AMINO-ACIDS OF THE HUMAN GROWTH-HORMONE RECEPTOR THAT ARE REQUIRED FOR PROLIFERATION AND JAK-STAT SIGNALING

Dimerization of the GH receptor after ligand binding leads to the rapid association and tyrosine phosphorylation of the intracellular kinase, Jak2, as well as to the tyrosine phosphorylation and activation of STAT protein(s). The tyrosine phosphorylation of these and other related proteins has been shown to be required for intracellular signaling by the interferon receptors. Carboxyl-terminal truncations of the human GH receptor have been used to demonstrate that a 54-amino acid, intracellular region of the receptor is sufficient to signal cell proliferation in response to ligand binding. In this work, we examine 10 single and double amino acid mutations of this 54-amino acid region for their ability to signal the proliferation of stably transfected Ba/F3 cells, an interleukin-3 dependent pro-B cell line. The mutation of either of two proline residues or of a lysine residue abolished the GH-induced proliferation. These amino acids are located in or adjacent to a proline-rich sequence known as box 1 that is weakly conserved in other receptors of the GH/cytokine receptor family. The tyrosine phosphorylation of Jak2, as well as the activation of transcription factors (as judged by electrophoretic mobility shift assays), was also induced by GH in the transfected Ba/F3 cells. A consistent pattern of proliferation, Jak2 phosphorylation, and transcription factor activation was found for the 10 GH receptor mutants, a finding that is consistent with the hypothesis that the Jak-STAT pathway is required for the signaling of proliferation in these cells.