Identification of amino acids in transmembrane domains of mutated cytokine receptor-like factor 2 and interleukin-7 receptor α required for constitutive signal transduction.

Cytokine receptor-like factor 2 (CRLF2) and interleukin-7 receptor α (IL-7Rα) form a receptor for thymic stromal lymphopoietin (TSLP). A somatic mutation consisting of the substitution of five amino acids (SLLLL) in the transmembrane domain of CRLF2 with three amino acids, including glutamic acid, isoleucine, and methionine (insEIM), which has been identified in acute lymphocytic leukemia, causes the TSLP-independent dimerization with IL-7Rα and activation. However, the dimerization mechanism remains unclear. In this study, we examined the involvement of the amino acids in the transmembrane domains of EIM CRLF2 and IL-7Rα in TSLP-independent activation. HEK293 cells were transfected with vectors encoding CRLF2 and IL-7Rα, or their mutants, in which the amino acid of the transmembrane domain was replaced with alanine. STAT5 phosphorylation was detected using western blotting, and receptor dimerization was analyzed using the NanoBiT assay. The substitution of glutamic acid within the insEIM mutation for alanine failed to cause the STAT5 phosphorylation in the absence of TSLP. Moreover, the alanine substation of the specific leucine residues in the transmembrane domains of both CRLF2 and IL-7Rα abrogated the TSLP-independent signal transduction and dimerization. The mutation of IL-7Rα W264 partially reduced the phosphorylation of STAT5 without affecting receptor dimerization. These results suggest that the amino acids in the transmembrane domains of EIM CRLF2 and IL-7Rα play at least three possible functions: interaction through hydrogen bonds, hydrophobic interaction, and signal transduction. Our findings contribute to a better understanding of the function of the transmembrane domains of cytokine receptors in their dimerization and signal transduction.

Effect of N-glycosylation on constitutive signal transduction by mutated cytokine receptor-like factor 2.

BACKGROUND: Cytokine receptor-like factor 2 (CRLF2) is a subunit of the receptor for thymic stromal lymphopoietin (TSLP). A somatic mutation (insEIM) in the transmembrane domains of CRLF2 has been identified in acute lymphocytic leukemia (ALL), and Glu-Ile-Met (EIM) CRLF2 induces constitutive activation of signals. However, the signaling mechanism remains unclear. METHODS: HEK293 cells were transfected with expression vectors encoding wild-type (WT), insEIM CRLF2, or their mutants which N-glycosylation site was replaced with a glutamine. Cell surface expression of CRLF2 was assessed by flow cytometry. Total CRLF2 and phosphorylated signal transducer and activator of transcription 5 (STAT5) were detected by western blotting. RESULTS: Three major species of CRLF2 (53-, 57- and 58-kDa) were identified. Deglycosylation analysis revealed that they were modified with complex-type and oligomannose-type glycans. The expression of both WT and EIM CRLF2 decreased in N-acetylglucosaminyltransferase (GnT)-I (MGAT1) knockout (KO) cells and slightly decreased in α1,6-fucosyltransferase (Fut8) KO cells compared to that in the control cells. In GnT-I or Fut8 KO cells, WT CRLF2 did not induce ligand-independent activation. Both WT and EIM CRLF2 contained four N-glycosylation sites. N55 of CRLF2 was required for the cell surface expression and activation by EIM CRLF2. CONCLUSIONS: We found that N-glycosylation of CRLF2 plays crucial roles for its cell surface expression and signaling. However, N-glycan processing in the Golgi apparatus does not seem to be essential for ligand-independent activation of EIM CRLF2. GENERAL SIGNIFICANCE: Our studies provide a crucial role of glycosylation in the cell surface expression of receptors.