Engineering Metamorphic Chemokine Lymphotactin/XCL1 into the GAG-Binding, HIV-Inhibitory Dimer Conformation

Metamorphic proteins under a computational microscope: Lessons from a fold-switching RfaH protein

Metamorphic proteins constitute unexpected paradigms of the protein folding problem, as their sequences encode two alternative folds, which reversibly interconvert within biologically relevant timescales to trigger different cellular responses. Once considered a rare aberration, metamorphism may be common among proteins that must respond to rapidly changing environments, exemplified by NusG-like proteins, the only transcription factors present in every domain of life. RfaH, a specialized paralog of bacterial NusG, undergoes an all-α to all-β domain switch to activate expression of virulence and conjugation genes in many animal and plant pathogens and is the quintessential example of a metamorphic protein. The dramatic nature of RfaH structural transformation and the richness of its evolutionary history makes for an excellent model for studying how metamorphic proteins switch folds. Here, we summarize the structural and functional evidence that sparked the discovery of RfaH as a metamorphic protein, the experimental and computational approaches that enabled the description of the molecular mechanism and refolding pathways of its structural interconversion, and the ongoing efforts to find signatures and general properties to ultimately describe the protein metamorphome.

Design and discovery of metamorphic proteins

Metamorphic proteins are single amino acid sequences that reversibly interconvert between multiple, dramatically different native structures, often with distinct functions. Since the discovery of the first metamorphic proteins in the early 2000s, several additional metamorphic proteins have been identified, and it was suggested that up to 4% of proteins in the PDB may switch folds. Metamorphic proteins have been found to share common features such as marginal thermostability and inconsistencies in predicted secondary structures. Outstanding challenges in the field include the search for more metamorphic proteins and the design of new proteins that switch folds. Identification of novel metamorphic proteins in nature will improve therapeutic targeting of fold-switching proteins involved in human pathology and will enhance the design of protein-based therapies. Designed fold switching proteins have applications as biosensors, molecular switches, molecular machines, and self-assembling systems.

Specific binding-induced modulation of the XCL1 metamorphic equilibrium

The metamorphic protein XCL1 switches between two distinct native structures with different functions in the human immune system. This structural interconversion requires complete rearrangement of all hydrogen bonding networks, yet fold-switching occurs spontaneously and reversibly in solution. One structure occupies the canonical α-β chemokine fold and binds XCL1's cognate G-protein coupled receptor, while the other structure occupies a dimeric, all-β fold that binds glycosaminoglycans and has antimicrobial activity. Both of these functions are important for the biologic role of XCL1 in the immune system, and each structure is approximately equally populated under near-physiologic conditions. Recent work has begun to illuminate XCL1's role in combatting infection and cancer. However, without a way to control XCL1's dynamic structural interconversion, it is difficult to study the role of XCL1 fold-switching in human health and disease. Thus, a molecular tool that can regulate the fractional population of the two XCL1 structures is needed. Here, we find by heparin affinity chromatography and NMR that an engineered XCL1 variant called CC5 can trigger a dose-dependent shift in XCL1's metamorphic equilibrium such that the receptor binding structure is depleted, and the antimicrobial structure is more heavily populated. This shift likely occurs due to formation of XCL1-CC5 heterodimers in which both protomers occupy the β-sheet structure. These findings lay the groundwork for future studies seeking to understand the functional role of XCL1 metamorphosis, as well as studies screening for a drug-like molecule that can therapeutically target XCL1 by tuning its metamorphic equilibrium. Moreover, the proof of concept presented here suggests that protein metamorphosis is druggable, opening numerous avenues for controlling biological function of metamorphic proteins by altering the population of their multiple native states.

Metamorphic Protein Folding Encodes Multiple Anti-Candida Mechanisms in XCL1

Candida species cause serious infections requiring prolonged and sometimes toxic therapy. Antimicrobial proteins, such as chemokines, hold great interest as potential additions to the small number of available antifungal drugs. Metamorphic proteins reversibly switch between multiple different folded structures. XCL1 is a metamorphic, antimicrobial chemokine that interconverts between the conserved chemokine fold (an α–β monomer) and an alternate fold (an all-β dimer). Previous work has shown that human XCL1 kills C. albicans but has not assessed whether one or both XCL1 folds perform this activity. Here, we use structurally locked engineered XCL1 variants and Candida killing assays, adenylate kinase release assays, and propidium iodide uptake assays to demonstrate that both XCL1 folds kill Candida, but they do so via different mechanisms. Our results suggest that the alternate fold kills via membrane disruption, consistent with previous work, and the chemokine fold does not. XCL1 fold-switching thus provides a mechanism to regulate the XCL1 mode of antifungal killing, which could protect surrounding tissue from damage associated with fungal membrane disruption and could allow XCL1 to overcome candidal resistance by switching folds. This work provides inspiration for the future design of switchable, multifunctional antifungal therapeutics.

Metamorphic proteins: the Janus proteins of structural biology

The structural paradigm that the sequence of a protein encodes for a unique three-dimensional native fold does not acknowledge the intrinsic plasticity encapsulated in conformational free energy landscapes. Metamorphic proteins are a recently discovered class of biomolecules that illustrate this plasticity by folding into at least two distinct native state structures of comparable stability in the absence of ligands or cofactors to facilitate fold-switching. The expanding list of metamorphic proteins clearly shows that these proteins are not mere aberrations in protein evolution, but may have actually been a consequence of distinctive patterns in selection pressure such as those found in virus–host co-evolution. In this review, we describe the structure–function relationships observed in well-studied metamorphic protein systems, with specific focus on how functional residues are sequestered or exposed in the two folds of the protein. We also discuss the implications of metamorphosis for protein evolution and the efforts that are underway to predict metamorphic systems from sequence properties alone.

Gill transcriptomes reveal expression changes of genes related with immune and ion transport under salinity stress in silvery pomfret (Pampus argenteus)

Salinity is a major ecological factor in the marine environment, and extremely important for the survival, development, and growth of fish. In this study, gill transcriptomes were examined by high-throughput sequencing at three different salinities (12 ppt as low salinity, 22 ppt as control salinity, and 32 ppt as high salinity) in an importantly economical fish silvery pomfret. A total of 187 genes were differentially expressed, including 111 up-regulated and 76 down-regulated transcripts in low-salinity treatment group and 107 genes differentially expressed, including 74 up-regulated and 33 down-regulated transcripts in high-salinity treatment group compared with the control group, respectively. Some pathways including NOD-like receptor signaling pathway, cytokine-cytokine receptor interaction, Toll-like receptor pathway, cardiac muscle contraction, and vascular smooth muscle contraction were significantly enriched. qPCR analysis further confirmed that mRNA expression levels of immune (HSP90A, IL-1β, TNFα, TLR2, IP-10, MIG, CCL19, and IL-11) and ion transport-related genes (WNK2, NPY2R, CFTR, and SLC4A2) significantly changed under salinity stress. Low salinity stress caused more intensive expression changes of immune-related genes than high salinity. These results imply that salinity stress may affect immune function in addition to regulating osmotic pressure in silvery pomfret.

The chemokine lymphotactin and its recombinant variants in oral cancer cell regulation

BACKGROUND

The expression of XCR1 receptor and its metamorphic ligand lymphotactin (hLtn) has been shown in cancers but their precise role in tumorigenesis is poorly understood including the significance of the physiologically existing hLtn monomeric (CC3) and dimeric (W55D) confirmations where the latter thought to function as the receptor antagonist. The aim of this study was to explore the functional role of bioengineered hLtn variants and the role of fibroblasts in XCR1/hLtn expression regulation in oral cancer cells (OCCL).

MATERIAL AND METHODS

qRT-PCR and flow cytometry were performed to evaluate mRNA and protein expression of XCR1 and hLtn. Recombinant hLtn variants (wild-type, CC3 and W55D mutant) were designed, expressed, purified and evaluated using proliferation, adhesion and chemotaxis assays. XCR1 and hLtn expression regulation by fibroblasts was determined using indirect co-culture. XCR1 and hLtn expression in primary and metastatic OSCC tissue was assessed using immunohistochemistry.

RESULTS

hLtn caused a significant decrease in OCCL XCR1 surface protein expression. hLtn CC3 mutant was highly functional facilitating proliferation and migration. Conditioned media from primary cancer-associated and senescent fibroblasts significantly upregulated XCR1 and hLtn mRNA expression in OCCL. Immunohistochemistry revealed higher XCR1 and hLtn expression in metastatic tumour deposits and surrounding stroma compared to primary OSCC tissue.

CONCLUSIONS

The development of hLtn biological mutants, regulation of XCR1 expression by its ligand hLtn and crosstalk with fibroblasts are novel findings suggesting an important role for the XCR1/hLtn axis within the OSCC tumour microenvironment. These discoveries build upon previous studies and suggest that the hLtn/XCR1 axis has a significant role in stromal crosstalk and OSCC progression.

Switchable Membrane Remodeling and Antifungal Defense by Metamorphic Chemokine XCL1

Antimicrobial peptides (AMPs) are a class of molecules which generally kill pathogens via preferential cell membrane disruption. Chemokines are a family of signaling proteins that direct immune cell migration and share a conserved α–β tertiary structure. Recently, it was found that a subset of chemokines can also function as AMPs, including CCL20, CXCL4, and XCL1. It is therefore surprising that machine learning based analysis predicts that CCL20 and CXCL4’s α-helices are membrane disruptive, while XCL1’s helix is not. XCL1, however, is the only chemokine known to be a metamorphic protein which can interconvert reversibly between two distinct native structures (a β-sheet dimer and the α–β chemokine structure). Here, we investigate XCL1’s antimicrobial mechanism of action with a focus on the role of metamorphic folding. We demonstrate that XCL1 is a molecular “Swiss army knife” that can refold into different structures for distinct context-dependent functions: whereas the α–β chemokine structure controls cell migration by binding to G-Protein Coupled Receptors (GPCRs), we find using small angle X-ray scattering (SAXS) that only the β-sheet and unfolded XCL1 structures can induce negative Gaussian curvature (NGC) in membranes, the type of curvature topologically required for membrane permeation. Moreover, the membrane remodeling activity of XCL1’s β-sheet structure is strongly dependent on membrane composition: XCL1 selectively remodels bacterial model membranes but not mammalian model membranes. Interestingly, XCL1 also permeates fungal model membranes and exhibits anti-Candida activity in vitro, in contrast to the usual mode of antifungal defense which requires Th17 mediated cell-based responses. These observations suggest that metamorphic XCL1 is capable of a versatile multimodal form of antimicrobial defense.

Targeting Chemokine-Glycosaminoglycan Interactions to Inhibit Inflammation

Leukocyte migration into tissues depends on the activity of chemokines that form concentration gradients to guide leukocytes to a specific site. Interaction of chemokines with their specific G protein-coupled receptors (GPCRs) on leukocytes induces leukocyte adhesion to the endothelial cells, followed by extravasation of the leukocytes and subsequent directed migration along the chemotactic gradient. Interaction of chemokines with glycosaminoglycans (GAGs) is crucial for extravasation in vivo. Chemokines need to interact with GAGs on endothelial cells and in the extracellular matrix in tissues in order to be presented on the endothelium of blood vessels and to create a concentration gradient. Local chemokine retention establishes a chemokine gradient and prevents diffusion and degradation. During the last two decades, research aiming at reducing chemokine activity mainly focused on the identification of inhibitors of the interaction between chemokines and their cognate GPCRs. This approach only resulted in limited success. However, an alternative strategy, targeting chemokine-GAG interactions, may be a promising approach to inhibit chemokine activity and inflammation. On this line, proteins derived from viruses and parasites that bind chemokines or GAGs may have the potential to interfere with chemokine-GAG interactions. Alternatively, chemokine mimetics, including truncated chemokines and mutant chemokines, can compete with chemokines for binding to GAGs. Such truncated or mutated chemokines are characterized by a strong binding affinity for GAGs and abrogated binding to their chemokine receptors. Finally, Spiegelmers that mask the GAG-binding site on chemokines, thereby preventing chemokine-GAG interactions, were developed. In this review, the importance of GAGs for chemokine activity in vivo and strategies that could be employed to target chemokine-GAG interactions will be discussed in the context of inflammation.

Turncoat Polypeptides: We Adapt to Our Environment

A common interpretation of Anfinsen's hypothesis states that one amino acid sequence should fold into a single, native, ordered state, or a highly similar set thereof, coinciding with the global minimum in the folding-energy landscape, which, in turn, is responsible for the function of the protein. However, this classical view is challenged by many proteins and peptide sequences, which can adopt exchangeable, significantly dissimilar conformations that even fulfill different biological roles. The similarities and differences of concepts related to these proteins, mainly chameleon sequences, metamorphic proteins, and switch peptides, which are all denoted herein "turncoat" polypeptides, are reviewed. As well as adding a twist to the conventional view of protein folding, the lack of structural definition adds clear versatility to the activity of proteins and can be used as a tool for protein design and further application in biotechnology and biomedicine.

Structure-function guided modeling of chemokine-GPCR specificity for the chemokine XCL1 and its receptor XCR1

Chemokines interact with their G protein-coupled receptors (GPCRs) through a two-step, two-site mechanism and, through this interaction, mediate various homeostatic and immune response mechanisms. Upon initial recognition of the chemokine by the receptor, the amino terminus of the chemokine inserts into the orthosteric pocket of the GPCR, causing conformational changes that trigger intracellular signaling. There is considerable structural and functional evidence to suggest that the amino acid composition and length of the chemokine amino terminus is critical for GPCR activation, complementing the size and amino acid composition of the orthosteric pocket. However, very few structures of a native chemokine-receptor complex have been solved. Here, we used a hybrid approach that combines structure-function data with Rosetta modeling to describe key contacts within a chemokine-GPCR interface. We found that the extreme amino-terminal residues of the chemokine XCL1 (Val¹, Gly², Ser³, and Glu⁴) contribute a large fraction of the binding energy to its receptor XCR1, whereas residues near the disulfide bond-forming residue Cys¹¹ modulate XCR1 activation. Alterations in the XCL1 amino terminus changed XCR1 activation, as determined by assessing inositol triphosphate accumulation, intracellular calcium release, and directed cell migration. Computational analysis of XCL1-XCR1 interactions revealed functional contacts involving Glu⁴ of XCL1 and Tyr¹¹⁷ and Arg²⁷³ of XCR1. Subsequent mutation of Tyr¹¹⁷ and Arg²⁷³ led to diminished binding and activation of XCR1 by XCL1. These findings demonstrate the utility of a hybrid approach, using biological data and homology modeling, to study chemokine-GPCR interactions.

Duplication of a Single Strand in a β-Sheet Can Produce a New Switching Function in a Photosensory Protein

Duplication of a single β-strand that forms part of a β-sheet in photoactive yellow protein (PYP) was found to produce two approximately isoenergetic protein conformations, in which either the first or the second copy of the duplicated β-strand participates in the β-sheet. Whereas one conformation (big-loop) is more stable at equilibrium in the dark, the other conformation (long-tail) is populated after recovery from blue light irradiation. By appending a recognition motif (E-helix) to the C-terminus of the protein, we show that β-strand duplication, and the resulting possibility of β-strand slippage, can lead to a new switchable protein-protein interaction. We suggest that β-strand duplication may be a general means of introducing two-state switching activity into protein structures.

Unfolding the Mysteries of Protein Metamorphosis

Since the proposal of Anfinsen’s thermodynamic hypothesis in 1963, our understanding of protein folding and dynamics has gained significant appreciation of its nuance and complexity. Intrinsically disordered proteins, chameleonic sequences, morpheeins, and metamorphic proteins have broadened the protein folding paradigm. Here, we discuss noncanonical protein folding patterns, with an emphasis on metamorphic proteins, and we review known metamorphic proteins that occur naturally and that have been engineered in the laboratory. Finally, we discuss research areas surrounding metamorphic proteins that are primed for future exploration, including evolution, drug discovery, and the quest for previously unrecognized metamorphs. As we enter an age where we are capable of complex bioinformatic searches and de novo protein design, we are primed to search for previously unrecognized metamorphic proteins and to design our own metamorphs to act as targeted, switchable drugs; biosensors; and more.

Molecular characterization of a CC motif chemokine 19-like gene in ayu (Plecoglossus altivelis) and its role in leukocyte trafficking

The CC motif chemokine 19 (CCL19) functions in acute inflammation by recruiting lymphocytes and other cells. However, CCL19 has only been investigated in few fish species. In this study, we characterized a CCL19-like molecule (PaCCL19l) in ayu (Plecoglossus altivelis), a teleost fish. Sequence analysis revealed that PaCCL19l was most closely related to Atlantic salmon (Salmon salar) CCL19l1, which belonged to the fish CCL19a.1 subcluster. PaCCL19l was constitutively expressed in the tested ayu tissues and peripheral blood mononuclear cells (PBMCs), with the highest transcript level in PBMCs. Upon infection with Vibrio anguillarum, the expressions of PaCCL19l in the head kidney, liver, spleen, PBMCs, and monocytes/macrophages (MO/MΦ) were dramatically up-regulated. Recombinant PaCCL19l (rPaCCL19l) exhibited a significant effect on the chemotaxis of lymphocytes and MO/MΦ in vitro and in vivo. Meanwhile, rPaCCL19l exerted a high chemotaxic activity for lipopolysaccharide (LPS)-stimulated MO/MΦ (M1-type), but not for cyclic adenosine monophosphate (cAMP)-stimulated MO/MΦ (M2-type). When ayu MO/MΦ was treated with rPaCCL19l along with Vibrio anguillarum infection, the mRNA expression of proinflammatory cytokines (IL-1β, TNFα, IL-6, IL-12b, and IFN-γ) was up-regulated, while that of anti-inflammatory cytokines (IL-10, TGFβ, and IL-22) was down-regulated. Ayu MO/MΦ treated with anti-PaCCL19l IgG gave the opposite result. These results implicated that PaCCL19l is involved in the selective chemotaxis of ayu immune cells and promotes the host at a pro-inflammatory state.

Depot-specific inflammation with decreased expression of ATM2 in white adipose tissues induced by high-margarine/lard intake

A high-fat diet has been recognized as an important risk factor of obesity, with variable impacts of different fatty acid compositions on the physiological process. To understand the effects of a high-margarine/lard diet, which is a major source of trans fatty acids (TFAs)/ saturated fatty acids (SFAs), elaidic acid as a biomarker of margarine intake was used to screen affected adipokines on mature human adipocytes in vitro. Weaned male Wistar rats were fed a high-fat diet enriched with margarine/lard to generate obesity-prone (OP) and obesity-resistant (OR) models, which were then used to explore the inflammatory responses of depot-specific white adipose tissue. Adiposity, glucose and lipid metabolism parameters and macrophage cell markers were also compared in vivo. In the subcutaneous depot, a high-margarine diet induced elevated IL-6, MCP-1 and XCL1 expression levels in both M-OP and M-OR groups. High-lard diet-fed rats displayed higher protein expression levels of MCP-1 and XCL1 compared with the control group. In the epididymal depot, significantly elevated IL-6 production was observed in M-OP rats, and high-lard diet-fed rats displayed elevated IL-6 and decreased XCL1 expression. In the retroperitoneal depot, a high-margarine diet caused higher IL-6 and MCP-1 expression levels, a high-lard diet caused elevated IL-6 expression in L-OP/L-OR rats, and elevated XCL1 expression was observed only in L-OP rats. In general, CD206 mRNA levels were notably down-regulated by high-fat diet feeding in the above-mentioned depots. CD11c mRNA levels were slightly upregulated in the subcutaneous depot of OP rats fed a high-margarine/lard diet. In the epidydimal depot, higher expression levels of F4/80 and CD206 mRNA were observed only in high-margarine diet-fed OP rats. These results suggest that depot-specific inflammation with decreased expression of adipose tissue anti-inflammatory M2-type (ATM2) macrophages could be induced by high-margarine/lard intake.

Progress in research on C-chemokine XCL1

XCL1, also known as lymphotactin, is the only known member of the C-type-chemokine family, which is produced mainly by CD8⁺ T cells and natural killer cells. XCL1 has a unique amino acid sequence feature and two interchangeable conformations, which makes XCL1 different from other chemokines in structure and function. The XCL1-specific receptor, XCR1, is a member of the G-protein-coupled receptor family and plays an important role in the negative selection of T cells in the thymus and in the initiation of cross-antigen presentation and mediation of cytotoxic immune responses. XCL1 can regulate the balance of the immune system and maintain intestinal immune homeostasis, and it is involved in a variety of diseases such as autoimmune diseases, nephritis, tuberculosis and human immunodeficiency virus infection. In recent years, the selective expression of XCR1 on CD8⁺ DCs with strong cross antigen-presention ability has been proved, which has led to studies using XCL1 for mucosal immunization, antitumor immunotherapy and targeted vaccine development.

Cytokines Elevated in HIV Elite Controllers Reduce HIV Replication In Vitro and Modulate HIV Restriction Factor Expression

A subset of HIV-infected individuals termed elite controllers (ECs) maintain CD4⁺ T cell counts and control viral replication in the absence of antiretroviral therapy (ART). Systemic cytokine responses may differentiate ECs from subjects with uncontrolled viral replication or from those who require ART to suppress viral replication. We measured 87 cytokines in four groups of women: 73 ECs, 42 with pharmacologically suppressed viremia (ART), 42 with uncontrolled viral replication (noncontrollers [NCs]), and 48 HIV-uninfected (NEG) subjects. Four cytokines were elevated in ECs but not NCs or ART subjects: CCL14, CCL21, CCL27, and XCL1. In addition, median stromal cell-derived factor-1 (SDF-1) levels were 43% higher in ECs than in NCs. The combination of the five cytokines suppressed R5 and X4 virus replication in resting CD4⁺ T cells, and individually SDF-1β, CCL14, and CCL27 suppressed R5 virus replication, while SDF-1β, CCL21, and CCL14 suppressed X4 virus replication. Functional studies revealed that the combination of the five cytokines upregulated CD69 and CCR5 and downregulated CXCR4 and CCR7 on CD4⁺ T cells. The CD69 and CXCR4 effects were driven by SDF-1, while CCL21 downregulated CCR7. The combination of the EC-associated cytokines induced expression of the anti-HIV host restriction factors IFITM1 and IFITM2 and suppressed expression of RNase L and SAMHD1. These results identify a set of cytokines that are elevated in ECs and define their effects on cellular activation, HIV coreceptor expression, and innate restriction factor expression. This cytokine pattern may be a signature characteristic of HIV-1 elite control, potentially important for HIV therapeutic and curative strategies.IMPORTANCE Approximately 1% of people infected with HIV control virus replication without taking antiviral medications. These subjects, termed elite controllers (ECs), are known to have stronger immune responses targeting HIV than the typical HIV-infected subject, but the exact mechanisms of how their immune responses control infection are not known. In this study, we identified five soluble immune signaling molecules (cytokines) in the blood that were higher in ECs than in subjects with typical chronic HIV infection. We demonstrated that these cytokines can activate CD4⁺ T cells, the target cells for HIV infection. Furthermore, these five EC-associated cytokines could change expression levels of intrinsic resistance factors, or molecules inside the target cell that fight HIV infection. This study is significant in that it identified cytokines elevated in subjects with a good immune response against HIV and defined potential mechanisms as to how these cytokines could induce resistance to the virus in target cells.

Mechanisms of Regulation of the Chemokine-Receptor Network

The interactions of chemokines with their G protein-coupled receptors promote the migration of leukocytes during normal immune function and as a key aspect of the inflammatory response to tissue injury or infection. This review summarizes the major cellular and biochemical mechanisms by which the interactions of chemokines with chemokine receptors are regulated, including: selective and competitive binding interactions; genetic polymorphisms; mRNA splice variation; variation of expression, degradation and localization; down-regulation by atypical (decoy) receptors; interactions with cell-surface glycosaminoglycans; post-translational modifications; oligomerization; alternative signaling responses; and binding to natural or pharmacological inhibitors.

Chemokines and their receptors: insights from molecular modeling and crystallography

Chemokines are small secreted proteins that direct cell migration in development, immunity, inflammation, and cancer. They do so by binding and activating specific G protein coupled receptors on the surface of migrating cells. Despite the importance of receptor:chemokine interactions, their structural basis remained unclear for a long time. In 2015, the first atomic resolution insights were obtained with the publication of X-ray structures for two distantly related receptors bound to chemokines. In conjunction with experiment-guided molecular modeling, the structures suggest a conserved receptor:chemokine complex architecture, while highlighting the diverse details and functional roles of individual interaction epitopes. Novel findings promote the development and detailed structural interpretation of the canonical two-site hypothesis of receptor:chemokine recognition, and suggest new avenues for pharmacological modulation of chemokine receptors.

A Requirement for Metamorphic Interconversion in the Antimicrobial Activity of Chemokine XCL1

Chemokines make up a superfamily of ∼50 small secreted proteins (8-12 kDa) involved in a host of physiological processes and disease states, with several previously shown to have direct antimicrobial activity comparable to that of defensins in efficacy. XCL1 is a unique metamorphic protein that interconverts between the canonical chemokine fold and a novel all-β-sheet dimer. Phylogenetic analysis suggests that, within the chemokine family, XCL1 is most closely related to CCL20, which exhibits antibacterial activity. The in vitro antimicrobial activity of WT-XCL1 and structural variants was quantified using a radial diffusion assay (RDA) and in solution bactericidal assays against Gram-positive and Gram-negative species of bacteria. Comparisons of WT-XCL1 with variants that limit metamorphic interconversion showed a loss of antimicrobial activity when restricted to the conserved chemokine fold. These results suggest that metamorphic folding of XCL1 is required for potent antimicrobial activity.

Transferring the C-terminus of the chemokine CCL21 to CCL19 confers enhanced heparin binding

Chemokines direct the migration of cells during various immune processes and are involved in many disease states. For example, CCL19 and CCL21, through activation of the CCR7 receptor, recruit dendritic cells and naïve T-cells to the secondary lymphoid organs aiding in balancing immune response and tolerance. However, CCL19 and CCL21 can also direct the metastasis of CCR7 expressing cancers. Chemokine binding to glycosaminoglycans, such as heparin, is as important to chemokine function as receptor activation. CCL21 is unique in that it contains an extended C-terminus not found in other chemokines like CCL19. Deletion of this extended C-terminus reduces CCL21's affinity for heparin and transferring the CCL21 C-terminus to CCL19 enhances heparin binding mainly through non-specific, electrostatic interactions.

New paradigms in chemokine receptor signal transduction: Moving beyond the two-site model

Chemokine receptor (CKR) signaling forms the basis of essential immune cellular functions, and dysregulated CKR signaling underpins numerous disease processes of the immune system and beyond. CKRs, which belong to the seven transmembrane domain receptor (7TMR) superfamily, initiate signaling upon binding of endogenous, secreted chemokine ligands. Chemokine-CKR interactions are traditionally described by a two-step/two-site mechanism, in which the CKR N-terminus recognizes the chemokine globular core (i.e. site 1 interaction), followed by activation when the unstructured chemokine N-terminus is inserted into the receptor TM bundle (i.e. site 2 interaction). Several recent studies challenge the structural independence of sites 1 and 2 by demonstrating physical and allosteric links between these supposedly separate sites. Others contest the functional independence of these sites, identifying nuanced roles for site 1 and other interactions in CKR activation. These developments emerge within a rapidly changing landscape in which CKR signaling is influenced by receptor PTMs, chemokine and CKR dimerization, and endogenous non-chemokine ligands. Simultaneous advances in the structural and functional characterization of 7TMR biased signaling have altered how we understand promiscuous chemokine-CKR interactions. In this review, we explore new paradigms in CKR signal transduction by considering studies that depict a more intricate architecture governing the consequences of chemokine-CKR interactions.