The three-dimensional solution structure of RANTES

N-terminal Backbone Pairing Shifts in CCL5-12AAA14 Dimer Interface: Structural Significance of the FAY Sequence

CC-type chemokine ligand 5 (CCL5) has been known to regulate immune responses by mediating the chemotaxis of leukocytes. Depending on the environment, CCL5 forms different orders of oligomers to interact with targets and create functional diversity. A recent CCL5 trimer structure revealed that the N-terminal conversed F12-A13-Y14 (¹²FAY¹⁴) sequence is involved in CCL5 aggregation. The CCL5-¹²AAA¹⁴ mutant with two mutations had a deficiency in the formation of high-order oligomers. In the study, we clarify the respective roles of F12 and Y14 through NMR analysis and structural determination of the CCL5-¹²AAA¹⁴ mutant where F12 is involved in the dimer assembly and Y14 is involved in aggregation. The CCL5-¹²AAA¹⁴ structure contains a unique dimer packing. The backbone pairing shifts for one-residue in the N-terminal interface, when compared to the native CCL5 dimer. This difference creates a new structural orientation and leads to the conclusion that F12 confines the native CCL5 dimer configuration. Without F12 anchoring in the position, the interfacial backbone pairing is permitted to slide. Structural plasticity occurs in the N-terminal interaction. This is the first case to report this structural rearrangement through mutagenesis. The study provides a new idea for chemokine engineering and complements the understanding of CCL5 oligomerization and the role of the ¹²FAY¹⁴ sequence.

Integrative Model to Coordinate the Oligomerization and Aggregation Mechanisms of CCL5

CC-type chemokine ligand 5 (CCL5) is involved in the pathogenesis of many inflammatory conditions. Under physiological conditions, CCL5 oligomerization and aggregation are considered to be responsible for its inflammatory properties. The structural basis of CCL5 oligomerization remains controversial because the current oligomer models contain no consensus interactions. In this study, NMR and biophysical analyses proposed evidence that the CC-type CCL5 dimer acts as the basic unit to constitute the oligomer and that CCL5 oligomerizes alternatively through E66-K25 and E66-R44/K45 interactions. In addition, a newly determined trimer structure, constituted by CCL5 and the E66S mutant, reported an interfacial interaction through the N-terminal ¹²FAY¹⁴ sequence. The interaction contributes to CCL5 aggregation and precipitation but not to oligomerization. In accordance with the observations, an integrative model explains the CCL5 oligomerization and aggregation mechanism in which CCL5 assembly consists of two types of dimer-dimer interactions and one aggregation mechanism. For full-length CCL5, the molecular accumulation triggers oligomerization through the E66-K25 and E66-R44/K45 interactions, and the ¹²FAY¹⁴ interaction acts as a secondary effect to derive aggregation and precipitation. In contrast, the E66-R44/K45 interaction might dominate in CCL5 N-terminal truncations, and the interaction would lead to the filament-like formation in solution.

Predicting the Structures of Glycans, Glycoproteins, and Their Complexes

Complex carbohydrates are ubiquitous in nature, and together with proteins and nucleic acids they comprise the building blocks of life. But unlike proteins and nucleic acids, carbohydrates form nonlinear polymers, and they are not characterized by robust secondary or tertiary structures but rather by distributions of well-defined conformational states. Their molecular flexibility means that oligosaccharides are often refractory to crystallization, and nuclear magnetic resonance (NMR) spectroscopy augmented by molecular dynamics (MD) simulation is the leading method for their characterization in solution. The biological importance of carbohydrate-protein interactions, in organismal development as well as in disease, places urgency on the creation of innovative experimental and theoretical methods that can predict the specificity of such interactions and quantify their strengths. Additionally, the emerging realization that protein glycosylation impacts protein function and immunogenicity places the ability to define the mechanisms by which glycosylation impacts these features at the forefront of carbohydrate modeling. This review will discuss the relevant theoretical approaches to studying the three-dimensional structures of this fascinating class of molecules and interactions, with reference to the relevant experimental data and techniques that are key for validation of the theoretical predictions.

The solution structure of monomeric CCL5 in complex with a doubly sulfated N-terminal segment of CCR5

The inflammatory chemokine CCL5, which binds the chemokine receptor CCR5 in a two-step mechanism so as to activate signaling pathways in hematopoetic cells, plays an important role in immune surveillance, inflammation, and development as well as in several immune system pathologies. The recently published crystal structure of CCR5 bound to a high-affinity variant of CCL5 lacks the N-terminal segment of the receptor that is post-translationally sulfated and is known to be important for high-affinity binding. Here, we report the NMR solution structure of monomeric CCL5 bound to a synthetic doubly sulfated peptide corresponding to the missing first 27 residues of CCR5. Our structures show that two sulfated tyrosine residues, sY10 and sY14, as well as the unsulfated Y15 form a network of strong interactions with a groove on a surface of CCL5 that is formed from evolutionarily conserved basic and hydrophobic amino acids. We then use our NMR structures, in combination with available crystal data, to create an atomic model of full-length wild-type CCR5:CCL5. Our findings reveal the structural determinants involved in the recognition of CCL5 by the CCR5 N terminus. These findings, together with existing structural data, provide a complete structural framework with which to understand the specificity of receptor:chemokine interactions.

DATABASE

Structural data are available in the PDB under the accession number 6FGP.

O-GalNAcylation of RANTES Improves Its Properties as a Human Immunodeficiency Virus Type 1 Entry Inhibitor

Many human proteins have the potential to be developed as therapeutic agents. However, side effects caused by direct administration of natural proteins have significantly slowed expansion of protein therapeutics into the clinic. Post-translational modifications (PTMs) can improve protein properties, but because of significant knowledge gaps, we are considerably limited in our ability to apply PTMs to generate better protein therapeutics. Here, we seek to fill the gaps by studying the PTMs of a small representative chemotactic cytokine, RANTES. RANTES can inhibit HIV-1 infection by competing with it for binding to receptor CCR5 and stimulating CCR5 endocytosis. Unfortunately, RANTES can induce strong signaling, leading to severe inflammatory side effects. We apply a chemical biology approach to explore the potential of post-translationally modified RANTES as safe inhibitors of HIV-1 infection. We synthesized and systematically tested a library of RANTES isoforms for their ability to inhibit inflammatory signaling and prevent HIV-1 infection of primary human cells. Through this research, we revealed that most of the glycosylated variants have decreased inflammation-associated properties and identified one particular glyco variant, a truncated RANTES containing a Galβ1-3GalNAc disaccharide α-linked to Ser4, which stands out as having the best overall properties: relatively high HIV-1 inhibition potency but also weak inflammatory properties. Moreover, our results provided a structural basis for the observed changes in the properties of RANTES. Taken together, this work highlights the potential importance of glycosylation as an alternative strategy for developing CCR5 inhibitors to treat HIV-1 infection and, more generally, for reducing or eliminating unwanted properties of therapeutic proteins.

Structural basis for oligomerization and glycosaminoglycan binding of CCL5 and CCL3

CC chemokine ligand 5 (CCL5) and CCL3 are critical for immune surveillance and inflammation. Consequently, they are linked to the pathogenesis of many inflammatory conditions and are therapeutic targets. Oligomerization and glycosaminoglycan (GAG) binding of CCL5 and CCL3 are vital for the functions of these chemokines. Our structural and biophysical analyses of human CCL5 reveal that CCL5 oligomerization is a polymerization process in which CCL5 forms rod-shaped, double-helical oligomers. This CCL5 structure explains mutational data and offers a unified mechanism for CCL3, CCL4, and CCL5 assembly into high-molecular-weight, polydisperse oligomers. A conserved, positively charged BBXB motif is key for the binding of CC chemokines to GAG. However, this motif is partially buried when CCL3, CCL4, and CCL5 are oligomerized; thus, the mechanism by which GAG binds these chemokine oligomers has been elusive. Our structures of GAG-bound CCL5 and CCL3 oligomers reveal that these chemokine oligomers have distinct GAG-binding mechanisms. The CCL5 oligomer uses another positively charged and fully exposed motif, KKWVR, in GAG binding. However, residues from two partially buried BBXB motifs along with other residues combine to form a GAG-binding groove in the CCL3 oligomer. The N termini of CC chemokines are shown to be involved in receptor binding and oligomerization. We also report an alternative CCL3 oligomer structure that reveals how conformational changes in CCL3 N termini profoundly alter its surface properties and dimer-dimer interactions to affect GAG binding and oligomerization. Such complexity in oligomerization and GAG binding enables intricate, physiologically relevant regulation of CC chemokine functions.

The Interaction of Heparin Tetrasaccharides with Chemokine CCL5 Is Modulated by Sulfation Pattern and pH

Interactions between chemokines such as CCL5 and glycosaminoglycans (GAGs) are essential for creating haptotactic gradients to guide the migration of leukocytes into inflammatory sites, and the GAGs that interact with CCL5 with the highest affinity are heparan sulfates/heparin. The interaction between CCL5 and its receptor on monocytes, CCR1, is mediated through residues Arg-17 and -47 in CCL5, which overlap with the GAG-binding (44)RKNR(47) "BBXB" motifs. Here we report that heparin and tetrasaccharide fragments of heparin are able to inhibit CCL5-CCR1 binding, with IC50 values showing strong dependence on the pattern and extent of sulfation. Modeling of the CCL5-tetrasaccharide complexes suggested that interactions between specific sulfate and carboxylate groups of heparin and residues Arg-17 and -47 of the protein are essential for strong inhibition; tetrasaccharides lacking the specific sulfation pattern were found to preferentially bind CCL5 in positions less favorable for inhibition of the interaction with CCR1. Simulations of a 12-mer heparin fragment bound to CCL5 indicated that the oligosaccharide preferred to interact simultaneously with both (44)RKNR(47) motifs in the CCL5 homodimer and engaged residues Arg-47 and -17 from both chains. Direct engagement of these residues by the longer heparin oligosaccharide provides a rationalization for its effectiveness as an inhibitor of CCL5-CCR1 interaction. In this mode, histidine (His-23) may contribute to CCL5-GAG interactions when the pH drops just below neutral, as occurs during inflammation. Additionally, an examination of the contribution of pH to modulating CCL5-heparin interactions suggested a need for careful interpretation of experimental results when experiments are performed under non-physiological conditions.

Elucidating a key anti-HIV-1 and cancer-associated axis: the structure of CCL5 (Rantes) in complex with CCR5

CCL5 (RANTES) is an inflammatory chemokine which binds to chemokine receptor CCR5 and induces signaling. The CCL5:CCR5 associated chemotactic signaling is of critical biological importance and is a potential HIV-1 therapeutic axis. Several studies provided growing evidence for the expression of CCL5 and CCR5 in non-hematological malignancies. Therefore, the delineation of the CCL5:CCR5 complex structure can pave the way for novel CCR5-targeted drugs. We employed a computational protocol which is primarily based on free energy calculations and molecular dynamics simulations, and report, what is to our knowledge, the first computationally derived CCL5:CCR5 complex structure which is in excellent agreement with experimental findings and clarifies the functional role of CCL5 and CCR5 residues which are associated with binding and signaling. A wealth of polar and non-polar interactions contributes to the tight CCL5:CCR5 binding. The structure of an HIV-1 gp120 V3 loop in complex with CCR5 has recently been derived through a similar computational protocol. A comparison between the CCL5 : CCR5 and the HIV-1 gp120 V3 loop : CCR5 complex structures depicts that both the chemokine and the virus primarily interact with the same CCR5 residues. The present work provides insights into the blocking mechanism of HIV-1 by CCL5.

Characterization of structure, dynamics, and detergent interactions of the anti-HIV chemokine variant 5P12-RANTES

RANTES (CCL5) is a chemokine that recruits immune cells to inflammatory sites by interacting with the G-protein coupled receptor CCR5, which is also the primary coreceptor used together with CD4 by HIV to enter and infect target cells. Ligands of CCR5, including chemokines and chemokine analogs, are capable of blocking HIV entry, and studies of their structures and interactions with CCR5 will be key to understanding and optimizing HIV inhibition. The RANTES derivative 5P12-RANTES is a highly potent HIV entry inhibitor that is being developed as a topical HIV prevention agent (microbicide). We have characterized the structure and dynamics of 5P12-RANTES by solution NMR. With the exception of the nine flexible N-terminal residues, 5P12-RANTES has the same structure as wild-type RANTES but unlike the wild-type, does not dimerize via its N-terminus. To prepare the ground for interaction studies with detergent-solubilized CCR5, we have also investigated the interaction of RANTES and 5P12-RANTES with various commonly used detergents. Both RANTES variants are stable in Cymal-5, DHPC, Anzergent-3-12, dodecyltrimethylammonium chloride, and a DDM/CHAPS/CHS mixture. Fos-Cholines, dodecyldimethylglycine, and sodium dodecyl-sulfate denature both RANTES variants at low pH, whereas at neutral pH the stability is considerably higher. The onset of Fos-Choline-12-induced denaturation and the denatured state were characterized by circular dichroism and NMR. The detergent interaction starts below the critical micelle concentration at a well-defined mixed hydrophobic/positive surface region of the chemokine, which overlaps with the dimer interface. An increase of Fos-Choline-12 concentration above the critical micelle concentration causes a transition to a denatured state with a high α-helical content.

Chemokine oligomerization in cell signaling and migration

Chemokines are small proteins best known for their role in controlling the migration of diverse cells, particularly leukocytes. Upon binding to their G-protein-coupled receptors on the leukocytes, chemokines stimulate the signaling events that cause cytoskeletal rearrangements involved in cell movement, and migration of the cells along chemokine gradients. Depending on the cell type, chemokines also induce many other types of cellular responses including those related to defense mechanisms, cell proliferation, survival, and development. Historically, most research efforts have focused on the interaction of chemokines with their receptors, where monomeric forms of the ligands are the functionally relevant state. More recently, however, the importance of chemokine interactions with cell surface glycosaminoglycans has come to light, and in most cases appears to involve oligomeric chemokine structures. This review summarizes existing knowledge relating to the structure and function of chemokine oligomers, and emerging methodology for determining structures of complex chemokine assemblies in the future.

Glycosaminoglycan analogs as a novel anti-inflammatory strategy

Heparin, a glycosaminoglycan (GAG), has both anti-inflammatory and anti-coagulant properties. The clinical use of heparin against inflammation, however, has been limited by concerns about increased bleeding. While the anti-coagulant activity of heparin is well understood, its anti-inflammatory properties are less so. Heparin is known to bind to certain cytokines, including chemokines, small proteins which mediate inflammation through their control of leukocyte migration and activation. Molecules which can interrupt the chemokine-GAG interaction without inhibiting coagulation could therefore, represent a new class of anti-inflammatory agents. In the present study, two approaches were undertaken, both focusing on the heparin-chemokine relationship. In the first, a structure based strategy was used: after an initial screening of potential small molecule binders using protein NMR on a target chemokine, binding molecules were optimized through structure-based design. In the second approach, commercially available short oligosaccharides were polysulfated. In vitro, these molecules prevented chemokine-GAG binding and chemokine receptor activation without disrupting coagulation. However, in vivo, these compounds caused variable results in a murine peritoneal recruitment assay, with a general increase of cell recruitment. In more disease specific models, such as antigen-induced arthritis and delayed-type hypersensitivity, an overall decrease in inflammation was noted, suggesting that the primary anti-inflammatory effect may also involve factors beyond the chemokine system.

Design and receptor interactions of obligate dimeric mutant of chemokine monocyte chemoattractant protein-1 (MCP-1)

Chemokine-receptor interactions regulate leukocyte trafficking during inflammation. CC chemokines exist in equilibrium between monomeric and dimeric forms. Although the monomers can activate chemokine receptors, dimerization is required for leukocyte recruitment in vivo, and it remains controversial whether dimeric CC chemokines can bind and activate their receptors. We have developed an obligate dimeric mutant of the chemokine monocyte chemoattractant protein-1 (MCP-1) by substituting Thr(10) at the dimer interface with Cys. Biophysical analysis showed that MCP-1(T10C) forms a covalent dimer with similar structure to the wild type MCP-1 dimer. Initial cell-based assays indicated that MCP-1(T10C) could activate chemokine receptor CCR2 with potency reduced 1 to 2 orders of magnitude relative to wild type MCP-1. However, analysis of size exclusion chromatography fractions demonstrated that the observed activity was due to a small proportion of MCP-1(T10C) being monomeric and highly potent, whereas the majority dimeric form could neither bind nor activate CCR2 at concentrations up to 1 μM. These observations help to reconcile previous conflicting results and indicate that dimeric CC chemokines do not bind to their receptors with affinities approaching those of the corresponding monomeric chemokines.

Oligomeric structure of the chemokine CCL5/RANTES from NMR, MS, and SAXS data

CCL5 (RANTES) is a proinflammatory chemokine known to activate leukocytes through its receptor, CCR5. Although the monomeric form of CCL5 is sufficient to cause cell migration in vitro, CCL5's propensity for aggregation is essential for migration in vivo, T cell activation and apoptosis, and HIV entry into cells. However, there is currently no structural information on CCL5 oligomers larger than the canonical CC chemokine dimer. In this study the solution structure of a CCL5 oligomer was investigated using an integrated approach, including NMR residual dipolar couplings to determine allowed relative orientations of the component monomers, SAXS to restrict overall shape, and hydroxyl radical footprinting and NMR cross-saturation experiments to identify interface residues. The resulting model of the CCL5 oligomer provides a basis for explaining the disaggregating effect of E66 and E26 mutations and suggests mechanisms by which glycosaminoglycan binding may promote oligomer formation and facilitate cell migration in vivo.

Sulfotyrosine recognition as marker for druggable sites in the extracellular space

Chemokine signaling is a well-known agent of autoimmune disease, HIV infection, and cancer. Drug discovery efforts for these signaling molecules have focused on developing inhibitors targeting their associated G protein-coupled receptors. Recently, we used a structure-based approach directed at the sulfotyrosine-binding pocket of the chemokine CXCL12, and thereby demonstrated that small molecule inhibitors acting upon the chemokine ligand form an alternative therapeutic avenue. Although the 50 members of the chemokine family share varying degrees of sequence homology (some as little as 20%), all members retain the canonical chemokine fold. Here we show that an equivalent sulfotyrosine-binding pocket appears to be conserved across the chemokine superfamily. We monitored sulfotyrosine binding to four representative chemokines by NMR. The results suggest that most chemokines harbor a sulfotyrosine recognition site analogous to the cleft on CXCL12 that binds sulfotyrosine 21 of the receptor CXCR4. Rational drug discovery efforts targeting these sites may be useful in the development of specific as well as broad-spectrum chemokine inhibitors.

Biology and clinical relevance of chemokines and chemokine receptors CXCR4 and CCR5 in human diseases

Chemokines and their receptors are implicated in a wide range of human diseases, including acquired immune deficiency syndrome (AIDS). The entry of human immunodeficiency virus type 1 (HIV-1) into a cell is initiated by the interaction of the virus's surface envelope proteins with two cell surface components of the target cell, namely CD4 and a chemokine co-receptor, usually CXCR4 or CCR5. Typical anti-HIV-1 agents include protease and reverse transcriptase inhibitors, but the targets of these agents tend to show rapid mutation rates. As such, strategies based on HIV-1 co-receptors have appeal because they target invariant host determinants. Chemokines and their receptors are also of general interest since they play important roles in numerous physiological and pathological processes in addition to AIDS. Therefore, intensive basic and translational research is ongoing for the dissection of their structure - function relationships in an effort to understand the molecular mechanism of chemokine - receptor interactions and signal transductions across cellular membranes. This paper reviews and discusses recent advances and the translation of new knowledge and discoveries into novel interventional strategies for clinical application.

Structural NMR of protein oligomers using hybrid methods

Solving structures of native oligomeric protein complexes using traditional high-resolution NMR techniques remains challenging. However, increased utilization of computational platforms, and integration of information from less traditional NMR techniques with data from other complementary biophysical methods, promises to extend the boundary of NMR-applicable targets. This article reviews several of the techniques capable of providing less traditional and complementary structural information. In particular, the use of orientational constraints coming from residual dipolar couplings and residual chemical shift anisotropy offsets are shown to simplify the construction of models for oligomeric complexes, especially in cases of weak homo-dimers. Combining this orientational information with interaction site information supplied by computation, chemical shift perturbation, paramagnetic surface perturbation, cross-saturation and mass spectrometry allows high resolution models of the complexes to be constructed with relative ease. Non-NMR techniques, such as mass spectrometry, EPR and small angle X-ray scattering, are also expected to play increasingly important roles by offering alternative methods of probing the overall shape of the complex. Computational platforms capable of integrating information from multiple sources in the modeling process are also discussed in the article. And finally a new, detailed example on the determination of a chemokine tetramer structure will be used to illustrate how a non-traditional approach to oligomeric structure determination works in practice.

Molecular engineering of RANTES peptide mimetics with potent anti-HIV-1 activity

The chemokine receptor CCR5 is utilized as a critical coreceptor by most primary HIV-1 strains. While the lack of structural information on CCR5 has hampered the rational design of specific inhibitors, mimetics of the chemokines that naturally bind CCR5 can be molecularly engineered. We used a structure-guided approach to design peptide mimetics of the N-loop and β1-strand regions of regulated on activation normal T-cell-expressed and secreted (RANTES)/CCL5, which contain the primary molecular determinants of HIV-1 blockade. Rational modifications were sequentially introduced into the N-loop/β1-strand sequence, leading to the generation of mimetics with potent activity against a broad spectrum of CCR5-specific HIV-1 isolates (IC(50) range: 104-640 nM) but lacking activity against CXCR4-specific HIV-1 isolates. Functional enhancement was initially achieved with the stabilization of the N loop in the β-extended conformation adopted in full-length RANTES, as confirmed by nuclear magnetic resonance (NMR) analysis. However, the most dramatic increase in antiviral potency resulted from the engraftment of an in silico-optimized linker segment designed using de novo structure-prediction algorithms to stabilize the C-terminal α-helix and experimentally validated by NMR. Our mimetics exerted CCR5-antagonistic effects, demonstrating that the antiviral and proinflammatory functions of RANTES can be uncoupled. RANTES peptide mimetics provide new leads for the development of safe and effective HIV-1 entry inhibitors.

MCP-1: chemoattractant with a role beyond immunity: a review

BACKGROUND

Monocyte Chemoattractant Protein (MCP)-1, a potent monocyte attractant, is a member of the CC chemokine subfamily. MCP-1 exerts its effects through binding to G-protein-coupled receptors on the surface of leukocytes targeted for activation and migration. Role of MCP-1 and its receptor CCR2 in monocyte recruitment during infection or under other inflammatory conditions is well known.

METHOD

A comprehensive literature search was conducted from the websites of the National Library of Medicine (http://www.ncbl.nlm.nih.gov) and Pubmed Central, the US National Library of Medicine's digital archive of life sciences literature (http://www.pubmedcentral.nih.gov/). The data was assessed from books and journals that published relevant articles in this field.

RESULT

Recent and ongoing research indicates the role of MCP-1 in various allergic conditions, immunodeficiency diseases, bone remodelling, and permeability of blood - brain barrier, atherosclerosis, nephropathies and tumors.

CONCLUSION

MCP-1 plays an important role in pathogenesis of various disease states and hence MCP-1 inhibition may have beneficial effects in such conditions.

Characterization of the chemokine CXCL11-heparin interaction suggests two different affinities for glycosaminoglycans

Chemokines orchestrate the migration of leukocytes in the context of homeostasis and inflammation. In addition to interactions of chemokines with receptors on migrating cells, these processes require interactions of chemokines with glycosaminoglycans (GAGs) for cell surface localization. Most chemokines are basic proteins with Arg/Lys/His residue clusters functioning as recognition epitopes for GAGs. In this study we characterized the GAG-binding epitopes of the chemokine I-TAC/CXCL11. Four separate clusters of basic residues were mutated to alanine and tested for their ability to bind to GAGs in vitro and to activate the receptor, CXCR3. Mutation of a set of basic residues in the C-terminal helix (the 50s cluster, (57)KSKQAR(62)) along with Lys(17), significantly impaired heparin binding in vitro, identifying these residues as components of the dominant epitope. However, this GAG mutant retained nearly wild type receptor binding affinity, and its ability to induce cell migration in vitro was only mildly perturbed. Nevertheless, the mutant was unable to induce cell migration in vivo, establishing a requirement of CXCL11 for GAG binding for in vivo function. These studies also led to some interesting findings. First, CXCL11 exhibits conformational heterogeneity, as evidenced by the doubling of peaks in its HSQC spectra. Second, it exhibits more than one affinity state for both heparin and CXCR3, which may be related to its structural plasticity. Finally, although the binding affinities of chemokines for GAGs are typically weaker than interactions with receptors, the high affinity GAG binding state of CXCL11 is comparable with typical receptor binding affinities, suggesting some unique properties of this chemokine.

NMR analysis of the structure, dynamics, and unique oligomerization properties of the chemokine CCL27

Chemokines have two essential interactions in vivo, with G protein-coupled receptors, which activate intracellular signaling pathways, and with glycosaminoglycans (GAGs), which are involved in cell surface localization and transport. Although it has been shown that chemokines bind and activate their respective G protein-coupled receptors as monomers, many chemokines oligomerize upon GAG binding, and the ability to oligomerize and bind GAGs is required for in vivo function. In this study, we investigated the structure, dynamics, and oligomerization behavior of cutaneous T-cell-attracting chemokine (CTACK, also known as CCL27) by NMR. (15)N relaxation and translational self-diffusion rates indicate that CCL27 oligomerizes, but in contrast to many other chemokines that form relatively discrete oligomers, CCL27 transitions between monomer, dimer, and tetramer species over a relatively narrow concentration range. A three-dimensional structure determination was pursued under conditions where CCL27 is primarily dimeric, revealing the standard motif for a chemokine monomer. Analysis of chemical shift perturbations of (1)H-(15)N HSQC spectra, relaxation-dispersion experiments, and filtered nuclear Overhauser effects suggest that CCL27 does not adopt a discrete CXC or CC dimer motif. Instead, CCL27 has uncommon oligomerization behavior, where several equilibria involving relatively low affinity interactions between different interfaces seem to be simultaneously at work. However, interaction with heparin avidly promotes oligomerization under conditions where CCL27 is monomeric by itself. We hypothesize that the plasticity in the oligomerization state may enable CCL27 to adopt different oligomeric structures, depending on the nature of the GAG binding partner, thereby providing a mechanism for increased diversity and specificity in GAG-binding and GAG-related functions.

Structural and functional studies of the potent anti-HIV chemokine variant P2-RANTES

The N-terminal region of the chemokine RANTES is critical for its function. A synthesized N-terminally modified analog of RANTES, P2-RANTES, was discovered using a phage display selection against living CCR5-expressing cells, and has been reported to inhibit HIV-1 env-mediated cell-cell fusion at subnanomolar levels (Hartley et al. J Virol 2003;77:6637-6644). In the present study we produced this protein using E. coli overexpression and extensively studied its structure and function. The x-ray crystal structure of P2-RANTES was solved and refined at 1.7 A resolution. This protein was found to be predominantly a monomer in solution by analytical ultracentrifugation, but a tetramer in the crystal. In studies of glycosaminoglycan binding, P2-RANTES was found to be significantly less able to bind heparin than wild type RANTES. We also tested this protein for receptor internalization where it was shown to be functional, in cell-cell fusion assays where recombinant P2-RANTES was a potent fusion inhibitor (IC(50) = 2.4 +/- 0.8 nM), and in single round infection assays where P2-RANTES inhibited at subnanomolar levels. Further, in a modified fusion assay designed to test specificity of inhibition, P2-RANTES was also highly effective, with a 65-fold improvement over the fusion inhibitor C37, which is closely related to the clinically approved inhibitor T-20. These studies provide detailed structural and functional information for this novel N-terminally modified chemokine mutant. This information will be very useful in the development of more potent anti-HIV agents. PDB Accession Number: 2vxw.

The superior folding of a RANTES analogue expressed in lactobacilli as compared to mammalian cells reveals a promising system to screen new RANTES mutants

Development of effective topical microbicides for the prevention of HIV-1 sexual transmission represents a primary goal for the control of the AIDS pandemic. The viral coreceptor CCR5, used by the vast majority of primary HIV-1 isolates, is considered a primary target molecule. RANTES and its derivatives are the most suitable protein-based compounds to fight HIV-1 via CCR5 targeting. Yet, receptor activation should be avoided to prevent pro-inflammatory effects and possibly provide anti-inflammatory properties. C1C5 RANTES is a chemokine mutant that exhibits high anti-HIV-1 potency coupled with CCR5 antagonism. However, the need for the formation of an N-terminal intramolecular disulfide bridge between non-natural cysteine residues at positions 1 and 5 represents a challenge for the correct folding of this protein in recombinant expression systems, a crucial step towards its development as a microbicide against HIV-1. We report here a rare case of superior folding in a prokaryote as compared to an eukaryotic expression system. Production of C1C5 RANTES was highly impaired in CHO cells, with a dramatic yield reduction compared to that of wild type RANTES and secretion of the molecule as disulfide-linked dimer. Conversely, a human vaginal isolate of Lactobacillus jensenii engineered to secrete C1C5 RANTES provided efficient delivery of the monomeric protein. This and other reports on successful secretion of complex proteins indicate that lactic acid bacteria are an excellent system for the expression of therapeutic proteins, which can be used as a platform for the engineering of conceptually novel RANTES mutants with potent anti-HIV-1 activity.

Disrupting functional interactions between platelet chemokines inhibits atherosclerosis in hyperlipidemic mice

Atherosclerosis is characterized by chronic inflammation of the arterial wall due to chemokine-driven mononuclear cell recruitment. Activated platelets can synergize with chemokines to exacerbate atherogenesis; for example, by deposition of the chemokines platelet factor-4 (PF4, also known as CXCL4) and RANTES (CCL5), triggering monocyte arrest on inflamed endothelium. Homo-oligomerization is required for the recruitment functions of CCL5, and chemokine heteromerization has more recently emerged as an additional regulatory mechanism, as evidenced by a mutual modulation of CXCL8 and CXCL4 activities and by enhanced monocyte arrest resulting from CCL5-CXCL4 interactions. The CCL5 antagonist Met-RANTES reduces diet-induced atherosclerosis; however, CCL5 antagonism may not be therapeutically feasible, as suggested by studies using Ccl5-deficient mice which imply that direct CCL5 blockade would severely compromise systemic immune responses, delay macrophage-mediated viral clearance and impair normal T cell functions. Here we determined structural features of CCL5-CXCL4 heteromers and designed stable peptide inhibitors that specifically disrupt proinflammatory CCL5-CXCL4 interactions, thereby attenuating monocyte recruitment and reducing atherosclerosis without the aforementioned side effects. These results establish the in vivo relevance of chemokine heteromers and show the potential of targeting heteromer formation to achieve therapeutic effects.

Rational design of novel HIV-1 entry inhibitors by RANTES engineering

The discovery that the CC chemokines RANTES, MIP-1alpha and MIP-1beta act as potent natural inhibitors of HIV-1, the causative agent of AIDS, and the subsequent identification of CCR5 as a major virus coreceptor have triggered a wealth of basic and applied research approaches aimed at developing safe and effective viral entry inhibitors. Some of these efforts have focused on RANTES engineering with the goal of enhancing the antiviral activity of the native molecule while reducing or abrogating its inflammatory properties. The wavefront generated a decade ago is still on its course, with a flow of promising leads constantly emerging and being evaluated in preclinical studies. Here, we present an overview of this rapidly evolving field, highlighting the most important features of RANTES molecular architecture and structure-function relationships.

The synergy of ChemMatrix resin and pseudoproline building blocks renders RANTES, a complex aggregated chemokine

Traditionally, solid-phase synthesis has relied on polystyrene-based resins for the synthesis of all kinds of peptides. However, due to their high hydrophobicity, these resins have certain limitations, particularly in the synthesis of complex peptides, and in such cases, poly(ethylene glycol) (PEG)-based resins are often found to give superior results. Another powerful strategy for expediting the assembly of complex peptides is to employ pseudoproline dipeptides. These derivatives disrupt the interactions among chains that are usually the cause of poor coupling yields in aggregated sequences. Here we report on an efficient stepwise solid-phase synthesis of RANTES (1-68) by combining the advantages of the totally PEG-based ChemMatrix resin and pseudoproline dipeptides.

Recognition of RANTES by extracellular parts of the CCR5 receptor

The chemokine RANTES (regulated upon activation, normal T-cell expressed and secreted) is a natural ligand of CCR5, one of the major HIV-1 coreceptors. It is secreted as part of the immune response to human immunodeficiency virus 1 (HIV-1) and inhibits infection by CCR5-dependent (R5) HIV-1 isolates. We have investigated the interaction of RANTES with several peptides derived from the extracellular domains of CCR5 by heteronuclear NMR spectroscopy in aqueous solution. We show that a peptide comprising the first 25 amino acid residues of the CCR5 N-terminal domain and sulfated at the Y10 and Y14 side-chains binds with micromolar affinity exclusively to the monomeric form of RANTES. In contrast to the tight binding of the sulfated peptide, the affinity of the same peptide in non-sulfated form was reduced by more than two orders of magnitude. Peptides derived from the CCR5 extracellular loops ECL1, ECL2 and ECL3 showed only very moderate and mostly non-specific binding. Chemical shift mapping of the interaction of the sulfated N-terminal peptide reveals a contiguous binding surface on RANTES, which comprises amino acid residues of the first beta-strand, the N-loop, the fourth beta-strand and the turns around residues 30 and 40. This binding surface largely overlaps with the dimer interface and is strongly positively charged, providing a rationale for the exclusive binding of the monomer to the peptide and the requirement of the negative sulfate groups at the Y10 and Y14 side-chains. The binding surface also largely overlaps with the segments that were identified previously as crucial for HIV blockade by peptide scanning and mutagenesis studies. These data offer new insights into the structure-function relation of the RANTES-CCR5 interaction and may be helpful for the design of novel HIV-1 inhibitors.

On-column refolding of recombinant chemokines for NMR studies and biological assays

We have applied an efficient solid-phase protein refolding method to the milligram scale production of natively folded recombinant chemokine proteins. Chemokines are intensely studied proteins because of their roles in immune system regulation, response to inflammation, fetal development, and numerous disease states including, but not limited to, HIV-1/AIDS, cancer metastasis, Crohn's disease, asthma and arthritis. Many investigators use recombinant chemokines for research purposes, however these proteins partition almost exclusively to the inclusion body fraction when produced in Escherichia coli. A major hurdle is to correctly refold the chemokine and oxidize the two highly conserved disulfide bonds found in nearly all chemokines. Conventional methods for oxidation and refolding by dialysis or extreme dilution are effective but slow and yield large volumes of dilute chemokine. Here we use an on-column approach for rapid refolding and oxidation of four chemokines, CXCL12/SDF-1alpha (stromal cell-derived factor-1alpha), CCL5/RANTES, XCL1/lymphotactin, and CX3CL1/fractalkine. NMR spectra of SDF-1alpha, RANTES, lymphotactin, and fractalkine indicate these chemokines adopt native structures. On-column refolded SDF-1alpha is fully active in an intracellular calcium flux assay. Our success with multiple SDF-1alpha mutants and members of all four chemokine subfamilies suggests that on-column refolding is a robust method for preparative-scale production of recombinant chemokine proteins.

SMM-chemokines: a class of unnatural synthetic molecules as chemical probes of chemokine receptor biology and leads for therapeutic development

Chemokines and their receptors play important roles in numerous physiological and pathological processes. To develop natural chemokines into receptor probes and inhibitors of pathological processes, the lack of chemokine-receptor selectivity must be overcome. Here, we apply chemical synthesis and the concept of modular modifications to generate unnatural synthetically and modularly modified (SMM)-chemokines that have high receptor selectivity and affinity, and reduced toxicity. A proof of the concept was shown by transforming the nonselective viral macrophage inflammatory protein-II into new analogs with enhanced selectivity and potency for CXCR4 or CCR5, two principal coreceptors for human immunodeficiency virus (HIV)-1 entry. These new analogs provided insights into receptor binding and signaling mechanisms and acted as potent HIV-1 inhibitors. These results support the concept of SMM-chemokines for studying and controlling the function of other chemokine receptors.

Crystal structure of viral macrophage inflammatory protein I encoded by Kaposi's sarcoma-associated herpesvirus at 1.7A

Viral macrophage inflammatory protein I (vMIP-I) is a chemokine encoded by the Kaposi's sarcoma-associated herpesvirus (KSHV) that selectively activates the CC chemokine receptor 8 (CCR8), for which the endogenous ligand is CCL1. The crystal structure of vMIP-I was determined at 1.7A for comparison with other chemokines, especially those that bind CCR8, such as vMIP-II from KSHV, a CCR8 antagonist and the closest homolog (40% identical). vMIP-I has a typical chemokine fold consisting of an extended N-terminal loop, followed by a three-stranded antiparallel beta-sheet and a C-terminal alpha-helix. The four molecules in the asymmetric unit comprise two MIP-1beta-like dimers. Electrostatic surface representations of CCR8-binding chemokines reveal only minor areas of correlating surface potential, which must be reconciled with promiscuity in receptor and glycosaminoglycan (GAG) binding. In addition, the biological relevance of chemokine oligomerization is examined by comparing the oligomeric states of all chemokine structures deposited to date in the RCSB PDB.

A structural model of a seven-transmembrane helix receptor: The Duffy antigen/receptor for chemokine (DARC)

The Duffy antigen/receptor for chemokine (DARC) is an erythrocyte receptor for malaria parasites (Plasmodium vivax and Plasmodium knowlesi) and for chemokines. In contrast to other chemokine receptors, DARC is a promiscuous receptor that binds chemokines of both CC and CXC classes. The four extracellular domains (ECDs) of DARC are essential for its interaction with chemokines, whilst the first (ECD1) is sufficient for the interaction with malaria erythrocyte-binding protein. In this study, we elaborate and analyze structural models of the DARC. The construction of the 3D models is based on a comparative modeling process and on the use of many procedures to predict transmembrane segments and to detect far homologous proteins with known structures. Threading, ab initio, secondary structure and Protein Blocks approaches are used to build a very large number of models. The conformational exploration of the ECDs is performed with simulated annealing. The second and fourth ECDs are strongly constrained. On the contrary, the ECD1 is highly flexible, but seems composed of three consecutive regions: a small beta-sheet, a linker region and a structured loop. The chosen structural models encompass most of the biochemical features and reflect the known experimental data. They may be used to analyze functional interaction properties.

Dimerization of chemokine receptors and its functional consequences

It became clear over the recent years that most, if not all, G protein-coupled receptors (GPCR) are able to form dimers or higher order oligomers. Chemokine receptors make no exception to this new rule and both homo- and heterodimerization were demonstrated for CC and CXC receptors. Functional analyses demonstrated negative binding cooperativity between the two subunits of a dimer. The consequence is that only one chemokine can bind with high affinity onto a receptor dimer. In the context of receptor activation, this implies that the motions of helical domains triggered by the binding of agonists induce correlated changes in the other protomer. The impact of the chemokine dimerization process in terms of co-receptor function and drug development is discussed.

The X-ray structure of RANTES: heparin-derived disaccharides allows the rational design of chemokine inhibitors

The biological activity of chemokines requires interactions with cell surface proteoglycans. We have determined the structure of the chemokine RANTES (regulated on activation normal T cell expressed) in the presence of heparin-derived disaccharide analogs by X-ray crystallography. These structures confirm the essential role of the BBXB motif in the interaction between the chemokine and the disaccharide. Unexpected interactions were observed in the 30s loop and at the amino terminus. Mutant RANTES molecules were designed to abrogate these interactions and their biological activity examined in vivo. The K45E mutant within the BBXB motif lost the capacity to bind heparin and the ability to elicit cellular recruitment. The Y3A mutant maintained its capacity to bind heparin but was unable to elicit cellular recruitment. Finally, a tetrasaccharide is the smallest oligosaccharide which effectively abolishes the ability of RANTES to recruit cells in vivo. These crystallographic structures provide a description of the molecular interaction of a chemokine with glycosaminoglycans.

Alternative conformations of HIV-1 V3 loops mimic beta hairpins in chemokines, suggesting a mechanism for coreceptor selectivity

The V3 loop of the HIV-1 envelope glycoprotein gp120 is involved in binding to the CCR5 and CXCR4 coreceptors. The structure of an HIV-1(MN) V3 peptide bound to the Fv of the broadly neutralizing human monoclonal antibody 447-52D was solved by NMR and found to be a beta hairpin. This structure of V3(MN) was found to have conformation and sequence similarities to beta hairpins in CD8 and CCR5 ligands MIP-1alpha, MIP-1beta, and RANTES and differed from the beta hairpin of a V3(IIIB) peptide bound to the strain-specific murine anti-gp120(IIIB) antibody 0.5beta. In contrast to the structure of the bound V3(MN) peptide, the V3(IIIB) peptide resembles a beta hairpin in SDF-1, a CXCR4 ligand. These data suggest that the 447-52D-bound V3(MN) and the 0.5beta-bound V3(IIIB) structures represent alternative V3 conformations responsible for selective interactions with CCR5 and CXCR4, respectively.

Antagonist of secondary lymphoid-tissue chemokine (CCR ligand 21) prevents the development of chronic graft-versus-host disease in mice

The use of receptor antagonists for chemokines is an alternative approach to blocking chemokine actions and has the potential to provide novel therapeutics. We determined the receptor antagonist properties of murine N-terminally truncated secondary lymphoid tissue chemokine (SLC)/6Ckine/CCR ligand 21 analogs and evaluated the preventive effects of SLC antagonists on chronic graft-vs-host disease (GVHD) in a murine model by blocking the homing of donor CCR7-expressing T cells into the recipient's lymphoid organs. SLC analogs truncated >4 aa residues from the N terminus showed a loss of chemotaxis and Ca2+ influx of CCR7-expressing cells and also inhibited SLC-stimulated chemotaxis and SLC-induced Ca2+ influx completely. To determine whether SLC antagonist inhibits the development of chronic GVHD, chronic GVHD was induced by injecting DBA/2 spleen cells into (C57BL/6 x DBA/2) F1 mice. Total numbers of spleen cells and host B cells, serum levels of IgE, and of total IgG and IgG1 of anti-DNA Abs in SLC antagonist-treated GVHD mice were significantly lower than those in control PBS-treated GVHD mice. This was due to a reduction in the levels of activated donor CD4+ T cells and a decrease in IL-4 production, resulting in a reduction in the numbers of activated host B cells. Therefore, our results suggest that SLC antagonist has beneficial effects for the prevention of chronic GVHD.

The expression of the cytomegalovirus chemokine receptor homolog US28 sequesters biologically active CC chemokines and alters IL-8 production

We hypothesized that US28, a cytomegalovirus (CMV) CC chemokine receptor homolog, plays a role in modulating the host antiviral defense. Monocyte chemotaxis was induced by supernatants from fibroblasts infected with a US28 deletion mutant of CMV (CMV Delta US28) due to endogenously produced CC chemokines MCP-1 and RANTES. However, these chemokines were sequestered from the supernatants of CMV-infected cells that did express US28. US28 was also capable of sequestering exogenously added RANTES. Surprisingly, cells infected with CMV Delta US28 transcribed and secreted increased levels IL-8, a CXC chemokine, when compared to CMV-infected cells. Finally, because chemokines are potent mediators of immune cell migration through the endothelium, we characterized the CC chemokine binding potential of CMV-infected endothelial cells. We propose that US28 functions as a 'chemokine sink' by sequestering endogenously and exogenously produced chemokines and alters the production of the CXC chemokine IL-8, suggesting that CMV could significantly alter the inflammatory milieu surrounding infected cells.

Cosine modulated HSQC: a rapid determination of (3)J(HNHalpha) scalar couplings in (15)N-labeled proteins

A two-dimensional HSQC-based NMR method, (15)N-COSMO-HSQC, is presented for the rapid determination of homonuclear (3)J(HNHalpha) couplings in (15)N-labeled proteins in solution. Scalar couplings are extracted by comparing the intensity of two separate datasets recorded with and without decoupling of the (3)J(HNHalpha) during a preparation period. The scalar couplings are introduced through a cosine modulation of the peak intensities. The experiment relies on a BIRD sandwich to selectively invert all amide protons H(N) and is very simple to implement. (3)J(HNHalpha) couplings were determined using both the (15)N-COSMO-HSQC and quantitative-J on (15)N-labeled chemokine RANTES. The two experiments show well-correlated values.

Structural rearrangement of human lymphotactin, a C chemokine, under physiological solution conditions

NMR spectra of human lymphotactin (hLtn), obtained under various solution conditions, have revealed that the protein undergoes a major conformational rearrangement dependent on temperature and salt concentration. At high salt (200 mm NaCl) and low temperature (10 degrees C), hLtn adopts a chemokine-like fold, which consists of a three-stranded antiparallel beta-sheet and a C-terminal alpha-helix (Kuloğlu, E. S., McCaslin, D. R., Kitabwalla, M., Pauza, C. D., Markley, J. L., and Volkman, B. F. (2001) Biochemistry 40, 12486-12496). We have used NMR spectroscopy, sedimentation equilibrium, and intrinsic fluorescence to monitor the reversible conformational change undergone by hLtn as a function of temperature and ionic strength. We have used two-, three- and four-dimensional NMR spectroscopy of isotopically enriched protein samples to determine structural properties of the conformational state stabilized at 45 degrees C and 0 mm NaCl. Patterns of NOEs and (1)H(alpha) and (13)C chemical shifts show that hLtn rearranges under these conditions to form a four-stranded, antiparallel beta-sheet with a pattern of hydrogen bonding that is completely different from that of the chemokine fold stabilized at 10 degrees C and 200 mm NaCl. The C-terminal alpha-helix observed at 10 degrees C and 200 mm NaCl, which is conserved in other chemokines, is absent at 45 degrees C and no salt, and the last 38 residues of the protein are completely disordered, as indicated by heteronuclear (15)N-(1)H NOEs. Temperature dependence of the tryptophan fluorescence of hLtn in low and high salt confirmed that the chemokine conformation is stabilized by increased ionic strength. Sedimentation equilibrium analytical ultracentrifugation showed that hLtn at 40 degrees C in the presence of 100 mm NaCl exists mainly as a dimer. Under near physiological conditions of temperature, pH, and ionic strength, both the chemokine-like and non-chemokine-like conformations of hLtn are significantly populated. The functional relevance of this structural interconversion remains to be elucidated.

Structure, function, and inhibition of chemokines

Chemokines are the largest family of cytokines in human immunophysiology. These proteins are defined by four invariant cysteines and are categorized based on the sequence around the first two cysteines, which leads to two major and two minor subfamilies. Chemokines function by activating specific G protein-coupled receptors, which results in, among other functions, the migration of inflammatory and noninflammatory cells to the appropriate tissues or compartments within tissues. Some of these proteins and receptors have been implicated or shown to be involved in inflammation, autoimmune diseases, and infection by HIV-1. The three-dimensional structure of each monomer is virtually identical, but the quaternary structure of chemokines is different for each subfamily. Structure-function studies reveal several regions of chemokines to be involved in function, with the N-terminal region playing a dominant role. A number of proteins and small-molecule antagonists have been identified that inhibit chemokine activities. In this review, we discuss aspects of the structure, function, and inhibition of chemokines.

Monomeric solution structure of the prototypical 'C' chemokine lymphotactin

Lymphotactin, the sole identified member of the C class of chemokines, specifically attracts T lymphocytes and natural killer cells. This 93-residue protein lacks 2 of the 4 conserved cysteine residues characteristic of the other 3 classes of chemokines and possesses an extended carboxyl terminus, which is required for chemotactic activity. We have determined the three-dimensional solution structure of recombinant human lymphotactin by NMR spectroscopy. Under the conditions used for the structure determination, lymphotactin was predominantly monomeric; however, pulsed field gradient NMR self-diffusion measurements and analytical ultracentrifugation revealed evidence of dimer formation. Sequence-specific chemical shift assignments were determined through analysis of two- and three-dimensional NMR spectra of (15)N- and (13)C/(15)N-enriched protein samples. Input for the torsion angle dynamics calculations used in determining the structure included 1258 unique NOE-derived distance constraints and 60 dihedral angle constraints obtained from chemical-shift-based searching of a protein conformational database. The ensemble of 20 structures chosen to represent the structure had backbone and heavy atom rms deviations of 0.46 +/- 0.11 and 1.02 +/- 0.14 A, respectively. The results revealed that human lymphotactin adopts the conserved chemokine fold, which is characterized by a three-stranded antiparallel beta-sheet and a C-terminal alpha-helix. Two regions are dynamically disordered as evidenced by (1)H and (13)C chemical shifts and [(15)N]-(1)H NOEs: residues 1-9 of the amino terminus and residues 69-93 of the C-terminal extension. A functional role for the C-terminal extension, which is unique to lymphotactin, remains to be elucidated.

CCR2 and CCR5 receptor-binding properties of herpesvirus-8 vMIP-II based on sequence analysis and its solution structure

Human herpesvirus-8 (HHV-8) is the infectious agent responsible for Kaposi's sarcoma and encodes a protein, macrophage inflammatory protein-II (vMIP-II), which shows sequence similarity to the human CC chemokines. vMIP-II has broad receptor specificity that crosses chemokine receptor subfamilies, and inhibits HIV-1 viral entry mediated by numerous chemokine receptors. In this study, the solution structure of chemically synthesized vMIP-II was determined by nuclear magnetic resonance. The protein is a monomer and possesses the chemokine fold consisting of a flexible N-terminus, three antiparallel beta strands, and a C-terminal alpha helix. Except for the N-terminal residues (residues 1-13) and the last two C-terminal residues (residues 73-74), the structure of vMIP-II is well-defined, exhibiting average rmsd of 0.35 and 0.90 A for the backbone heavy atoms and all heavy atoms of residues 14-72, respectively. Taking into account the sequence differences between the various CC chemokines and comparing their three-dimensional structures allows us to implicate residues that influence the quaternary structure and receptor binding and activation of these proteins in solution. The analysis of the sequence and three-dimensional structure of vMIP-II indicates the presence of epitopes involved in binding two receptors CCR2 and CCR5. We propose that vMIP-II was initially specific for CCR5 and acquired receptor-binding properties to CCR2 and other chemokine receptors.

Molecular determinants for CC-chemokine recognition by a poxvirus CC-chemokine inhibitor

Poxviruses express a family of secreted proteins that bind with high affinity to chemokines and antagonize the interaction with their cognate G protein-coupled receptors (GPCRs). These viral inhibitors are novel in structure and, unlike cellular chemokine receptors, are able to specifically interact with most, if not all, CC-chemokines. We therefore sought to define the structural features of CC-chemokines that facilitate this broad-spectrum interaction. Here, we identify the residues present on human monocyte chemoattractant protein-1 (MCP-1) that are required for high-affinity interaction with the vaccinia virus 35-kDa CC-chemokine binding protein (VV-35kDa). Not only do these residues correspond to those required for interaction with the cognate receptor CCR2b but they are also conserved among many CC-chemokines. Thus, the results provide a structural basis for the ability of VV-35kDa to promiscuously recognize CC-chemokines and block binding to their receptors.

NMR solution structure of murine CCL20/MIP-3alpha, a chemokine that specifically chemoattracts immature dendritic cells and lymphocytes through its highly specific interaction with the beta-chemokine receptor CCR6

CCL20/MIP-3alpha is a beta-chemokine expressed in the thymus, skin, and intestinal epithelial cells that exclusively binds and activates the CCR6 receptor in both mice and humans. The strict receptor binding specificity of CCL20 is exceptional; other chemokines and their receptors bind promiscuously with multiple partners. Toward determining the structural basis for the selective receptor specificity of CCL20, we have determined its three-dimensional structure by 1H NMR spectroscopy. CCL20 exhibits the same monomeric structure previously described for other chemokines: a three-stranded beta-sheet and an overlying alpha-helix. The CCL20 receptor selectivity could arise from the rigid conformation of the N-terminal DCCL motif as well as the groove between the N-loop and the beta2-beta3 hairpin, which is significantly narrower in CCL20 than in other chemokines. Similar structural features are seen in human beta-defensin 2, a small nonchemokine polypeptide reported to selectively bind and activate CCR6, which stresses their importance for the specific binding of both CCL20 and beta-defensin 2 to CCR6. CCL20's structure will be useful to design tools aimed to modulate its important biological functions.

Identification of human macrophage inflammatory proteins 1alpha and 1beta as a native secreted heterodimer

Chemokines are secreted proteins that function as chemoattractants for leukocytes. The chemokines macrophage inflammatory protein 1alpha and 1beta (MIP-1alpha and MIP-1beta) now have been shown to be secreted from activated human monocytes and peripheral blood lymphocytes (PBLs) as a heterodimer. Immunoprecipitation and immunoblot analysis revealed that antibodies to either MIP-1alpha or MIP-1beta precipitated a protein complex containing both MIP-1alpha and MIP-1beta under normal conditions from culture supernatants and lysates of these cells. Mass spectrometry of the complexes, precipitated from the culture supernatants of monocytes and PBLs, revealed the presence of NH(2)-terminal truncated MIP-1alpha (residues 5-70) together with either intact MIP-1beta or NH(2)-terminal truncated MIP-1beta (residues 3-69), respectively. The secreted MIP-1alpha/beta heterodimers were dissociated into their component monomers under acidic conditions. Exposure of monocytes or PBLs to monensin induced the accumulation of heterodimers composed of NH(2)-terminal truncated MIP-1alpha and full-length MIP-1beta in the Golgi complex. The mixing of recombinant chemokines in vitro demonstrated that heterodimerization of MIP-1alpha and MIP-1beta is specific and that it occurs at physiological conditions, pH 7.4, and in the range of nanomolar concentrations. The data presented here provide the first biochemical evidence for the existence of chemokine heterodimers under natural conditions. Formation of heterodimers of MIP-1alpha/beta may have an impact on intracellular signaling events that contribute to CCR5 and possibly to other chemokine receptor functions.

The BBXB motif of RANTES is the principal site for heparin binding and controls receptor selectivity

The chemokine RANTES (regulated on activation normal T cell expressed and secreted; CCL5) binds selectively to glycosaminoglycans (GAGs) such as heparin, chondroitin sulfate, and dermatan sulfate. The primary sequence of RANTES contains two clusters of basic residues, (44)RKNR(47) and (55)KKWVR(59). The first is a BBXB motif common in heparin-binding proteins, and the second is located in the loop directly preceding the C-terminal helix. We have mutated these residues to alanine, both as point mutations as well as triple mutations of the 40s and 50s clusters. Using a binding assay to heparin beads with radiolabeled proteins, the (44)AANA(47) mutant demonstrated an 80% reduction in its capacity to bind heparin, whereas the (55)AAWVA(59) mutant retained full binding capacity. Mutation of the (44)RKNR(47) site reduced the selectivity of RANTES binding to different GAGs. The mutants were tested for their integrity by receptor binding assays on CCR1 and CCR5 as well as their ability to induce chemotaxis in vitro. In all assays the single point mutations and the triple 50s cluster mutation caused no significant difference in activity compared with the wild type sequence. However, the triple 40s mutant showed a 80-fold reduction in affinity for CCR1, despite normal binding to CCR5. It was only able to induce monocyte chemotaxis at micromolar concentrations. The triple 40s mutant was also able to inhibit HIV-1 infectivity, but consistent with its abrogated GAG binding capacity, it no longer induced enhanced infectivity at high concentrations.