Crystal Structure of Interleukin-21 Receptor (IL-21R) Bound to IL-21 Reveals That Sugar Chain Interacting with WSXWS Motif Is Integral Part of IL-21R

Abstract

Background:The class I cytokine IL-21 exerts pleiotropic effects on innate and adaptive immunity. Results: We obtained the crystal structure of the partially glycosylated IL-21 receptor (IL-21R) bound to IL-21. Conclusion: A sugar chain is an integral part of IL-21R. Significance: This structure offers an insight into the putative role of the class I cytokine receptor signature motif. IL-21 is a class I cytokine that exerts pleiotropic effects on both innate and adaptive immune responses. It signals through a heterodimeric receptor complex consisting of the IL-21 receptor (IL-21R) and the common ␥-chain. A hallmark of the class I cytokine receptors is the class I cytokine receptor signature motif (WSXWS). The exact role of this motif has not been determined yet; however, it has been implicated in diverse functions, including ligand binding, receptor internalization, proper folding, and export, as well as signal transduction. Furthermore, the WXXW motif is known to be a consensus sequence for C-mannosylation. Here, we present the crystal structure of IL-21 bound to IL-21R and reveal that the WSXWS motif of IL-21R is C-mannosylated at the first tryptophan. We furthermore demonstrate that a sugar chain bridges the two fibronectin domains that constitute the extracellular domain of IL-21R and anchors at the WSXWS motif through an extensive hydrogen bonding network, including mannosylation. The glycan thus transforms the V-shaped receptor into an A-frame. This finding offers a novel structural explanation of the role of the class I cytokine signature motif.IL-21 is a class I cytokine with a four-helix bundle structure arranged in an up-up-down-down topology typical for the class I cytokines (1). It exerts pleiotropic effects on both innate and adaptive immune responses. IL-21 is secreted by activated CD4 ϩ T cells, in particular T H 17 and T follicular helper cells, as well as natural killer cells (2). Not only do both T H 17 and T follicular helper cells produce IL-21, but this cytokine also plays an important role in promoting the development of T H 17 and T follicular helper cells by a feed-forward mechanism (3-8). Furthermore, IL-21 cooperates with other cytokines to increase the cytotoxicity of CD8 ϩ T cells and promotes proliferation of CD8 ϩ cells in the presence of antigens (9). IL-21 also influences antibody production by B cells (10). Recent studies demonstrated that IL-21 produced by CD4 ϩ cells is critical for the ability of CD8 ϩ T cells to control viral infection (11-13). The ability of IL-21 to augment immunity has spurred substantial interest in the therapeutic use of IL-21, and it is currently being evaluated in a number of clinical trials against, for example, metastatic melanoma and renal cancer (14).IL-21 signals through a heterodimeric receptor complex consisting of the private chain IL-21 receptor (IL-21R) 2 and the common ␥-chain (␥C), the latter being shared by IL-2, IL-4, . Upon binding of IL-21 to the receptor complex and subsequent receptor activation, signaling occurs through the Jak-STAT ...

Structural insights into the common γ‐chain family of cytokines and receptors from the interleukin‐7 pathway

“…Complex structures of the γ c interleukins and their receptors have been solved: IL‐2 bound to its α‐receptor, IL‐2Rα, its β‐receptor, IL‐2Rβ, and γ c ; IL‐4 bound to its α‐receptor (IL‐4Rα) ; IL‐4 bound to IL‐4Rα and γ c ; IL‐7 bound its α‐receptor, IL‐7Rα ; IL‐15 bound to its α‐receptor, IL‐15Rα ; and IL‐21 bound to its α‐receptor, IL‐21Rα ( Fig. ).…”

“…IL‐7 and IL‐7Rα exhibit structural features similar to the other γ c interleukins and their receptors, and IL‐7 interacts with IL‐7Rα using the same secondary structures used by other γ c interleukins and their receptors. Both IL‐7 and IL‐7Rα, like the other γ c interleukins and their receptors, are glycoproteins composed of numerous asparagines that can be attached to N‐linked glycans, or serines/threonines attached to O‐linked glycans, or the first tryptophan of the WSXWS sequence motif attached to a C‐mannose . Also similar to other γ c IL‐specific receptors , IL‐7Rα self‐associates to form homodimers incapable of signaling .…”

Distinct C- mannosylation of netrin receptor thrombospondin type 1 repeats by mammalian DPY19L1 and DPY19L3

“…The CHO cell knockouts generated here will now allow the study of the effects of C-mannosylation on various proteins. In addition to TSR-containing proteins, another important class of proteins that are C-mannosylated are the type I cytokine receptors (39,40). Many other unrelated proteins have WxxW or WxxC consensus sequences (36), suggesting a possible more widespread role for C-mannosylation.…”

Adenovirus-mediated interleukin 21 gene transfer enhances antitumor immunity and reduces tumorigenicity of Hepa1–6 in mice

“…It effectively enhances T cell proliferation and the killing function of NK cells, resulting in a strong antitumor immune response (4,5). Its receptor (IL-21R) is widely expressed in various cell types within the immune system, including NK cells, B cells, T cells, macrophages and dendritic cells (6–8). The widespread lymphoid distribution of the IL-21R leads to pleiotropic action of IL-21 in the innate and adaptive immune responses (6–8).…”

“…Its receptor (IL-21R) is widely expressed in various cell types within the immune system, including NK cells, B cells, T cells, macrophages and dendritic cells (6–8). The widespread lymphoid distribution of the IL-21R leads to pleiotropic action of IL-21 in the innate and adaptive immune responses (6–8). For this reason, researchers are becoming increasingly interested in IL-21.…”