General classes and functions of four-helix bundle cytokines

Interleukin-15 in kidney disease and therapeutics

PURPOSE OF REVIEW

Interleukin 15 (IL-15) is a member of the IL-2 family of common gamma chain receptor cytokines with well described anti-inflammatory, pro-survival and pro-proliferative signaling properties. The cytoprotective role of IL-15 in the kidney is now coming into focus with recent reports of its beneficial actions in various forms of kidney disease. This review will summarize what is currently known about IL-15 signaling in the kidney and highlight recent evidence of its beneficial effects on kidney physiology.

RECENT FINDINGS

IL-15 and its heterotrimeric receptor are expressed throughout the kidney. Like all IL-2 family cytokines, IL-15 can activate signaling through the Janus Kinase (JAK)/Signal transducer of activated T-cells (STAT), phosphoinositol-3 kinase (PI-3K)/AKT and mitogen activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathways and recent evidence suggests that STAT5B is an essential transcriptional mediator of prosurvival signaling in glomerular visceral epithelial cells (i.e. podocytes). IL-15 has also been shown to suppress pro-apoptotic signaling in models of acute kidney injury and pro-fibrotic signaling in models of chronic kidney disease.

SUMMARY

The cytoprotective properties of IL-15 suggest that it may have potential as a nonimmunosuppresive therapeutic for kidney disease. A novel class of IL-15 immunotherapies has emerged for the treatment cancer and some have demonstrated efficacy in clinical trials. These well tolerated IL-15 agonists could possibly be repurposed for the treatment of kidney disease and warrant further exploration.

Molecular Actors of Inflammation and Their Signaling Pathways: Mechanistic Insights from Zebrafish

Inflammation is a hallmark of the physiological response to aggressions. It is orchestrated by a plethora of molecules that detect the danger, signal intracellularly, and activate immune mechanisms to fight the threat. Understanding these processes at a level that allows to modulate their fate in a pathological context strongly relies on in vivo studies, as these can capture the complexity of the whole process and integrate the intricate interplay between the cellular and molecular actors of inflammation. Over the years, zebrafish has proven to be a well-recognized model to study immune responses linked to human physiopathology. We here provide a systematic review of the molecular effectors of inflammation known in this vertebrate and recapitulate their modes of action, as inferred from sterile or infection-based inflammatory models. We present a comprehensive analysis of their sequence, expression, and tissue distribution and summarize the tools that have been developed to study their function. We further highlight how these tools helped gain insights into the mechanisms of immune cell activation, induction, or resolution of inflammation, by uncovering downstream receptors and signaling pathways. These progresses pave the way for more refined models of inflammation, mimicking human diseases and enabling drug development using zebrafish models.

Structural and evolutionary exploration of the IL-3 family and its alpha subunit receptors

Interleukin-3 (IL-3) is a cytokine belonging to the family of common β (βc) and is involved in various biological systems. Its activity is mediated by the interaction with its receptor (IL-3R), a heterodimer composed of two distinct subunits: IL-3Rα and βc. IL-3 and its receptor, especially IL-3Rα, play a crucial role in pathologies like inflammatory diseases and therefore are interesting therapeutic targets. Here, we have performed an analysis of these proteins and their interaction based on structural and evolutionary information. We highlighted that IL-3 and IL-3Rα structural architectures are conserved across evolution and shared with other proteins belonging to the same βc family interleukin-5 (IL-5) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The IL-3Rα/IL-3 interaction is mediated by a large interface in which most residues are surprisingly not conserved during evolution and across family members. In spite of this high variability, we suggested small regions constituted by few residues conserved during the evolution in both proteins that could be important for the binding affinity.

The tertiary structure of γc cytokines dictates receptor sharing

The γc family of cytokines comprising interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15 and IL-2 is an important group of 4-helix bundle cytokines that signals through receptors incorporating the common gamma chain (γc). These cytokines are involved in lymphocyte biology and their specific functions are contingent on binding to cognate receptor chains. Here, we examined the structural relationships between γc cytokines, aiming to understand the basis for receptor chain usage and sharing. To that end, we obtained tertiary structures of human and mouse γc cytokines plus two other related cytokines, IL-13 and TSLP, which share receptors with IL-4 and IL-7, respectively. Subsequently, we compared the cytokine 3D-structures introducing a structural similarity score that grouped γc cytokines in a manner that mirrored the relationships dictated by receptor sharing. Unlike previously thought, we identified that IL-9 is more closely related to IL-2 and IL-15 than to IL-7, which is actually the most distant member of the γc family of cytokines. Moreover, we found that all the members of the γc family of cytokines share the topology of short-chain 4-helix bundle cytokines but IL-7 that with TSLP has the topology of long-chain 4-helix bundle cytokines. We also carried out Maximun-Likehood and Bayesian phylogenetic analyses that supported these results at the amino acid sequence level. Overall, our findings are of paramount relevance to understand receptor sharing among γc cytokines and can lead to the discovery of new cytokine receptor partners.

JAK1 Takes a FERM Hold of Type II Cytokine Receptors

Janus kinases (JAKs) initiate the intracellular signaling cascade triggered by exposure of cells to cytokines and interferons. In order to achieve this, JAKs are bound to the intracellular domain of specific cytokine receptors immediately adjacent to the cell membrane. In this issue of Structure, Ferrao et al. (2016) provide structural details of such an interaction and in doing so, identify for the first time the motif used by type II cytokine receptors to recruit JAK1.

The Structural Basis for Class II Cytokine Receptor Recognition by JAK1

JAK1 is a member of the Janus kinase (JAK) family of non-receptor tyrosine kinases that are activated in response to cytokines and interferons. Here, we present two crystal structures of the human JAK1 FERM and SH2 domains bound to peptides derived from the class II cytokine receptors IFN-λ receptor 1 and IL-10 receptor 1 (IFNLR1 and IL10RA). These structures reveal an interaction site in the JAK1 FERM that accommodates the so-called "box1" membrane-proximal receptor peptide motif. Biophysical analysis of the JAK1-IFNLR1 interaction indicates that the receptor box1 is the primary driver of the JAK1 interaction, and identifies residues conserved among class II receptors as important for binding. In addition, we demonstrate that a second "box2" receptor motif further stabilizes the JAK1-IFNLR1 complex. Together, these data identify a conserved JAK binding site for receptor peptides and elucidate the mechanism by which class II cytokine receptors interact with JAK1.

JAK2 activation by growth hormone and other cytokines

Growth hormone (GH) and structurally related cytokines regulate a great number of physiological and pathological processes. They do this by coupling their single transmembrane domain (TMD) receptors to cytoplasmic tyrosine kinases, either as homodimers or heterodimers. Recent studies have revealed that many of these receptors exist as constitutive dimers rather than being dimerized as a consequence of ligand binding, which has necessitated a new paradigm for describing their activation process. In the present study, we describe a model for activation of the tyrosine kinase Janus kinase 2 (JAK2) by the GH receptor homodimer based on biochemical data and molecular dynamics simulations. Binding of the bivalent ligand reorientates and rotates the receptor subunits, resulting in a transition from a form with parallel TMDs to one where the TMDs separate at the point of entry into the cytoplasm. This movement slides the pseudokinase inhibitory domain of one JAK kinase away from the kinase domain of the other JAK within the receptor dimer-JAK complex, allowing the two kinase domains to interact and trans-activate. This results in phosphorylation and activation of STATs and other signalling pathways linked to this receptor which then regulate postnatal growth, metabolism and stem cell activation. We believe that this model will apply to most if not all members of the class I cytokine receptor family, and will be useful in the design of small antagonists and agonists of therapeutic value.

Insights into cytokine-receptor interactions from cytokine engineering

Cytokines exert a vast array of immunoregulatory actions critical to human biology and disease. However, the desired immunotherapeutic effects of native cytokines are often mitigated by toxicity or lack of efficacy, either of which results from cytokine receptor pleiotropy and/or undesired activation of off-target cells. As our understanding of the structural principles of cytokine-receptor interactions has advanced, mechanism-based manipulation of cytokine signaling through protein engineering has become an increasingly feasible and powerful approach. Modified cytokines, both agonists and antagonists, have been engineered with narrowed target cell specificities, and they have also yielded important mechanistic insights into cytokine biology and signaling. Here we review the theory and practice of cytokine engineering and rationalize the mechanisms of several engineered cytokines in the context of structure. We discuss specific examples of how structure-based cytokine engineering has opened new opportunities for cytokines as drugs, with a focus on the immunotherapeutic cytokines interferon, interleukin-2, and interleukin-4.

Inhibitory GH receptor extracellular domain monoclonal antibodies: three-dimensional epitope mapping

GH receptor (GHR) mediates the anabolic and metabolic effects of GH. We previously characterized a monoclonal antibody (anti-GHR(ext-mAb)) that reacts with subdomain 2 of the rabbit GHR extracellular domain (ECD) and is a conformation-specific inhibitor of GH signaling in cells bearing rabbit or human GHR. Notably, this antibody has little effect on GH binding and also inhibits inducible metalloproteolysis of the GHR that occurs in the perimembranous ECD stem region. In the current study, we demonstrate that anti-GHR(ext-mAb) inhibits GH-dependent cellular proliferation and also inhibits hepatic GH signaling in vivo in mice that adenovirally express rabbit GHR, as assessed with our noninvasive bioluminescence hepatic signaling assay. A separate monoclonal antibody (anti-GHR(mAb 18.24)) is a sister clone of anti-GHR(ext-mAb). Here, we demonstrate that anti-GHR(mAb 18.24) also inhibits rabbit and human GHR signaling and inducible receptor proteolysis. Further, we use a random PCR-generated mutagenic expression system to map the three-dimensional epitopes in the rabbit GHR ECD for both anti-GHR(ext-mAb) and anti-GHR(mAb 18.24). We find that each of the two antibodies has similar, but nonidentical, discontinuous epitopes that include regions of subdomain 2 encompassing the dimerization interface. These results have fundamental implications for understanding the role of the dimerization interface and subdomain 2 in GHR activation and regulated GHR metalloproteolysis and may inform development of therapeutics that target GHR.

Structure and function of interleukin-17 family cytokines

The recently identified interleukin-17 (IL-17) cytokines family, which comprises six members in mammals (IL-17A-F), plays essential roles in the host immunity against infectious diseases and chronic inflammatory diseases. The three-dimensional structures containing IL-17A or IL-17F have become available and revealed the unique structural features of IL-17s as well as their receptors. Molecular modeling in this review shows that IL-17s may adopt a "cysteine knot" fold commonly seen in nerve growth factor (NGF) and other neurotrophins. Further modeling analysis unmasks a signature interaction feature of the IL-17F/IL-17RA complex, where a small loop of IL-17RA slots into the deep groove of the interface of IL-17F homodimer. This is quite different from the interaction between the best known four-helix cytokines and their cognate receptors. On the other hand, structure of IL-17A and its monoclonal antibody (CAT-2200) shows that, albeit that the antigenic epitope of IL-17A resides outside of the IL-17A homodimer interface, its physical proximity to the receptor binding groove may explain that antibody blockage would be achieved by interfering with the ligand-receptor interaction. This review is to summarize the advance in understanding the structure and function of IL-17 family cytokines, focusing mainly on IL-17A, IL-17F and IL-17E, in the hope of gaining better knowledge of immunotherapeutic strategies against various inflammatory diseases.

Non-core region modulates interleukin-11 signaling activity: generation of agonist and antagonist variants

Human interleukin-11 (hIL-11) is a pleiotropic cytokine administered to patients with low platelet counts. From a structural point of view hIL-11 belongs to the long-helix cytokine superfamily, which is characterized by a conserved core motif consisting of four α-helices. We have investigated the region of hIL-11 that does not belong to the α-helical bundle motif, and that for the purpose of brevity we have termed "non-core region." The primary sequence of the interleukin was altered at various locations within the non-core region by introducing glycosylation sites. Functional consequences of these modifications were examined in cell-based as well as biophysical assays. Overall, the data indicated that the non-core region modulates the function of hIL-11 in two ways. First, the majority of muteins displayed enhanced cell-stimulatory properties (superagonist behavior) in a glycosylation-dependent manner, suggesting that the non-core region is biologically designed to limit the full potential of hIL-11. Second, specific modification of a predicted mini α-helix led to cytokine inactivation, demonstrating that this putative structural element belongs to site III engaging a second copy of cell-receptor gp130. These findings have unveiled new and unexpected elements modulating the biological activity of hIL-11, which may be exploited to develop more versatile medications based on this important cytokine.

Structural biology of shared cytokine receptors

Recent structural information for complexes of cytokine receptor ectodomains bound to their ligands has significantly expanded our understanding of the macromolecular topology and ligand recognition mechanisms used by our three principal shared cytokine signaling receptors-gp130, gamma(c), and beta(c). The gp130 family receptors intricately coordinate three structurally unique cytokine-binding sites on their four-helix bundle cytokine ligands to assemble multimeric signaling complexes. These organizing principles serve as topological blueprints for the entire gp130 family of cytokines. Novel structures of gamma(c) and beta(c) complexes show us new twists, such as the use of a nonstandard sushi-type alpha receptors for IL-2 and IL-15 in assembling quaternary gamma(c) signaling complexes and an antiparallel interlocked dimer in the GM-CSF signaling complex with beta(c). Unlike gp130, which appears to recognize vastly different cytokine surfaces in chemically unique fashions for each ligand, the gamma(c)-dependent cytokines appear to seek out some semblance of a knobs-in-holes shape recognition code in order to engage gamma(c) in related fashions. We discuss the structural similarities and differences between these three shared cytokine receptors, as well as the implications for transmembrane signaling.

Structural organization of a full-length gp130/LIF-R cytokine receptor transmembrane complex

gp130 is a shared receptor for at least nine cytokines and can signal either as a homodimer or as a heterodimer with Leukemia Inhibitory Factor Receptor (LIF-R). Here, we biophysically and structurally characterize the full-length, transmembrane form of a quaternary cytokine receptor complex consisting of gp130, LIF-R, the cytokine Ciliary Neurotrophic Factor (CNTF), and its alpha receptor (CNTF-Ralpha). Thermodynamic analysis indicates that, unlike the cooperative assembly of the symmetric gp130/Interleukin-6/IL-6Ralpha hexameric complex, CNTF/CNTF-Ralpha heterodimerizes gp130 and LIF-R via noncooperative energetics to form an asymmetric 1:1:1:1 complex. Single particle electron microscopic analysis of the full-length gp130/LIF-R/CNTF-Ralpha/CNTF quaternary complex elucidates an asymmetric structural arrangement, in which the receptor extracellular and transmembrane segments join as a continuous, rigid unit, poised to sensitively transduce ligand engagement to the membrane-proximal intracellular signaling regions. These studies also enumerate the organizing principles for assembly of the "tall" class of gp130 family cytokine receptor complexes including LIF, IL-27, IL-12, and others.

Jak3 negatively regulates dendritic-cell cytokine production and survival

Cytokines are critical in regulating the development and function of diverse cells. Janus kinase 3 (Jak3) is a tyrosine kinase expressed in hematopoietic cells that associates with the common gamma chain (gammac) and is required for signaling for a family of cytokines including interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15, and IL-21; deficiency of either Jak3 or gammac results in severe combined immunodeficiency (SCID). While Jak3 is essential for lymphoid-cell development, the potential roles for Jak3 in regulating dendritic cells (DCs) were unclear. Herein, we show that although CD8+CD11c+ splenic DCs are absent in Jak3-/- mice, bone marrow-derived DCs developed normally in vitro from Jak3-/- precursor cells. In fact, the survival of Jak3-/- DCs was enhanced, and they expressed lower levels of proapoptotic proteins. Jak3-/- DCs exhibited normal antigen uptake and up-regulation of costimulatory molecules. However, Jak3-/- DCs produced more IL-12 and IL-10 in response to Toll-like receptor ligands, which correlated with enhanced T helper 1 (Th1) differentiation in vivo. In summary, Jak3 is not essential for DC development but unexpectedly appears to be an important negative regulator. These results may be relevant clinically for patients with SCID who have undergone hematopoietic stem cell transplantation and for patients who might be treated with a Jak3 inhibitor.

De novo design of an IL-4 antagonist and its structure at 1.9 A

An IL-4 antagonist was designed based on structural and biochemical analysis of unbound IL-4 and IL-4 in complex with its high-affinity receptor (IL-4Ralpha). Our design strategy sought to capture a protein-protein interaction targeting the high affinity that IL-4 has for IL-4Ralpha. This strategy has impact due to the potential relevance of IL-4Ralpha as a drug target in the treatment of asthma. To mimic the IL-4 binding surface, critical side chains for receptor binding were identified, and these side chains were transplanted onto a previously characterized, de novo-designed four-helix protein called designed helical protein 1 (DHP-1). This first-generation design resolved the ambiguity previously described for the connectivity between helices in DHP-1 and resulted in a protein capable of binding to IL-4Ralpha. The second-generation antagonist was based upon further molecular modeling, and it succeeded in binding IL-4Ralpha better than the first-generation. This protein, termed DHP-14-AB, yielded a protein with a cooperative unfolding transition (DeltaGu0=8.1 kcal/mol) and an IC50 of 27 microM when in competition with IL-4 whereas DHP-1 had no affinity for IL-4Ralpha. The crystal structure of DHP-14-AB was determined to 1.9-A resolution and was compared with IL-4. This comparison revealed how design strategies targeting protein-protein interactions require high-resolution 3D data and the incorporation of orientation-specific information at the level of side-chains and secondary structure element interactions.

Shared cytokine signaling receptors: structural insights from the gp130 system

The vast majority of cytokine signaling is mediated by "shared" receptors that form central signaling components of higher-order complexes incorporating ligand-specific receptors. These include the common gamma chain (gamma(c)), common beta chain (beta(c)), and gp130, as well as others. These receptors have the dual tasks of cross-reactive cytokine recognition, and formation of precisely oriented multimeric signaling assemblies. Currently, detailed structural information on a shared receptor complex exists only for gp130, which is a highly pleiotropic shared cytokine signaling receptor essential for mammalian cell growth and homeostasis. To date, more than 10 different four-helix bundle ligands have been identified that incorporate gp130, or one of its close relatives such as LIF receptor, into functional oligomeric signaling complexes. In this review we summarize our current knowledge of shared receptor recognition and activation, with a focus on gp130. We discuss recent structural and functional information to analyze overall architectural assemblies of gp130 cytokine complexes and probe the basis for the extreme cross-reactivity of gp130 for its multiple cytokine ligands.

Sugar coating extends half-lives and improves effectiveness of cytokine hormones

Recombinant human erythropoietin (rhEPO) is an effective and widely used therapeutic agent that is produced by bioengineering. Modification of the rhEPO protein by glycoengineering increased its already abundant N-glycosylation, which enhances its erythropoietic activity in vivo by decreasing its metabolic clearance. Elliott et al. recently reported increased in vivo activities of thrombopoietin (Mpl ligand) and leptin following carbohydrate addition to both, which suggests that such glycoengineering could be applied to a variety of hormones, cytokines and growth factors.

Site2 binding energetics of the regulatory step of growth hormone-induced receptor homodimerization

Receptor signaling in the growth hormone (GH)-growth hormone receptor (GHR) system is controlled through a sequential two-step hormone-induced dimerization of two copies of the extracellular domain (ECD) of the receptor. The regulatory step of this process is the binding of the second ECD (ECD2) to the stable preassociated 1 : 1 GH/ECD1 complex on the cell surface. To determine the energetics that governs this step, the binding kinetics of 38 single- and double-alanine mutants in the hGH Site2 contact with ECD2 were measured by using trimolecular surface plasmon resonance (TM-SPR). We find that the Site2 interface of hGH does not have a distinct binding hot-spot region, and the most important residues are not spatially clustered, but rather are distributed over the whole binding surface. In addition, it was determined through analysis of a set of pairwise double alanine mutations that there is a significant degree of negative cooperativity among Site2 residues. Residues that show little effect or even improved binding on substitution with alanine, when paired with D116A-hGH, display significant negative cooperativity. Because most of these pairwise mutated residues are spatially separated by >or=10 A, this indicates that the Site2 binding interface of the hGH-hGHR ternary complex displays both structural and energetic malleability.

A structural template for gp130-cytokine signaling assemblies

The gp130-cytokine system has been fertile ground for protein structure-function studies aimed at elucidating the basis of ligand recognition and receptor activation. A number of longstanding questions involve the mechanism of the stepwise assembly of the active signaling complexes, as well as the structure of the gp130-cytokine complexes. It has been clear from functional studies that the paradigm of gp130-cyokine recognition will differ substantially from the classical homo-dimeric systems, typified by human growth hormone (hGH) and its receptor. Recently, a crystal structure of a viral interleukin-6 (vIL-6), complexed with the D1D2D3 domains of the gp130 extracellular domain, has resolved many of these questions, and reconciled much of the functional and mutagenesis data which have existed for a variety of gp130-cytokines. In this review, we discuss the structure of the vIL-6/gp130 complex in some detail and suggest that the geometry of this complex will be a common structural template utilized by other gp130-cytokines, as well as cytokines from distinct signaling systems.

Sequence conservation in families whose members have little or no sequence similarity: the four-helical cytokines and cytochromes

Proteins for which there are good structural, functional and genetic similarities that imply a common evolutionary origin, can have sequences whose similarities are low or undetectable by conventional sequence comparison procedures. Do these proteins have sequence conservation beyond the simple conservation of hydrophobic and hydrophilic character at specific sites and if they do what is its nature? To answer these questions we have analysed the structures and sequences of two superfamilies: the four-helical cytokines and cytochromes c'-b(562). Members of these superfamilies have sequence similarities that are either very low or not detectable. The cytokine superfamily has within it a long chain family and a short chain family. The sequences of known representative structures of the two families were aligned using structural information. From these alignments we identified the regions that conserve the same main-chain conformation: the common core (CC). For members of the same family, the CC comprises some 50% of the individual structures; for the combination of both families it is 30%. We added homologous sequences to the structural alignment. Analysis of the residues occurring at sites within the CCs showed that 30% have little or no conservation, whereas about 40% conserve the polar/neutral or hydrophobic/neutral character of their residues. The remaining 30% conserve hydrophobic residues with strong or medium limitations on their volume variations. Almost all of these residues are found at sites that form the "buried spine" of each helix (at sites i, i+3, i+7, i+10, etc., or i, i+4, i+7, i+11, etc.) and they pack together at the centre of each structure to give a pattern of residue-residue contacts that is almost absolutely conserved. These CC conserved hydrophobic residues form only 10-15% of all the residues in the individual structures.A similar analysis of the cytochromes c'-b(562), which bind haem and have a very different function to that of the cytokines, gave very similar results. Again some 30% of the CC residues have hydrophobic residues with strong or medium conservation. Most of these form the buried spine of each helix and play the same role as those in the cytokines. The others, and some spine residues bind the haem co-factor.

Molecular evolution of growth hormone (GH) in Cetartiodactyla: cloning and characterization of the gene encoding GH from a primitive ruminant, the chevrotain (Tragulus javanicus)

In mammals the sequence of pituitary growth hormone (GH) is generally strongly conserved, indicating a slow basal rate of molecular evolution. However, on two occasions, during the evolution of primates and that of cetartiodactyls, the rate of evolution has increased dramatically (25 to 50-fold) so that the sequences of human and ruminant GHs differ markedly from those of other mammalian GHs. To define further the burst of GH evolution that occurred in cetartiodactyls, the GH gene of the chevrotain (Tragulus javanicus) has been cloned and characterized by use of genomic DNA and a polymerase chain reaction technique. Two very similar gene sequences, which probably reflect allelic variation, were isolated. The deduced sequence for the mature chevrotain GH differs from that of the bovine or red deer GH at only two to three residues, and phylogenetic analysis shows that the burst of rapid evolution of GH that occurred in the Cetartiodactyla must have been completed before the divergence of the Tragulidae and the advanced ruminants (Pecora). The rate of evolution during this burst must therefore have been greater than previously estimated. In other aspects (including signal sequence, 5' upstream sequence, and synonymous substitutions in the coding sequence), the chevrotain GH gene differs considerably from the GH genes of other ruminants and here there is no evidence for the period of accelerated evolution that is seen for GH itself.

Signaling by type I and II cytokine receptors: ten years after

Discovered during the past ten years, Janus kinases and signal transducers and activators of transcription have emerged as critical elements in cytokine signaling and immunoregulation. Recently, knockout mice for all the members of these families have been generated, with remarkably specific outcomes. Equally exciting is the discovery of a new class of inhibitors, the suppressor of cytokine signaling family. The phenotypes of mice deficient in these molecules are also striking, underscoring the importance of negative regulation in cytokine signaling.

Crystal structure of human stem cell factor: implication for stem cell factor receptor dimerization and activation

Stem cell factor (SCF) plays important roles in hematopoiesis and the survival, proliferation, and differentiation of mast cells, melanocytes, and germ cells. SCF mediates its biological effects by binding to and activating a receptor tyrosine kinase designated c-kit or SCF receptor. In this report we describe the 2.3-A crystal structure of the functional core of recombinant human SCF. SCF is a noncovalent homodimer composed of two slightly wedged protomers. Each SCF protomer exhibits an antiparallel four-helix bundle fold. Dimerization is mediated by extensive polar and nonpolar interactions between the two protomers with a large buried surface area. Finally, we have identified a hydrophobic crevice and a charged region at the tail of each protomer that functions as a potential receptor-binding site. On the basis of these observations, a model for SCF small middle dotc-kit complex formation and dimerization is proposed.