This is the D2 domain in cytokine-binding module 1 (CBM1) found in Leukemia inhibitory factor receptor (LIFR) and OSM receptors (OSMR). LIFR has an extracellular region with a modular structure containing two cytokine-binding modules (CBM) separated by an Ig-like domain and followed by three membrane-proximal fibronectin type-III (FNIII) domains. The D2 domain in CBM1 shows structural similarity to the corresponding CBM domains of both gp130 and IL-6Ralpha because it contains conserved structural features like the WSXWS motif [1] [2]. The WSXWS motif in cytokine receptors is a molecular switch involved in receptor activation [3].

1: Structural organization of a full-length gp130/LIF-R cytokine receptor transmembrane complex. Skiniotis G, Lupardus PJ, Martick M, Walz T, Garcia KC; Mol Cell. 2008;31:737-748. PMID:18775332

2: An unusual cytokine:Ig-domain interaction revealed in the crystal structure of leukemia inhibitory factor (LIF) in complex with the LIF receptor. Huyton T, Zhang JG, Luo CS, Lou MZ, Hilton DJ, Nicola NA, Garrett TP; Proc Natl Acad Sci U S A. 2007;104:12737-12742. PMID:17652170

3: The WSXWS motif in cytokine receptors is a molecular switch involved in receptor activation: insight from structures of the prolactin receptor. Dagil R, Knudsen MJ, Olsen JG, O'Shea C, Franzmann M, Goffin V, Teilum K, Breinholt J, Kragelund BB; Structure. 2012;20:270-282. PMID:22325776