMC with Nef, suggesting that Nef interferes with activation of p56lck and as a consequence of signalling via the IL-2 receptor. Further evidence for Nef interfering with cell proliferation was the decreased production of the proto-oncogene c-myb, which is required for cell cycle progression, in Nef-treated MT-2 cells. In contrast to the binding characteristics and biological effects of Nef27, the alternate 25-kDa isoform of Nef (Nef25) produced by translation from the second start codon of HIV nef pNL4-3 (57 nucleotide residues downstream) was shown to interact with only three cellular proteins of approximately 26, 28, and 56 kDa from PBMC and MT-2 cells, one of which was identified as p56lck. Also, proliferation and posttranslational modification of p56lck in response to IL-2 stimulation were not profoundly affected by treatment of PBMC with Nef25 compared with Nef27.(ABSTRACT TRUNCATED AT 400 WORDS)

Discovery and in vitro development of AIDS antiviral drugs as biopharmaceuticals

The goal of developing an effective drug against HIV-1 and AIDS has been approached by several routes, with enough encouraging results to stimulate further efforts. Compounds active against HIV-1 have been discovered for many of the functions in the reproductive cycle recognized as virus-specific targets. Discoveries have been made in cell-based assays as well as mechanistic assays, and the value of both types of assays in the drug discovery process has been discussed. Although the final test of a drug's efficacy comes in the clinical experience, submission of an antiviral compound to an in vitro developmental gauntlet can save much time, effort, expense, and human resource in the in vivo developmental regimen required prior to human use. Emergence of viral resistance to drugs in several structural classes has compromised their clinical efficacy, suggesting that development of other potential drugs in those classes may not be good investments. Strains of HIV-1 resistant to specific compound classes are used to categorize new active discoveries for possible developmental exclusion, and defining the mechanism of action of such a new compound may confirm the discouraging judgement. On the other hand, novel compounds which exhibit a broad range of activity in drug-resistant and other HIV-1 strains deserve greater scrutiny. Clinicians most likely will be hesitant to treat patients with compounds shown to act on virus-cell surface interactions, given the failure in the past of several such compounds in clinical studies. But a compound shown to have a unique and novel mechanism of action will be looked upon more favorably, and surviving other tests of potency, solubility, and stability will be unhesitatingly presented for in vivo development. The partial successes of drugs currently in clinical use against AIDS offers great encouragement that other more-effective, less-toxic drugs will be found. Exquisite techniques for identifying new targets on virus gene products, the selection of compounds on activity paradigms, and the enormous variety of compounds becoming available through synthesis libraries, all offer opportunities for anti-HIV drug discovery, which, in our view, cannot fail to present potent antiviral compounds which will survive the rigorous preclinical and clinical tests leading to a drug effective against AIDS.

Human immunodeficiency virus type 1 Nef associates with a cellular serine kinase in T lymphocytes

With T-cell lines constitutively expressing Nef from the SF2 strain of human immunodeficiency virus type 1 (HIV-1SF2) in the form of a hybrid CD8-Nef fusion protein or T-cell lines chronically infected with HIV-1SF2, a cellular serine kinase was found that specifically associates with Nef. Proteins of 62 kDa and 72 kDa, which coimmunoprecipitated with Nef, were phosphorylated in in vitro kinase assays. This Nef-associated serine kinase activity was not blocked by inhibitors of protein kinase C or protein kinase A and was lost when Nef was truncated at amino acid 94 or 99. These findings present evidence that a serine kinase activity is associated with Nef expressed in human T lymphocytes.