Structural reorganization of the interleukin-7 signaling complex

Abstract

We report here an unliganded receptor structure in the common gamma-chain (γ(c)) family of receptors and cytokines. The crystal structure of the unliganded form of the interleukin-7 alpha receptor (IL-7Rα) extracellular domain (ECD) at 2.15 Å resolution reveals a homodimer forming an "X" geometry looking down onto the cell surface with the C termini of the two chains separated by 110 Å and the dimer interface comprising residues critical for IL-7 binding. Further biophysical studies indicate a weak association of the IL-7Rα ECDs but a stronger association between the γ(c)/IL-7Rα ECDs, similar to previous studies of the full-length receptors on CD4(+) T cells. Based on these and previous results, we propose a molecular mechanism detailing the progression from the inactive IL-7Rα homodimer and IL-7Rα-γ(c) heterodimer to the active IL-7-IL-7Rα-γ(c) ternary complex whereby the two receptors undergo at least a 90° rotation away from the cell surface, moving the C termini of IL-7Rα and γ(c) from a distance of 110 Å to less than 30 Å at the cell surface. This molecular mechanism can be used to explain recently discovered IL-7- and γ(c)-independent gain-of-function mutations in IL-7Rα from B- and T-cell acute lymphoblastic leukemia patients. The mechanism may also be applicable to other γ(c) receptors that form inactive homodimers and heterodimers independent of their cytokines.

Fig. 1.

Crystal structure of an unliganded IL-7Rα homodimer. (A) Ribbon diagram of the IL-7Rα homodimer oriented looking down onto the cell surface. Chains A and B are colored purple and green, respectively. The six loop regions involved in the interface and ligand binding are labeled L1–L6. The disulfide bonds and the N- and O-glycans are represented as sticks and labeled. (B) Edge view of what the IL-7Rα homodimer structure would look like on a cell surface. The residues that can be N-glycosylated on IL-7Rα are displayed. Blacks lines represent unmodeled C-terminal residues. (C) Expansion of the binding interface formed in the IL-7Rα homodimer depicted as sticks or spheres. For clarity, only residues are labeled for the stick representations for the individual chains in the two views. The two insets are related by a 180° rotation around the y axis.

Fig. 2.

SPR binding studies of IL-7Rα and γ_c at 25 °C. (A) Steady-state binding curve for the weak self-association of the IL-7Rα homodimer by plotting R_max versus IL-7Rα concentration. (B) Binding kinetic sensorgrams of the association of γ_c with IL-7Rα are indicated with black lines. The sensorgrams were globally fit to a two-step conformational exchange, with two on- and off-rate constants depicted by red lines. RU, resonance unit.

Fig. 3.

Proposed signaling mechanism of the IL-7 pathway. Signaling model of IL-7Rα–IL-7–γ_c interactions indicating preassembly of the IL-7Rα homodimer and IL-7Rα–γ_c heterodimer from cell biological studies (12, 13). There are no structures of the IL-7Rα–γ_c heterodimer or the IL-7Rα–IL-7–γ_c ternary complex. The distance between the D2 domains of IL-7Rα and γ_c was an average of the γ_c structures with IL-2 [PDB ID code 2bfi (20); PDB ID code 2erj (29)] and IL-4 [PDB ID code 3bpl (23)].

Fig. 4.

Structural models of two IL-7Rα mutations found from patients with either B-ALL or T-ALL. (A) Ribbon diagram of the disulfide-linked homodimer of the IL-7Rα ECDs through an S165-to-C165 mutation from a B-ALL patient. (B) Ribbon diagram of IL-7Rα including the juxtamembrane and transmembrane regions. For clarity, only one of the IL-7Rα ECDs is shown. Insertion of three amino acids by the T-ALL mutation from patient JPSP7 likely causes extension of the transmembrane region into the extracellular face, resulting in a disulfide-linked homodimer of two receptor molecules by C225.