Crystal structure of the IL-2 signaling complex: paradigm for a heterotrimeric cytokine receptor

Abstract

IL-2 is a cytokine that functions as a growth factor and central regulator in the immune system and mediates its effects through ligand-induced hetero-trimerization of the receptor subunits IL-2R alpha, IL-2R beta, and gamma(c). Here, we describe the crystal structure of the trimeric assembly of the human IL-2 receptor ectodomains in complex with IL-2 at 3.0 A resolution. The quaternary structure is consistent with a stepwise assembly from IL-2/IL-2R alpha to IL-2/IL-2R alpha/IL-2R beta to IL-2/IL-2R alpha/IL-2R beta/gamma(c). The IL-2R alpha subunit forms the largest of the three IL-2/IL-2R interfaces, which, together with the high abundance of charge-charge interactions, correlates well with the rapid association rate and high-affinity interaction of IL-2R alpha with IL-2 at the cell surface. Surprisingly, IL-2R alpha makes no contacts with IL-2R beta or gamma(c), and only minor changes are observed in the IL-2 structure in response to receptor binding. These findings support the principal role of IL-2R alpha to deliver IL-2 to the signaling complex and act as regulator of signal transduction. Cooperativity in assembly of the final quaternary complex is easily explained by the extraordinarily extensive set of interfaces found within the fully assembled IL-2 signaling complex, which nearly span the entire length of the IL-2R beta and gamma(c) subunits. Helix A of IL-2 wedges tightly between IL-2R beta and gamma(c) to form a three-way junction that coalesces into a composite binding site for the final gamma(c) recruitment. The IL-2/gamma(c) interface itself exhibits the smallest buried surface and the fewest hydrogen bonds in the complex, which is consistent with its promiscuous use in other cytokine receptor complexes.

Fig. 1.

Architecture of the human IL-2R signaling complex. Shown are side (A) and top (B) views of the quaternary IL-2 signaling assembly. IL-2 binds to the elbow regions of IL-2Rβ and γ_c. IL-2Rα docks on top of this assembly without forming any contacts with the other two receptor subunits. Six N-linked carbohydrates (S1 to S6) are displayed as ball-and-stick models. S1 is wedged between D1 and D2 of IL-2Rβ and, thus, contributes to the stabilization of a specific D1/D2 interdomain angle.

Fig. 2.

Interface between IL-2Rα and IL-2. Open-book representation of the IL-2/IL-2Rα interface. The electrostatic potential was mapped onto the molecular surface and contoured at ±35kT/eV (blue/red). The interface features a hydrophobic center, flanked by a large number of salt bridges and other polar contacts. The strong electrostatic component of this interaction serves to rapidly capture IL-2 and, thus, to dominate the k_on rate of IL-2 binding to the IL-2R.

Fig. 3.

Interface of IL-2Rβ with IL-2 and γ_c. The interface of IL-2Rβ (cyan/red) and IL-2 (yellow/red) is intermediate in buried surface area among the three IL-2/IL-2R interfaces. Asp-20 and His-16 of IL-2 are located at the center of the binding region and are involved in many van der Waals contacts and hydrogen bonds. The interface features the highest number of hydrogen bonds, consistent with its high specificity and low k_off rate of IL-2 dissociation. The large contact area with γ_c is colored brown and includes two residues (red) that also interact with IL-2 (red residues).

Fig. 4.

Interface of γ_c with IL-2 and IL-2Rβ. This γ_c interface (green/red) with IL-2 (yellow/red) is characterized by the smallest total buried surface area and formation of only two hydrogen bonds. Strikingly, the elbow region of γ_c engages only partially with IL-2. Furthermore, γ_c exhibits a large elbow angle, which is atypical of class-I cytokine receptors. These structural properties are likely to contribute to promiscuity of this receptor subunit. The large contact area with IL-2Rβ is colored brown and includes three residues (red) which also interact with IL-2 (red residues).

Fig. 5.

Three-way junction between IL-2, IL-2Rβ, and γ_c. IL-2 (yellow ribbon representation), IL-2Rβ, and γ_c (the surfaces are colored as in Figs. 3 and 4) form a three-way junction at the heart of the high-affinity IL-2 signaling complex. The network of residues that mediate these contacts (colored red) provides a compelling structural basis for cooperativity in the IL-2/IL-2R complex assembly.