The Structure of the Interleukin-15α Receptor and Its Implications for Ligand Binding

Inken Lorenzen,Andrew J Dingley,Yannick Jacques,Joachim Grötzinger

doi:10.1074/jbc.m513118200

Inken Lorenzen, Andrew J Dingley + Show 2 more

Open Access

https://doi.org/10.1074/jbc.m513118200

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Abstract

Interleukin (IL)-15 is a member of the small four alpha-helix bundle family of cytokines. IL-15 was discovered by its ability to mimic IL-2-mediated T-cell proliferation. Both cytokines share the beta and gamma receptor chains of the IL-2 receptor for signal transduction. However, in addition, they target specific alpha chain receptors IL-15Ralpha and IL-2Ralpha, respectively. The exceptionally high affinity binding of IL-15 to IL-15Ralpha is mediated by its sushi domain. Here we present the solution structure of the IL-15Ralpha sushi domain solved by NMR spectroscopy and a model of its complex with IL-15. The model shows that, rather than the familiar hydrophobic forces dominating the interaction interface between cytokines and their cognate receptors, the interaction between the IL-15 and IL-15Ralpha complex involves a large network of ionic interactions. This type of interaction explains the exceptionally high affinity of the IL-15.IL-15Ralpha complex, which is essential for the biological effects of this important cytokine and which is not observed in other cytokine/cytokine receptor complexes.

Highlights

Both IL-2R␣ and IL-15R␣ use their sushi domains for ligand binding but with remarkably different affinities
IL-2 and IL-15 differ in their affinity for their specific ␣ receptors and in their tissue distribution
IL-2 is exclusively expressed by activated T-cells, whereas IL-15 has been described to be expressed in nonimmune and immune cells [6, 11]

Summary

EXPERIMENTAL PROCEDURES

Protein Expression, Purification, and Sample Preparation—The mature coding sequence of IL-15R␣ was originally cloned in pGEX2T (Amersham Biosciences). This clone was used as a template for amplification of the IL-15R␣ sushi domain coding region by PCR. The ratio of the intensity of the water and the corresponding 1H/15N diagonal peak (Ͻ0.1) in the 15N-edited NOE spectroscopy with water flipback is an indication of slow amide proton exchange. These amide groups are potential proton donors involved in hydrogen bonding. The programs RIBBONS [45] and GRASP [46] were used

RESULTS AND DISCUSSION

Ramachandran plotb

Binding constants of cytokines to their cognate receptors