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 lympho-poietin (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 alpha 1,6-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.