Structural biology of shared cytokine receptors

Abstract

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.

Figure 1

Diversity of shared cytokine-receptor interactions. Shared cytokine receptors γ_c (a), gp130 (b), and β_c (c) are represented schematically on a cell membrane. The respective interacting cytokines with known three-dimensional structures are shown with cylinder representations of the four-helix bundles. (Abbreviations: LIF, leukemia inhibitory factor; OSM, oncostatin-M; CNTF, cillary neurotrophic factor; CLC, cardiotrophin-like cytokine; CHR, cytokine-binding homology region; HHV-8, human herpes virus; GM-CSF, granulocyte-macrophage colony-stimulating factor.)

Figure 2

Diversity of shared receptor-receptor interactions. Shared cytokine receptors β_c (a), gp130 (b), and γ_c (c) and their various receptor partners are depicted. These complexes are formed by the combination of ligand-specific α and/or β receptors with shared cytokine receptors. (Abbreviations: TCCR, T cell cytokine receptor; TSLPR, thymic stromal-derived lymphopoietin receptor.)

Figure 3

Structures of human IL-6/IL-6Rα/gp130 hexameric complex (a), mouse LIFR in complex with human LIF (b), and human CNTF/CNTFRα/LIFR/gp130 (c) assembled from known crystallographic, biochemical, and electron microscopic data. In (a) the model shown was derived from the crystal structure of the IL-6 hexamer headpiece (12) together with the single-particle reconstruction of the entire extracellular complex (72). IL-6 signaling is mediated through homodimerization of gp130 in a symmetric hexameric arrangement with a nonsignaling IL-6Rα receptor. In (b) the model shown derives from the 4 Å crystal structure of the LIF/LIFR complex (14) missing the membrane-proximal domains that are depicted as cartoons. In (c), the quaternary LIFR/gp130/CNTF/CNTFRα complex is derived from a combination of the crystal structures of LIF/gp130 (13), CNTF (163), and a single-particle reconstruction of the entire quaternary complex (76). CNTF signals through the asymmetric heterodimerization of gp130 and LIFR and the nonsignaling CNTFα receptor. In panel (d), the assembly pathway for IL-6 signaling is depicted as elucidated from References , . IL-6 first engages IL-6Rα through a site I interaction to form a composite interface (site II) that recruits gp130. This trimeric structure can then engage a second trimer through two site III interfaces to form a productive signaling complex.

Figure 4

The 2:2:2 GM-CSF/GMRα/β_c complex viewed from the side (a) and from the top (b) (18). GMRα engages the cytokine GM-CSF via a canonical site I interaction, whereas the β_c receptor engages site II on GM-CSF by using a composite cytokine-binding homology region (CHR) interface generated by domain 1 (D1) of one β_c subunit and domain 4 (D4) of the second β_c.

Figure 5

Extracellular complex structures of IL-2/IL-2Rα/IL-2Rβ/γ_c (a) (15), IL-4/IL-4R α/γ_c (b), IL-4/IL-4Rα/IL-13Rα1 (c), and IL-13/IL-4Rα/IL-13Rα1 (d) (17) extracellular signaling complexes depicted on a cell membrane.

Figure 6

IL-2/IL-2Rα and IL-15/IL-15Rα binary complexes. (a) IL-2Rα is composed of two sushi modules (D1 and D2) that swap opposing β-strands, forming a noncanonical sushi fold topology (53). (b) IL-2Rα and IL-15Rα contact the dorsal surface of IL-2 and IL-15, respectively, with respect to the membrane (130, 131). (c) IL-2 quaternary complex and modeled IL-15 quaternary complexes. IL-2Rα presents IL-2 in cis, whereas IL-15Rα presents IL-15 in cis or trans.

Figure 7

The sequential assembly pathways of the IL-2/IL-2Rα/IL-2Rβ/γ_c quaternary (a), IL-4/IL-4Rα/γ_c (b), IL-4/IL-4Rα/IL-13Rα1 (c), and IL-13/IL-4Rα/IL-13Rα1 (d) ternary complexes. See Table 1 for the interaction affinity and thermodynamic parameters of each binding site.

Figure 8

Specific versus degenerate binding sites I and IIa in the IL-2/IL-2Rα/IL-2Rβ/γ_c and IL-4/IL-4Rα/γ_c complexes. (a) Left panel shows the binding site I between IL-2 helices A and C and IL-2Rβ loops in the elbow region. The corresponding binding site I in the IL-4 ternary complex is shown in right panel. (b) Binding site IIa in the IL-2 quaternary complex (γ_c/IL-2) and in the IL-4 ternary complex (γ_c/IL-4) are shown in left and right panels, respectively.

Figure 9

Cross-reactivity of the cytokine-binding homology region (CHR) of γ_c and gp130. (a) Contact surface of γ_c and gp130 when bound to IL-2, IL-4, HHV-8 IL-6, IL-6, and LIF. Hydrophobic residues are colored as blue surface area and hydrophilic residues as red surface area. The helices and the contacting residues for each of the cytokines are shown docked onto the surface of the receptor. (b) Surface representations showing the contact surface of each cytokine. Note that IL-2 and IL-4 have a similar distribution of hydrophobic and hydrophilic residues in the contact area. For gp130 family cytokines, HHV-8 IL-6 has primarily hydrophobic contact surface area. The contact area on IL-6 is more polar, and LIF has the most significant hydrophilic contact surface. (c) The packing environment of the common binding epitope residues Tyr-103 (γ_c) and Phe-169 (gp130) against the cytokines.