Specific interaction of CCR5 amino-terminal domain peptides containing sulfotyrosines with HIV-1 envelope glycoprotein gp120

Structural basis of chemokine receptor function--a model for binding affinity and ligand selectivity

Chemokine receptors play fundamental roles in human physiology from embryogenesis to inflammatory response. The receptors belong to the G-protein coupled receptor class, and are activated by chemokine ligands with a range of specificities and affinities that result in a complicated network of interactions. The molecular basis for function is largely a black box, and can be directly attributed to the lack of structural information on the receptors. Studies to date indicate that function can be best described by a two-site model, that involves interactions between the receptor N-domain and ligand N-terminal loop residues (site-I), and between receptor extracellular loop and the ligand N-terminal residues (site-II). In this review, we describe how the two-site model could modulate binding affinity and ligand selectivity, and also highlight some of the unique chemokine receptor features, and their role in function.

Interaction between the CCR5 chemokine receptors and microbial HSP70

Evidence is presented that the microbial 70-kD heat shock protein (HSP70) binds to CCR5 chemokine receptors in CCR5-transfected cell lines and in primary human cells. Significant CCR5-mediated calcium mobilization was stimulated by HSP70 and inhibited with TAK 779, which is a specific CCR5 antagonist. HSP70-mediated activation of the p38 MAPK phosphorylation signaling pathway was also demonstrated in CCR5-transfected HEK 293 cells. Direct binding of three extracellular peptides of CCR5 to HSP70 was demonstrated by surface plasmon resonance. Functional evidence of an interaction between HSP70, CCR5 and CD40 was shown by enhanced production of CCL5 by HEK 293 cells transfected with both CD40 and CCR5. Primary monocyte-derived immature DC stimulated with HSP70 produced IL-12 p40, which showed dose-dependent inhibition of >90% on treatment with both TAK 779 and anti-CD40 mAb. Stimulation of IL-12 p40 or TNF-alpha by HSP70 was related to the differential cell surface expression of CCR5 in primary human immature and mature DC, and those with the homozygous triangle DeltaDelta32 CCR5 mutation. These findings may be of significance in the interaction between HSP70 and immune responses of CCR5+ T cells in HIV-1 infection, as well as in inflammatory bowel disease.

A Tyrosine-sulfated Peptide Derived from the Heavy-chain CDR3 Region of an HIV-1-neutralizing Antibody Binds gp120 and Inhibits HIV-1 Infection

Sulfated tyrosines at the amino terminus of the principal HIV-1 coreceptor CCR5 play a critical role in its ability to bind the HIV-1 envelope glycoprotein gp120 and mediate HIV-1 entry. Human antibodies that recognize the CCR5-binding region of gp120 are also modified by tyrosine sulfation, which is necessary for their ability to neutralize HIV-1. Here we demonstrate that a sulfated peptide derived from the CDR3 region of one of these antibodies, E51, can efficiently bind gp120. Association of this peptide, pE51, with gp120 requires tyrosine sulfation and is enhanced by, but not dependent on, CD4. Alteration of any of four pE51 tyrosines, or alteration of gp120 residues 420, 421, or 422, critical for association with CCR5, prevents gp120 association with pE51. pE51 neutralizes HIV-1 more effectively than peptides based on the CCR5 amino terminus and may be useful as a fusion partner with other protein inhibitors of HIV-1 entry. Our data provide further insight into the association of the CCR5 amino terminus with gp120, show that a conserved, sulfate-binding region of gp120 is accessible to inhibitors in the absence of CD4, and suggest that soluble mimetics of CCR5 can be more effective than previously appreciated.

Modeling the structural basis of human CCR5 chemokine receptor function: from homology model building and molecular dynamics validation to agonist and antagonist docking

This article describes the construction and validation of a three-dimensional model of the human CCR5 receptor using a homology-based approach starting from the X-ray structure of the bovine rhodopsin receptor. The reliability of the model is assessed through molecular dynamics and docking simulations using both natural agonists and a synthetic antagonist. Some important structural and functional features of the receptor cavity and the extracellular loops are identified, in agreement with data available from site-directed mutagenesis. The results of this study help to explain the structural basis for the recognition, activation, and inhibition processes of CCR5 and may provide fresh insights for the design of HIV-1 entry blockers.

An immunoglobulin fusion protein based on the gp120-CD4 receptor complex potently inhibits human immunodeficiency virus type 1 in vitro

Fusion proteins containing immunoglobulin Fc domains attached to bioactive moieties have been developed as therapeutic agents against several diseases. Here, we describe the development and characteristics of a novel fusion protein (FLSC R/T-IgG1) that targets CCR5, the major coreceptor for HIV-1 during primary infection. FLSC R/T-IgG1 was expressed from a synthetic gene that linked a single chain gp120-CD4 complex containing an R5 gp120 sequence with the hinge-CH2-CH3 portion of human immunoglobulin gamma subtype 1. Purified FLSC R/T-IgG1 exhibited a molecular mass of 189 kDa under reducing conditions, which matched the expected size of one polypeptide chain. Chemically crosslinked or untreated FLSC R/T-IgG1 exhibited a mass of a 360-kDa polypeptide under reducing and nonreducing conditions, which indicated that the molecule adopts a disulfide-linked bivalent structure. The chimeric molecule bound specifically to CCR5-expressing cells and to peptides derived from the CCR5 N-terminus. Such binding was more efficient than what was obtained with a monomeric single chain gp120-CD4 complex. FLSC R/T-IgG1 binding to CCR5 was blocked by preincubation of coreceptor-expressing cells with CCR5 ligands and by antibody to the coreceptor binding domain of gp120. Conversely, FLSC R/T-IgG1 blocked the binding of chemokine to CCR5. However, FLSC R/T-IgG1 did not trigger intracellular Ca2+ mobilization in peripheral blood mononuclear cells. FLSC R/T-IgG1 potently neutralized primary R5 HIV-1 in both a PBMC-based assay and cell line-based assays but did not affect the replication of X4 viruses. These findings suggest that FLSC R/T-IgG1 might be used as a possible therapeutic agent against HIV.

CCR5 interactions with the variable 3 loop of gp120

The G-protein coupled receptor CCR5 functions pathologically as the primary co-receptor for macrophage tropic (R5) strains of HIV-1. The interactions responsible for co-receptor activity are unknown. Molecular-dynamics simulations of the extracellular and adjacent transmembrane domains of CCR5 were performed with explicit solvation utilizing a rhodopsin-based homology model. The functional unit of co-receptor binding was constructed via docking and molecular-dynamics simulation of CCR5 and the variable 3 loop of gp120, which is a dominant determinant of co-receptor utilization. The variable 3 loop was demonstrated to interact primarily with the amino terminus and the second extracellular loop of CCR5, providing novel structural information regarding the co-receptor-binding site. Alanine mutants that alter chemokine binding and co-receptor activity were examined. Molecular-dynamics simulations with and without the variable 3 loop of gp120 were able to rationalize the activities of these mutants successfully, providing support for the proposed model. Based on these results, the global complex of CCR5, gp120 including the V3 loop and CD4, was investigated. The utilization of computational analysis, in combination with molecular biological data, provides a powerful approach for understanding the use of CCR5 as a co-receptor by HIV-1.

Functional impact of HIV coreceptor-binding site mutations

The bridging sheet region of the gp120 subunit of the HIV-1 Env protein interacts with the major virus coreceptors, CCR5 and CXCR4. We examined the impact of mutations in and adjacent to the bridging sheet region of an X4 tropic HIV-1 on membrane fusion and entry inhibitor susceptibility. When the V3-loop of this Env was changed so that CCR5 was used, the effects of these same mutations on CCR5 use were assayed as well. We found that coreceptor-binding site mutations had greater effects on CXCR4-mediated fusion and infection than when CCR5 was used as a coreceptor, perhaps related to differences in coreceptor affinity. The mutations also reduced use of the alternative coreceptors CCR3 and CCR8 to varying degrees, indicating that the bridging sheet region is important for the efficient utilization of both major and minor HIV coreceptors. As seen before with a primary R5 virus strain, bridging sheet mutations increased susceptibility to the CCR5 inhibitor TAK-779, which correlated with CCR5 binding efficiency. Bridging sheet mutations also conferred increased susceptibility to the CXCR4 ligand AMD-3100 in the context of the X4 tropic Env. However, these mutations had little effect on the rate of membrane fusion and little effect on susceptibility to enfuvirtide, a membrane fusion inhibitor whose activity is dependent in part on the rate of Env-mediated membrane fusion. Thus, mutations that reduce coreceptor binding and enhance susceptibility to coreceptor inhibitors can affect fusion and enfuvirtide susceptibility in an Env context-dependent manner.

An aptamer that neutralizes R5 strains of human immunodeficiency virus type 1 blocks gp120-CCR5 interaction

We recently described the isolation and structural characterization of 2'-fluoropyrimidine-substituted RNA aptamers that bind to gp120 of R5 strains of human immunodeficiency virus type 1 and thereby potently neutralize the infectivity of phylogenetically diverse R5 strains. Here we investigate the physical basis of their antiviral action. We show that both N-linked oligosaccharides and the variable loops V1/V2 and V3 are not required for binding of one aptamer, B40, to gp120. Using surface plasmon resonance binding analyses, we show that the aptamer binds to the CCR5-binding site on gp120 in a relatively CD4-independent manner, providing a mechanistic explanation for its neutralizing potency.

A highly conserved arginine in gp120 governs HIV-1 binding to both syndecans and CCR5 via sulfated motifs

HIV-1 has maximized its utilization of syndecans. It uses them as in cis receptors to infect macrophages and as in trans receptors to infect T-lymphocytes. In this study, we investigated at a molecular level the mechanisms that control HIV-1-syndecan interactions. We found that a single conserved arginine (Arg-298) in the V3 region of gp120 governs HIV-1 binding to syndecans. We found that an amine group on the side chain of this residue is necessary for syndecan utilization by HIV-1. Furthermore, we showed that HIV-1 binds syndecans via a 6-O sulfation, demonstrating that this binding is not the result of random interactions between basic residues and negative charges, but the result of specific contacts between gp120 and a well defined sulfation in syndecans. Surprisingly, we found that Arg-298, which mediates HIV-1 binding to syndecans, also mediates HIV-1 binding to CCR5. We postulated that HIV-1 recognizes similar motifs on syndecans and CCR5. Supporting this hypothesis, we obtained several lines of evidence that suggest that the 6-O sulfation recognized by HIV-1 on syndecans mimics the sulfated tyrosines recognized by HIV-1 in the N terminus of CCR5. Our finding that CCR5 and syndecans are exploited by HIV-1 via a single determinant echoes the mechanisms by which chemokines utilize these two disparate receptors and suggests that the gp120/chemokine mimicry may represent a common strategy in microbial pathogenesis.

CCR5 N-terminal Region Plays a Critical Role in HIV-1 Inhibition by Toxoplasma gondii-derived Cyclophilin-18

Molecular mimicry of chemokine ligands has been described for several pathogens. Toxoplasma gondii produces a protein, cyclophilin-18 (C-18), which binds to the human immunodeficiency virus (HIV) co-receptor CCR5 and inhibits fusion and infection of T cells and macrophages by R5 viruses but not by X4 viruses. We recently identified structural determinants of C-18 required for anti-HIV activity (Yarovinsky, F., Andersen, J. F., King, L. R., Caspar, P., Aliberti, J., Golding, H., and Sher, A. (2004) J. Biol. Chem. 279, 53635-53642). Here we have elucidated the fine specificity of CCR5 residues involved in binding and HIV inhibitory potential of C-18. To delineate the regions of CCR5 involved in C-18 binding, we analyzed C-18 inhibition of cells expressing CXCR4/CCR5 chimeric receptors and CCR5 with a truncated N terminus (Delta2-19). These experiments identified a critical role for the N terminus of CCR5 in C-18 binding and anti-HIV activity. Studies with a large panel of CCR5 N-terminal peptides, including Tyr-sulfated analogues, truncated peptides, and alanine-scanning mutants, suggested that each of the 12-17 amino acids in the N terminus of CCR5 are essential for C-18 binding and inhibitory activity. Tyr sulfation did not improve C-18 reactivity. This finding is of interest because the same CCR5 N-terminal region was shown previously to play a key role in binding of HIV-1 envelope glycoproteins. The elucidation of the functional C-18-binding mechanism may help in the rational design of novel antiviral agents against HIV.

Functional mimicry of a human immunodeficiency virus type 1 coreceptor by a neutralizing monoclonal antibody

Interaction of the human immunodeficiency virus type 1 (HIV-1) gp120 envelope glycoprotein with the primary receptor, CD4, promotes binding to a chemokine receptor, either CCR5 or CXCR4. The chemokine receptor-binding site on gp120 elicits CD4-induced (CD4i) antibodies in some HIV-1-infected individuals. Like CCR5 itself, the CD4i antibody 412d exhibits a preference for CCR5-using HIV-1 strains and utilizes sulfated tyrosines to achieve binding to gp120. Here, we show that 412d binding requires the gp120 beta19 strand and the base of the V3 loop, elements that are important for the binding of the CCR5 N terminus. Two gp120 residues in the V3 loop base determined 412d preference for CCR5-using HIV-1 strains. A chimeric molecule in which the 412d heavy-chain third complementarity-determining loop sequence replaces the CCR5 N terminus functioned as an efficient second receptor, selectively supporting the entry of CCR5-using HIV-1 strains. Sulfation of N-terminal tyrosines contributed to the function of this chimeric receptor. These results emphasize the close mimicry of the CCR5 N terminus by the gp120-interactive region of a naturally elicited CD4i antibody.

The synthetic peptide derived from the NH2-terminal extracellular region of an orphan G protein-coupled receptor, GPR1, preferentially inhibits infection of X4 HIV-1

Several G protein-coupled receptors (GPCRs) serve as co-receptors for entry of human immunodeficiency virus type 1 (HIV-1) into target cells. Here we report that a synthetic peptide derived from the NH2-terminal extracellular region of an orphan GPCR, GPR1 (GPR1ntP-(1-27); MEDLEETLFEEFENYSYDLDYYSLESC), inhibited infection of not only an HIV-1 variant that uses GPR1 as a co-receptor, but also X4, R5, and R5X4 viruses. Among these HIV-1 strains tested, viruses that can utilize CXCR4 as their co-receptors were preferentially inhibited. Inhibition of early steps in X4 virus replication was also detected in the primary human peripheral blood lymphocytes. GPR1ntP-(1-27) directly interacted with recombinant X4 envelope glycoprotein (rgp120). This interaction was neither inhibited nor enhanced by the soluble CD4 (sCD4) but inhibited by the anti-third variable (V3) loop-specific monoclonal antibody and heparin known to bind to the V3 loop. Although the conformational changes in gp120, including the V3 loop, have been reported to be required for its interaction with a co-receptor after binding of gp120 to CD4, it has also been reported that the V3 loop is already exposed on the surface of virions before interaction with CD4. We found that GPR1ntP-(1-27) blocked binding of virus to the cells, and this peptide equally bound to rgp120 in the presence or absence of sCD4. Because we detected the binding of GPR1ntP-(1-27) to the highly purified virions even in the absence of sCD4, GPR1ntP-(1-27) probably recognized the V3 loop exposed on the virions, and this interaction was responsible for the anti-HIV-1 activity of GPR1ntP-(1-27).

Highly conserved beta16/beta17 beta-hairpin structure in human immunodeficiency virus type 1 YU2 gp120 is critical for CCR5 binding

Whereas gp120 CD4-induced structures have been largely documented and at least in part elucidated by crystallization, information about gp120 coreceptor-induced structures remains incomplete despite numerous studies. In this work, mutations were carried out in a selected internal region of HIV-1/YU2 gp120, proximal to the CD4-binding site, because of its highly conserved nature among retroviruses and its high structural stability. The targeted residues, belonging to the beta16/beta17 beta-hairpin, modulate gp120 binding to CD4 and gp120-CD4 complex binding to CCR5. Thus, it appears that this gp120 structure acts as a hinge between the CD4-binding site and the putative coreceptor binding structure. Substitution of amino acid residues like E381A did not affect gp120 binding to CD4 and did not induce significant structural changes in gp120, as demonstrated by epitope analysis, BIACORE analysis, and circular dichroism. Nevertheless, E381 has a critical influence on the maintenance of CCR5 coreceptor binding by forming a salt bridge with K207. Another important element of the beta-hairpin in this interaction is the probable hydrophobic link between F383 and I420. Altogether, these results suggest that the beta-hairpin structure likely governs interactions between the surface of gp120 with native CCR5 or the CCR5 amino-terminal domain (CCR5-Nt). The mutations within the beta-hairpin had a direct effect on the proximal surface of the bridging sheet, the putative CCR5 surface, and the gp120 YU2 HIV-1-CD4 binding site. These results on the gp120-CCR5-Nt binding mechanism contribute to our understanding of CCR5 and HIV-1 gp120 association and HIV-1 entry; they may also contribute to designing novel inhibitors.

Variants of human immunodeficiency virus type 1 that efficiently use CCR5 lacking the tyrosine-sulfated amino terminus have adaptive mutations in gp120, including loss of a functional N-glycan

By selecting the R5 human immunodeficiency virus type 1 (HIV-1) strain JR-CSF for efficient use of a CCR5 coreceptor with a badly damaged amino terminus [i.e., CCR5(Y14N)], we previously isolated variants that weakly utilize CCR5(Delta18), a low-affinity mutant lacking the normal tyrosine sulfate-containing amino-terminal region of the coreceptor. These previously isolated HIV-1(JR-CSF) variants contained adaptive mutations situated exclusively in the V3 loop of their gp120 envelope glycoproteins. We now have weaned the virus from all dependency on the CCR5 amino terminus by performing additional selections with HeLa-CD4 cells that express only a low concentration of CCR5(Delta18). The adapted variants had additional mutations in their V3 loops, as well as one in the V2 stem (S193N) and four alternative mutations in the V4 loop that eliminated the same N-linked oligosaccharide from position N403. Assays using pseudotyped viruses suggested that these new gp120 mutations all made strong contributions to use of CCR5(Delta18) by accelerating a rate-limiting CCR5-dependent conformational change in gp41 rather than by increasing viral affinity for this damaged coreceptor. Consistent with this interpretation, loss of the V4 N-glycan at position N403 also enhanced HIV-1 use of a different low-affinity CCR5 coreceptor with a mutation in extracellular loop 2 (ECL2) [i.e., CCR5(G163R)], whereas the double mutant CCR5(Delta18,G163R) was inactive. We conclude that loss of the N-glycan at position N403 helps to convert the HIV-1 envelope into a hair-trigger form that no longer requires strong interactions with both the CCR5 amino terminus and ECL2 but efficiently uses either site alone. These results demonstrate a novel functional role for a gp120 N-linked oligosaccharide and a high degree of adaptability in coreceptor usage by HIV-1.

Chemokine receptor CCR5: insights into structure, function, and regulation

CC chemokine receptor 5 (CCR5) is a seven-transmembrane, G protein-coupled receptor (GPCR) which regulates trafficking and effector functions of memory/effector T-lymphocytes, macrophages, and immature dendritic cells. It also serves as the main coreceptor for the entry of R5 strains of human immunodeficiency virus (HIV-1, HIV-2). Chemokine binding to CCR5 leads to cellular activation through pertussis toxin-sensitive heterotrimeric G proteins as well as G protein-independent signalling pathways. Like many other GPCR, CCR5 is regulated by agonist-dependent processes which involve G protein coupled receptor kinase (GRK)-dependent phosphorylation, beta-arrestin-mediated desensitization and internalization. This review discusses recent advances in the elucidation of the structure and function of CCR5, as well as the complex mechanisms that regulate CCR5 signalling and cell surface expression.

Sulphated tyrosines mediate association of chemokines and Plasmodium vivax Duffy binding protein with the Duffy antigen/receptor for chemokines (DARC)

Plasmodium vivax is one of four Plasmodium species that cause human malaria. P. vivax and a related simian malaria parasite, Plasmodium knowlesi, invade erythrocytes by binding the Duffy antigen/receptor for chemokines (DARC) through their respective Duffy binding proteins. Here we show that tyrosines 30 and 41 of DARC are modified by addition of sulphate groups, and that the sulphated tyrosine 41 is essential for association of the Duffy binding proteins of P. vivax (PvDBP) and P. knowlesi (PkDaBP) with DARC-expressing cells. These sulphated tyrosines also participate in the association of DARC with each of its four known chemokine ligands. Alteration of tyrosine 41 to phenylalanine interferes with MCP-1, RANTES and MGSA association with DARC, but not with that of IL8. In contrast, alteration of tyrosine 30 to phenylalanine interferes with the association of IL8 with DARC. A soluble sulphated amino-terminal domain of DARC, but not one modified to phenylalanine at residue 41, can be used to block the association of PvDBP and PkDaBP with red blood cells, with an IC50 of approximately 5 nM. These data are consistent with a role for tyrosine sulphation in the association of many or most chemokines with their receptors, and identify a key molecular determinant of erythrocyte invasion by P. vivax.

The structures of the unique sulfotransferase retinol dehydratase with product and inhibitors provide insight into enzyme mechanism and inhibition

The structure of retinol dehydratase (DHR) from Spodoptera frugiperda, a member of the sulfotransferase superfamily, in complexes with the inactive form of the cofactor PAP 3'-phosphoadenosine 5'-phosphate (PAP) and (1) the product of the reaction with retinol anhydroretinol (AR), (2) the retinoid inhibitor all-trans-4-oxoretinol (OR), and (3) the potent steroid inhibitor androsterone (AND) have been determined and compared to the enzyme complex with PAP and retinol. The structures show that the geometry of the active-site amino acids is largely preserved in the various complexes. However, the beta-ionone rings of the retinoids are oriented differently with respect to side chains that have been shown to be important for the enzymatic reaction. In addition, the DHR:PAP:AND complex reveals a novel mode for steroid binding that contrasts significantly with that for steroid binding in other sulfotransferases. The molecule is displaced and rotated approximately 180 degrees along its length so that there is no acceptor hydroxyl in close proximity to the site of sulfate transfer. This observation explains why steroids are potent inhibitors of retinol dehydratase activity, rather than substrates for sulfonation. Most of the steroid-protein contacts are provided by the alpha-helical cap that distinguishes this member of the superfamily. This observation suggests that in addition to providing a chemical environment that promotes the dehydration of a sulfonated intermediate, the cap may also serve to minimize a promiscuous sulfotransferases activity.

Characterization of noncovalent protein-ligand complexes and associated enzyme intermediates of GlcNAc-6-O-sulfotransferase by electrospray ionization FT-ICR mass spectrometry

In this study, a GlcNAc-6-O-Sulfotransferase, NodST and its complexation with the substrate 3'-phosphoadenosine 5'-phosphosulfate (PAPS) and the inhibitor 3'-phosphoadenosine 5'-phosphate (PAP) were studied using Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. In addition, using isotopically labeled substrate, we have successfully confirmed a sulfated enzyme intermediate, which was predicted by the MS kinetic measurement. It is also shown that information regarding solution binding affinities can be obtained using electrospray ionization (ESI)-FTICR mass spectrometry. The relative binding constants, Kd(PAPS)/Kd(PAP), derived from the solution and gas phase were very similar, which suggests that the binding domain of this particular enzyme system, given known structures of other sulfotransferases, may be preserved during the transmission of the complex from solution to the gas phase.

Rapid Discovery of Potent Sulfotransferase Inhibitors by Diversity-Oriented Reaction in Microplates Followed by in situ Screening

Rapid diversity-oriented microplate library synthesis and in situ screening with a high-throughput fluorescence-based assay were used to develop potent inhibitors of beta-arylsulfotransferase IV (beta-AST-IV). This strategy leads to facile inhibitor synthesis and study as it allows protecting-group manipulation and product isolation from other library components to be avoided. Through repeated library formation, three aspects of inhibitor makeup, the identities of the two binding groups and the length of the linker between them, were independently optimized. Several potent inhibitors were obtained, one of which was determined to have an inhibition constant K(i) of 5 nM. This compound is the most potent beta-AST-IV inhibitor developed to date, with a K(i) value more than five orders of magnitude lower than the Michaelis constant K(m) for the substrate whose binding it inhibits.

Comparison of SIVmac239(352-382) and SIVsmmPBj41(360-390) enterotoxic synthetic peptides

To characterize the active domain of the simian immunodeficiency virus (SIV) surface unit (SU) enterotoxin, peptides corresponding to the V3 loop of SIVmac239 (SIVmac) and SIVsmmPBj41 (SIVpbj) were synthesized and examined for enterotoxic activity, alpha-helical structure, and interaction(s) with model membranes. SIVmac and SIVpbj induced a dose-dependent diarrhea in 6-8-day-old mouse pups similar to full-length SU. The peptides mobilized [Ca(2+)](i) in HT-29 cells with distinct oscillations and elevated inositol triphosphate levels. Circular dichroism analyses showed the peptides were predominantly random coil in buffer, but increased in alpha-helical content when placed in a hydrophobic environment or with cholesterol-containing membrane vesicles that are rich in anionic phospholipids. None of the peptides underwent significant secondary structural changes in the presence of neutral vesicles indicating ionic interactions were important. These data show that the SIV SU enterotoxic domain localizes in part to the V3 loop region and interacts with anionic membrane domains on the host cell surface.

Structural basis of tyrosine sulfation and VH-gene usage in antibodies that recognize the HIV type 1 coreceptor-binding site on gp120

The conserved surface of the HIV-1 gp120 envelope glycoprotein that binds to the HIV-1 coreceptor is protected from humoral recognition by multiple layers of camouflage. Here we present sequence and genomic analyses for 12 antibodies that pierce these defenses and determine the crystal structures of 5. The data reveal mechanisms and atomic-level details for three unusual immune features: posttranslational mimicry of coreceptor by tyrosine sulfation of antibody, an alternative molecular mechanism controlling such sulfation, and highly selective V(H)-gene usage. When confronted by extraordinary viral defenses, the immune system unveils novel adaptive capabilities, with tyrosine sulfation enhancing the vocabulary of antigen recognition.

Contrasting use of CCR5 structural determinants by R5 and R5X4 variants within a human immunodeficiency virus type 1 primary isolate quasispecies

Macrophagetropic R5 human immunodeficiency virus type 1 (HIV-1) isolates often evolve into dualtropic R5X4 variants during disease progression. The structural basis for CCR5 coreceptor function has been studied in a limited number of prototype strains and suggests that R5 and R5X4 Envs interact differently with CCR5. However, differences between unrelated viruses may reflect strain-specific factors and do not necessarily represent changes resulting from R5 to R5X4 evolution of a virus in vivo. Here we addressed CCR5 domains involved in fusion for a large set of closely related yet functionally distinct variants within a primary isolate swarm, employing R5 and R5X4 Envs derived from the HIV-1 89.6(PI) quasispecies. R5 variants of 89.6(PI) could fuse using either N-terminal or extracellular loop CCR5 sequences in the context of CCR5/CXCR2 chimeras, similar to the unrelated R5 strain JRFL, but R5X4 variants of 89.6(PI) were highly dependent on the CCR5 N terminus. Similarly, R5 89.6(PI) variants and isolate JRFL tolerated N-terminal CCR5 deletions, but fusion by most R5X4 variants was markedly impaired. R5 89.6(PI) Envs also tolerated multiple extracellular domain substitutions, while R5X4 variants did not. In contrast to CCR5 use, fusion by R5X4 variants of 89.6(PI) was largely independent of the CXCR4 N-terminal region. Thus, R5 and R5X4 species from a single swarm differ in how they interact with CCR5. These results suggest that R5 Envs possess a highly plastic capacity to interact with multiple CCR5 regions and support the concept that viral evolution in vivo results from the emergence of R5X4 variants with the capacity to use the CXCR4 extracellular loops but demonstrate less-flexible interactions with CCR5 that are strongly dependent on the N-terminal region.

Specific inhibition of HIV-1 coreceptor activity by synthetic peptides corresponding to the predicted extracellular loops of CCR5

We used synthetic peptides to the extracellular loops (ECLs) of CCR5 to examine inhibitory effects on HIV infection/fusion with primary leukocytes and cells expressing recombinant CCR5. We show for the first time that peptides derived from the first, second, or third ECL caused dose-dependent inhibition of fusion and infection, although with varying potencies and specificities for envelope glycoproteins (Envs) from different strains. The first and third ECL peptides inhibited Envs from the R5 Ba-L strain and the R5X4 89.6 strain, whereas the second ECL peptide inhibited Ba-L but not 89.6 Env. None of the peptides affected fusion mediated by Env from the X4 LAV strain. Fusion mediated by Envs from several primary HIV-1 isolates was also inhibited by the peptides. These findings suggest that various HIV-1 strains use CCR5 domains in different ways. Experiments involving peptide pretreatment and washing, modulation of the expression levels of Env and CCR5, analysis of CCR5 peptide effects against different coreceptors, and inhibition of radiolabeled glycoprotein (gp) 120 binding to CCR5 suggested that the peptide-blocking activities reflect their interactions with gp120. The CCR5-derived ECL peptides thus provide a useful approach to analyze structure-function relationships involved in HIV-1 Env-coreceptor interactions and may have implications for the design of drugs that inhibit HIV infection.

Constitutive activation of CCR5 and CCR2 induced by conformational changes in the conserved TXP motif in transmembrane helix 2

CCR5 is a G protein-coupled receptor for RANTES, MIP-1alpha, MIP-1beta, and MCP-2 that functions as the front line coreceptor for human immunodeficiency virus type 1 infection. To elucidate the mechanism for CCR5 activation, this coreceptor was expressed in yeast coupled to the pheromone response pathway and a constitutively active mutant (CAM) was derived by random mutagenesis. Conversion of Thr-82 in the highly conserved TXP motif in transmembrane helix 2 to Pro, His, Tyr, Arg, or Lys conferred autonomous signaling activity in yeast and mammalian cells. This substitution also imparted constitutive signaling to CCR2 in yeast and mammalian cells, but not CCR1, CCR3, CCR4, CXCR2, or CXCR4. The CCR5-CAM, but not the CCR2-CAM had a reduction in ligand binding affinity. Whereas the amplitude of calcium mobilization induced by RANTES stimulation was lower in the CCR5-CAM than the wild-type (WT) receptor, MCP-1 induced a higher signal in the CCR2-CAM than in CCR2-WT. The chemotactic response of CCR5-CAM(T82P) to RANTES was similar to that of CCR5-WT, but CCR5-CAM(T82K) was dramatically decreased. The chemotactic response of CCR2-WT and CCR2-CAM(T94K) were similar. These findings extend insight into the role of the TXP motif in the mechanism for CCR5 signaling. CCR2, the receptor most closely genetically related to CCR5, shared a similar signaling mechanism, but other receptors containing the TXP motif did not. The expression of CCR5 and CCR2 in yeast and the availability of variants with autonomous signaling represent critical tools for characterizing receptor antagonists and developing approaches to block their role in human diseases.

CCR5 N-terminus peptides enhance X4 HIV-1 infection by CXCR4 up-regulation

The HIV-1 envelope glycoprotein gp120 interacts consecutively with CD4 and CCR5 to mediate the entry of R5-HIV-1 strains into target cells. The N-terminus of CCR5, which contains several sulfated tyrosines, plays a critical role in gp120-CCR5 binding and, consequently, in viral entry. Here, we demonstrate that a tyrosine sulfated peptide, reproducing the entire N-terminal extracellular region of CCR5, its unsulfated analogue, and a point-mutated peptide are unable to inhibit R5-HIV-1 mediated infection, competing with the entire CCR5 in the formation of gp120-CD4-CCR5 complex. Surprisingly, these peptides show the capability of enhancing HIV-1 infection caused by X4 strains through the up-regulation of both CD4 and CXCR4 receptors.

Tyrosine sulfation of human antibodies contributes to recognition of the CCR5 binding region of HIV-1 gp120

Sulfated tyrosines at the amino terminus of the principal HIV-1 coreceptor CCR5 play a critical role in its ability to bind the HIV-1 envelope glycoprotein gp120 and mediate HIV-1 infection. Here, we show that a number of human antibodies directed against gp120 are tyrosine sulfated at their antigen binding sites. Like that of CCR5, antibody association with gp120 is dependent on sulfate moieties, enhanced by CD4, and inhibited by sulfated CCR5-derived peptides. Most of these antibodies preferentially associate with gp120 molecules of CCR5-utilizing (R5) isolates and neutralize primary R5 isolates more efficiently than laboratory-adapted isolates. These studies identify a distinct subset of CD4-induced HIV-1 neutralizing antibodies that closely emulate CCR5 and demonstrate that tyrosine sulfation can contribute to the potency and diversity of the human humoral response.

Spying on HIV with SPR

The application of surface plasmon resonance (SPR)-based optical biosensors has contributed extensively to our understanding of functional aspects of HIV. SPR biosensors allow the analysis of real-time interactions of any biomolecule, be it protein, nucleic acid, lipid, carbohydrate or small molecule, without the need for intrinsic or extrinsic probes. As such, the technology has been used to analyze molecular interactions associated with every aspect of the viral life cycle, from basic studies of binding events occurring during docking, replication, budding and maturation to applied research related to vaccine and inhibitory drug development. Along the way, SPR biosensors have provided a unique and detailed view into the inner workings of HIV.

Mechanistic studies of beta-arylsulfotransferase IV

Sulfotransferases are an important class of enzymes that catalyze the transfer of a sulfuryl group to a hydroxyl or amine moiety on various molecules including small-molecule drugs, steroids, hormones, carbohydrates, and proteins. They have been implicated in a number of disease states but remain poorly understood, complicating the design of specific, small-molecule inhibitors. A linear free-energy analysis in both the forward and reverse directions indicates that the transfer of a sulfuryl group to an aryl hydroxyl group catalyzed by beta-arylsulfotransferase IV likely proceeds by a dissociative (sulfotrioxide-like) mechanism. Values for the Brønsted coefficients (beta(nuc) and beta(lg)) are +0.33 and -0.45, giving Leffler alpha values of 0.19 and 0.61 for the forward and reverse reactions, respectively.

Identification of gp120 binding sites on CXCR4 by using CD4-independent human immunodeficiency virus type 2 Env proteins

Human immunodeficiency virus (HIV) and simian (SIV) immunodeficiency virus entry is mediated by binding of the viral envelope glycoprotein (Env) to CD4 and chemokine receptors, CCR5 and/or CXCR4. CD4 induces extensive conformational changes that expose and/or induce formation of a chemokine receptor binding site on gp120. CD4-independent Env's of HIV type 1 (HIV-1), HIV-2, and SIV have been identified that exhibit exposed chemokine receptor binding sites and can bind directly to CCR5 or CXCR4 in the absence of CD4. While many studies have examined determinants for gp120-CCR5 binding, analysis of gp120-CXCR4 binding has been hindered by the apparently lower affinity of this interaction for X4-tropic HIV-1 isolates. We show here that gp120 proteins from two CD4-independent HIV-2 Env's, VCP and ROD/B, bind directly to CXCR4 with an apparently high affinity. By use of CXCR4 N-terminal deletion constructs, CXCR4-CXCR2 chimeras, and human-rat CXCR4 chimeras, binding determinants were shown to reside in the amino (N) terminus, extracellular loop 2 (ECL2), and ECL3. Alanine-scanning mutagenesis of charged residues, tyrosines, and phenylalanines in extracellular CXCR4 domains implicated multiple amino acids in the N terminus (E14/E15, D20, Y21, and D22), ECL2 (D187, R188, F189, Y190, and D193), and ECL3 (D262, E268, E277, and E282) in binding, although minor differences were noted between VCP and ROD/B. However, mutations in CXCR4 that markedly reduced binding did not necessarily hinder cell-cell fusion by VCP or ROD/B, especially in the presence of CD4. These gp120 proteins will be useful in dissecting determinants for CXCR4 binding and Env triggering and in evaluating pharmacologic inhibitors of the gp120-CXCR4 interaction.

A potent and highly selective sulfotransferase inhibitor

In the present work, we have used a newly developed, fluorescence-based assay to screen a library of >30 000 compounds as potential beta-arylsulfotransferase-IV inhibitors. A total of 11 inhibitors were discovered. Most of the compounds discovered showed low micromolar inhibition, but one of the compounds showed potent inhibition (Ki = 96 nM). The most potent of these inhibitors was tested against a variety of other purine binding enzymes and showed remarkable specificity.

Ionic interaction of the HIV-1 V3 domain with CCR5 and deregulation of T lymphocyte function

We have reported that the principal neutralizing domain of V3 of the HIV-1 gp120 induces an antigen-specific activation apoptosis of responding effector CD4+ T lymphocytes, a phenomenon inhibited by RANTES, an agonist of CCR5. Here, addressing the question of how a hypervariable region could induce such a selective reaction, we demonstrated that the magnitude of the activation phase was dependent on the number of basic amino acids present in the V3 peptide, an observation confirmed by using V3 peptides with appropriate basic amino acid substitutions. The relative position of the amino acids in the V3 peptide did not affect the biological phenomenon. Using surface plasmon resonance biosensor analysis, we also provided direct evidence of the influence of basic amino acids in the interaction between V3 and the amino terminal domain of CCR5. Sulphation of tyrosines in the CCR5 peptide was essential. Our results confirm gp120 modelling predictions and demonstrate simple molecular ionic interactions as capable of affecting key cell events, the wider biological implications of which need to be further explored.

Identification of conserved and variable structures in the human immunodeficiency virus gp120 glycoprotein of importance for CXCR4 binding

CD4 and the chemokine receptors, CXCR4 and CCR5, serve as receptors for human immunodeficiency virus type 1 (HIV-1). Binding of the HIV-1 gp120 envelope glycoprotein to the chemokine receptors normally requires prior interaction with CD4. Mapping the determinants on gp120 for the low-affinity interaction with CXCR4 has been difficult due to the nonspecific binding of this viral glycoprotein to cell surfaces. Here we examine the binding of a panel of gp120 mutants to paramagnetic proteoliposomes displaying CXCR4 on their surfaces. We show that the gp120 beta19 strand and third variable (V3) loop contain residues important for CXCR4 interaction. Basic residues from both elements, as well as a conserved hydrophobic residue at the V3 tip, contribute to CXCR4 binding. Removal of the gp120 V1/V2 variable loops allows the envelope glycoprotein to bind CXCR4 in a CD4-independent manner. These results indicate that although some variable gp120 residues contribute to the specific binding to CCR5 or CXCR4, gp120 elements common to CXCR4- or CCR5-using strains are involved in the interaction with both coreceptors.

Tyrosine-sulfated peptides functionally reconstitute a CCR5 variant lacking a critical amino-terminal region

Entry of most primary human immunodeficiency virus, type 1 (HIV-1) isolates into their target cells requires the cellular receptor CD4 and the G protein-coupled chemokine coreceptor CCR5. An acidic, tyrosine-rich, and tyrosine-sulfated domain of the CCR5 amino terminus plays a critical role in the ability of CCR5 to serve as an HIV-1 coreceptor, and tyrosine-sulfated peptides based on this region physically associate with the HIV-1 envelope glycoprotein gp120 and slow HIV-1 entry into CCR5-expressing cells. Here we show that the same tyrosine-sulfated peptides, but not their unsulfated analogs, can restore the HIV-1 coreceptor activity of a CCR5 variant lacking residues 2-17 of its amino terminus. Additionally, these sulfated peptides restored the ability of this CCR5 variant to mobilize calcium in response to the chemokines macrophage inflammatory factors 1alpha and 1beta. These observations show that a tyrosine-sulfated region of the CCR5 amino terminus can function independently to mediate association of chemokines and the HIV-1 envelope glycoprotein with the remaining domains of CCR5.

The crown and stem of the V3 loop play distinct roles in human immunodeficiency virus type 1 envelope glycoprotein interactions with the CCR5 coreceptor

Human immunodeficiency virus type 1 envelope glycoprotein gp120 interacts with CD4 and the CCR5 coreceptor in order to mediate viral entry. A CD4-induced surface on gp120, primarily composed of residues in the V3 loop and the C4 domain, interacts with CCR5. In the present study, we generated envelope glycoproteins comprising chimeric V3 loops and/or V3 loops with deletions and studied their binding to CCR5 amino-terminal domain (Nt)-based sulfopeptides and cell surface CCR5, as well as their ability to mediate viral entry. We thus delineated two functionally distinct domains of the V3 loop, the V3 stem and the V3 crown. The V3 stem alone mediates soluble gp120 binding to the CCR5 Nt. In contrast, both the V3 stem and crown are required for soluble gp120 binding to cell surface CCR5. Within the context of a virion, however, the V3 crown alone determines coreceptor usage. Our data support a two-site gp120-CCR5 binding model wherein the V3 crown and stem interact with distinct regions of CCR5 in order to mediate viral entry.

Tyrosine sulfation of CCR5 N-terminal peptide by tyrosylprotein sulfotransferases 1 and 2 follows a discrete pattern and temporal sequence

The CC-chemokine receptor 5 (CCR5) is the major coreceptor for the entry of macrophage-tropic (R5) HIV-1 strains into target cells. Posttranslational sulfation of tyrosine residues in the N-terminal tail of CCR5 is critical for high affinity interaction of the receptor with the HIV-1 envelope glycoprotein gp120 in complex with CD4. Here, we focused on defining precisely the sulfation pattern of the N terminus of CCR5 by using recombinant human tyrosylprotein sulfotransferases TPST-1 and TPST-2 to modify a synthetic peptide that corresponds to amino acids 2-18 of the receptor (CCR5 2-18). Analysis of the reaction products was made with a combination of reversed-phase HPLC, proteolytic cleavage, and matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS). We found that CCR5 2-18 is sulfated by both TPST isoenzymes leading to a final product with four sulfotyrosine residues. Sulfates were added stepwise to the peptide producing specific intermediates with one, two, or three sulfotyrosines. The pattern of sulfation in these intermediates suggests that Tyr-14 and Tyr-15 are sulfated first, followed by Tyr-10, and finally Tyr-3. These results represent a detailed analysis of the multiple sulfation reaction of a peptide substrate by TPSTs and provide a structural basis for understanding the role of tyrosine sulfation of CCR5 in HIV-1 coreceptor and chemokine receptor function.

Characterization of the anti-HIV effects of native lactoferrin and other milk proteins and protein-derived peptides

In a search for natural proteins with anti-HIV activity, we screened a large set of purified proteins from bovine milk and peptide fragments thereof. Because several charged proteins and peptides are known to inhibit the process of virus entry, we selected proteins with an unusual charge composition or hydrophobicity profile. In contrast with some chemically modified (strongly negative) milk proteins, unmodified alpha(s2)-, beta- and kappa-casein, as well as several negatively and positively charged fragments thereof, did not show significant inhibition of virus replication. In fact, HIV-1 replication was elevated in the presence of beta-casein or amphiphilic fragments thereof. Bovine lactoferrin (bLF), a milk protein of 80 kDa, showed considerable inhibitory activity against HIV-1 with an IC50 of 0.4 microM. Modest inhibition was obtained with lactoferricin, a highly positively charged loop domain of bLF, indicating that other domains within the native bLF protein may also be required for inhibition. bLF blocked HIV-1 variants that use either the CXCR4 or the CCR5 coreceptor. In order to obtain further insight into the mechanism of action of this antiviral protein, we selected a bLF-resistant HIV-1 variant. The bLF-resistance phenotype is mediated by the viral envelope protein, which contains two interesting mutations that have previously been associated with an altered virus-host interaction and a modified receptor-coreceptor interaction. These results demonstrate that bLF targets the HIV-1 entry process.

Anti-HIV-1 activity of cellulose acetate phthalate: synergy with soluble CD4 and induction of "dead-end" gp41 six-helix bundles

BACKGROUND

Cellulose acetate phthalate (CAP), a promising candidate microbicide for prevention of sexual transmission of the human immunodeficiency virus type 1 (HIV-1) and other sexually transmitted disease (STD) pathogens, was shown to inactivate HIV-1 and to block the coreceptor binding site on the virus envelope glycoprotein gp120. It did not interfere with virus binding to CD4. Since CD4 is the primary cellular receptor for HIV-1, it was of interest to study CAP binding to HIV-1 complexes with soluble CD4 (sCD4) and its consequences, including changes in the conformation of the envelope glycoprotein gp41 within virus particles.

METHODS

Enzyme-linked immunosorbent assays (ELISA) were used to study CAP binding to HIV-1-sCD4 complexes and to detect gp41 six-helix bundles accessible on virus particles using antibodies specific for the alpha-helical core domain of gp41.

RESULTS

1) Pretreatment of HIV-1 with sCD4 augments subsequent binding of CAP; 2) there is synergism between CAP and sCD4 for inhibition of HIV-1 infection; 3) treatment of HIV-1 with CAP induced the formation of gp41 six-helix bundles.

CONCLUSIONS

CAP and sCD4 bind to distinct sites on HIV-1 IIIB and BaL virions and their simultaneous binding has profound effects on virus structure and infectivity. The formation of gp41 six-helical bundles, induced by CAP, is known to render the virus incompetent for fusion with target cells thus preventing infection.

Grafting segments from the extracellular surface of CCR5 onto a bacteriorhodopsin transmembrane scaffold confers HIV-1 coreceptor activity

Components from the extracellular surface of CCR5 interact with certain macrophage-tropic strains of human immunodeficiency virus type 1 (HIV-1) to mediate viral fusion and entry. To mimic these viral interacting site(s), the amino-terminal and extracellular loop segments of CCR5 were linked in tandem to form concatenated polypeptides, or grafted onto a seven-transmembrane bacteriorhodopsin scaffold to generate several chimeras. The chimera studies identified specific regions in CCR5 that confer HIV-1 coreceptor function, structural rearrangements in the transmembrane region that may modulate this activity, and a role for the extracellular surface in folding and assembly. Methods developed here may be applicable to the dissection of functional domains from other seven-transmembrane receptors and form a basis for future structural studies.

CCR5 and CXCR4 usage by non-clade B human immunodeficiency virus type 1 primary isolates

CCR5 and CXCR4 usage has been studied extensively with a variety of clade B human immunodeficiency virus type 1 (HIV-1) isolates. The determinants of CCR5 coreceptor function are remarkably consistent, with a region critical for fusion and entry located in the CCR5 amino-terminal domain (Nt). In particular, negatively charged amino acids and sulfated tyrosines in the Nt are essential for gp120 binding to CCR5. The same types of residues are important for CXCR4-mediated viral fusion and entry, but they are dispersed throughout the extracellular domains of CXCR4, and their usage is isolate dependent. Here, we report on the determinants of CCR5 and CXCR4 coreceptor function for a panel of non-clade B isolates that are responsible for the majority of new HIV-1 infections worldwide. Consistent with clade B isolates, CXCR4 usage remains isolate dependent and is determined by the overall content of negatively charged and tyrosine residues. Residues in the Nt of CCR5 that are important for fusion and entry of clade B isolates are also important for the entry of all non-clade B HIV-1 isolates that we tested. Surprisingly, we found that in contrast to clade B isolates, a cluster of residues in the second extracellular loop of CCR5 significantly affects fusion and entry of all non-clade B isolates tested. This points to a different mechanism of CCR5 usage by these viruses and may have important implications for the development of HIV-1 inhibitors that target CCR5 coreceptor function.

Peptide interactions with G-protein coupled receptors

Peptide recognition by G-protein coupled receptors (GPCRs) is reviewed with an emphasis on the indirect approach used to determine the receptor-bound conformation of peptide ligands. This approach was developed in response to the lack of detailed structural information available for these receptors. Recent advances in the structural determination of rhodopsin (the GPCR of the visual system) by crystallography have provided a scaffold for homology modeling of the inactive state of a wide variety of GPCRs that interact with peptide messages. Additionally, the ability to mutate GPCRs and assay compounds of similar chemical structure to test a common binding site on the receptor provides a firm experimental basis for structure-activity studies. Recognition motifs, common in other well-studied systems such as proteolytic enzymes and major histocompatibility class receptors (MHC) are reviewed briefly to provide a basis of comparison. Finally, the development of true peptidomimetics is contrasted with nonpeptide ligands, discovered through combinatorial chemistry. In many systems, the evidence suggests that the peptide ligands bind at the interface between the transmembrane segments and the extracellular loops, while nonpeptide antagonists bind within the transmembrane segments. Plausible models of GPCRs and the mechanism by which they activate G-proteins on binding peptides are beginning to emerge.

Tyrosylprotein sulfotransferase inhibitors generated by combinatorial target-guided ligand assembly

Tyrosylprotein sulfotransferases (TPSTs) catalyze the sulfation of tyrosine residues within secreted and membrane-bound proteins. The modification modulates protein-protein interactions in the extracellular environment. Here we use combinatorial target-guided ligand assembly to discover the first known inhibitors of human TPST-2.

Adaptive mutations in the V3 loop of gp120 enhance fusogenicity of human immunodeficiency virus type 1 and enable use of a CCR5 coreceptor that lacks the amino-terminal sulfated region

To identify sites in gp120 that interact with the CCR5 coreceptor and to analyze the mechanisms of infection, we selected variants of the CCR5-dependent JRCSF molecular clone of human immunodeficiency virus type 1 (HIV-1) that adapted to replicate in HeLa-CD4 cells that express the mutant coreceptor CCR5(Y14N) or CCR5(G163R), which were previously shown to bind purified gp120-CD4 complexes only weakly. Correspondingly, these mutant CCR5s mediate infections of wild-type virus only at relatively high cell surface concentrations, demonstrating a concentration-dependent assembly requirement for infection. The plots of viral infectivity versus concentration of coreceptors had sigmoidal shapes, implying involvement of multiple coreceptors, with an estimated stoichiometry of four to six CCR5s in the active complexes. All of the adapted viruses had mutations in the V3 loops of their gp120s. The titers of recombinant HIV-1 virions with these V3 mutations were determined in previously described panels of HeLa-CD4 cell clones that express discrete amounts of CCR5(Y14N) or CCR5(G163R). The V3 loop mutations did not alter viral utilization of wild-type CCR5, but they specifically enhanced utilization of the mutant CCR5s by two distinct mechanisms. Several mutant envelope glycoproteins were highly fusogenic in syncytium assays, and these all increased the efficiency of infection of the CCR5(Y14N) or CCR5(G163R) clonal panels without enhancing virus adsorption onto the cells or viral affinity for the coreceptor. In contrast, V3 loop mutation N300Y was selected during virus replication in cells that contained only a trace of CCR5(Y14N) and this mutation increased the apparent affinity of the virus for this coreceptor, as indicated by a shift in the sigmoid-shaped infectivity curve toward lower concentrations. Surprisingly, N300Y increased viral affinity for the second extracellular loop of CCR5(Y14N) rather than for the mutated amino terminus. Indeed, the resulting virus was able to use a mutant CCR5 that lacks 16 amino acids at its amino terminus, a region previously considered essential for CCR5 coreceptor function. Our results demonstrate that the role of CCR5 in infection involves at least two steps that can be strongly and differentially altered by mutations in either CCR5 or the V3 loop of gp120: a concentration-dependent binding step that assembles a critical multivalent virus-coreceptor complex and a postassembly step that likely involves a structural rearrangement of the complex. The postassembly step can severely limit HIV-1 infections and is not an automatic consequence of virus-coreceptor binding, as was previously assumed. These results have important implications for our understanding of the mechanism of HIV-1 infection and the factors that may select for fusogenic gp120 variants during AIDS progression.

Molecular anatomy of CCR5 engagement by physiologic and viral chemokines and HIV-1 envelope glycoproteins: differences in primary structural requirements for RANTES, MIP-1 alpha, and vMIP-II Binding

Molecular analysis of CCR5, the cardinal coreceptor for HIV-1 infection, has implicated the N-terminal extracellular domain (N-ter) and regions vicinal to the second extracellular loop (ECL2) in this activity. It was shown that residues in the N-ter are necessary for binding of the physiologic ligands, RANTES (CCL5) and MIP-1 alpha (CCL3). vMIP-II, encoded by the Kaposi's sarcoma-associated herpesvirus, is a high affinity CCR5 antagonist, but lacks efficacy as a coreceptor inhibitor. Therefore, we compared the mechanism for engagement by vMIP-II of CCR5 to its interaction with physiologic ligands. RANTES, MIP-1 alpha, and vMIP-II bound CCR5 at high affinity, but demonstrated partial cross-competition. Characterization of 15 CCR5 alanine scanning mutants of charged extracellular amino acids revealed that alteration of acidic residues in the distal N-ter abrogated binding of RANTES, MIP-1 alpha, and vMIP-II. Whereas mutation of residues in ECL2 of CCR5 dramatically reduced the binding of RANTES and MIP-1 alpha and their ability to induce signaling, interaction with vMIP-II was not altered by any mutation in the exoloops of the receptor. Paradoxically, monoclonal antibodies to N-ter epitopes did not block chemokine binding, but those mapped to ECL2 were effective inhibitors. A CCR5 chimera with the distal N-ter residues of CXCR2 bound MIP-1 alpha and vMIP-II with an affinity similar to that of the wild-type receptor. Engagement of CCR5 by vMIP-II, but not RANTES or MIP-1 alpha blocked the binding of monoclonal antibodies to the receptor, providing additional evidence for a distinct mechanism for viral chemokine binding. Analysis of the coreceptor activity of randomly generated mouse-human CCR5 chimeras implicated residues in ECL2 between H173 and V197 in this function. RANTES, but not vMIP-II blocked CCR5 M-tropic coreceptor activity in the fusion assay. The insensitivity of vMIP-II binding to mutations in ECL2 provides a potential rationale to its inefficiency as an antagonist of CCR5 coreceptor activity. These findings suggest that the molecular anatomy of CCR5 binding plays a critical role in antagonism of coreceptor activity.

CD4-independent use of Rhesus CCR5 by human immunodeficiency virus Type 2 implicates an electrostatic interaction between the CCR5 N terminus and the gp120 C4 domain

Envelope glycoproteins (Envs) of human immunodeficiency virus type 2 (HIV-2) are frequently able to use chemokine receptors, CXCR4 or CCR5, in the absence of CD4. However, while these Envs are commonly dual-tropic, no isolate has been described to date that is CD4 independent on both CXCR4 and CCR5. In this report we show that a variant of HIV-2/NIHz, termed HIV-2/vcp, previously shown to utilize CXCR4 without CD4, is also CD4 independent on rhesus (rh) CCR5, but requires CD4 to fuse with human (hu) CCR5. The critical determinant for this effect was an acidic amino acid at position 13 in the CCR5 N terminus, which is an asparagine in huCCR5 and an aspartic acid in rhCCR5. Transferring the huCCR5 N terminus with an N13D substitution to CCR2b or CXCR2 was sufficient to render these heterologous chemokine receptors permissive for CD4-independent fusion. Chimeric Envs between HIV-2/vcp and a CD4-dependent clone of HIV-2/NIHz as well as site-directed Env mutations implicated a positively charged amino acid (lysine or arginine) at position 427 in the C4 region of the HIV-2/vcp env gene product (VCP) gp120 as a key determinant for this phenotype. Because CD4-independent use of CCR5 mapped to a negatively charged amino acid in the CCR5 N terminus and a positively charged amino acid in the gp120 C4 domain, an electrostatic interaction between these residues or domains is likely. Although not required for CD4-dependent fusion, this interaction may serve to increase the binding affinity of Env and CCR5 and/or to facilitate subsequent conformational changes that are required for fusion. Because the structural requirements for chemokine receptor use by HIV are likely to be more stringent in the absence of CD4, CD4-independent viruses should be particularly useful in dissecting molecular events that are critical for viral entry.

Ligand binding characteristics of CXCR4 incorporated into paramagnetic proteoliposomes

The G protein-coupled receptor CXCR4 is a coreceptor, along with CD4, for the human immunodeficiency virus type 1 (HIV-1) and has been implicated in breast cancer metastasis. We studied the binding of the HIV-1 gp120 envelope glycoprotein (gp) to CXCR4 but found that the gp120s from CXCR4-using HIV-1 strains bound nonspecifically to several cell lines lacking human CXCR4 expression. Therefore, we constructed paramagnetic proteoliposomes (CXCR4-PMPLs) containing pure, native CXCR4. CXCR4-PMPLs specifically bound the natural ligand, SDF-1alpha, and the gp120s from CXCR4-using HIV-1 strains. Conformation-dependent anti-CXCR4 antibodies and the CXCR4 antagonist AMD3100 blocked HIV-1 gp120 binding to CXCR4-PMPLs. The gp120-CXCR4 interaction was blocked by anti-gp120 antibodies directed against the third variable (V3) loop and CD4-induced epitopes, structures that have also been implicated in the binding of gp120 to the other HIV-1 coreceptor, CCR5. Compared with the binding of R5 HIV-1 gp120s to CCR5, the gp120-CXCR4 interaction exhibited a lower affinity (K(d) = 200 nm) and was dependent upon prior CD4 binding, even at low temperature. Thus, although similar regions of X4 and R5 HIV-1 gp120s appear to be involved in binding CXCR4 and CCR5, respectively, differences exist in nonspecific binding to cell surfaces, affinity for the chemokine receptor, and CD4 dependence at low temperature.

Inhibition of HIV-1 infection by synthetic peptide analogues derived from the NH(2)-Terminal extracellular region of GPR1

Several shortened peptide analogues of the N-terminal domain of GPR1, an orphan G protein-coupled receptor (GPCR), were prepared and their anti-HIV-1 activities were evaluated. Some of the prepared compounds, especially sulfated derivatives, showed potent inhibitory activity against a broad range of HIV-1, including T cell-tropic, dual cell-tropic and brain-derived (BT) cell-tropic HIV-1 strains.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.