Signal transduction from the granulocyte colony-stimulating factor receptor (G-CSF-R) involves the activation of the Janus tyrosine kinase/signal transducer and activator of transcription (Jak/STAT) pathway. G-CSF induces tyrosine phosphorylation of Jak1, Jak2, STAT1, and STAT3. The membrane-proximal region of G-CSF-R is sufficient for activation of Jaks. It is still unclear how STAT proteins are activated by G-CSF-R. We investigated the possible involvement of the C-terminal region of G-CSF-R in the recruitment of STAT proteins using BAF3 cell transfectants expressing wild type (WT) G-CSF-R, C-terminal deletion mutants and tyrosine-to-phenylalanine substitution mutants. Electrophoretic mobility shift assays with STAT-binding oligonucleotides (m67) showed that activation of WT G-CSF-R induces three distinct STAT complexes, namely STAT3 homodimers, STAT1-STAT3 heterodimers, and STAT1 homodimers. However, STAT1 homodimers and STAT1- STAT3 heterodimers were predominantly formed after activation of a C- terminal deletion mutant d685, which lacks all four conserved cytoplasmic tyrosine residues, located at positions 704, 729, 744, and 764. Antiphosphotyrosine immunoblots of STAT3 immunoprecipitates showed that activation of WT G-CSF-R induced phosphorylation of STAT3. In contrast, no phosphorylation of STAT3 was observed after activation of deletion mutant d685. These findings establish that the C-terminal region of G-CSF-R plays a major role in the activation of STAT3. By using tyrosine-to-phenylalanine substitution mutants of G-CSF-R, we further show that tyrosine 704, present in a YXXQ consensus sequence shown to be essential for STAT3 binding to gp130, is not exclusively involved in the activation of STAT3 by G-CSF-R.

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