DISTINCT REGIONS OF C-MPL CYTOPLASMIC DOMAIN ARE COUPLED TO THE JAK-STAT SIGNAL-TRANSDUCTION PATHWAY AND SHC PHOSPHORYLATION

c-Mpl, a member of the hematopoietic cytokine receptor family, is the receptor for thrombopoietin. To investigate signal transduction by c-M pl, a chimeric receptor, composed of the extracellular domain of human growth hormone receptor and the intracellular domain of c-Mpl, was in troduced into the interleukin 3-dependent cell line Ba/F3. In response to growth hormone, this chimeric receptor induced growth in the absen ce of interleukin 3. Deletion analysis of the 123-amino acid intracell ular domain indicated that the elements responsible for this effect ar e present within the 63 amino acids proximal to the transmembrane doma in, Mutation of the recently described box 1 motif abrogated the proli ferative response, Tyrosine phosphorylation of the tyrosine kinase JAK -2 and activation of STAT proteins were dependent on box 1 and sequenc es within 63 amino acids of the plasma membrane. STAT proteins activat ed by thrombopoietin in a megakaryocytic cell line were purified and s hown to be STAT1 and STAT3. A separate region located at the C terminu s of the c-Mpl intracellular domain nas found to be required for induc tion of Shc phosphorylation and c-fos mRNA accumulation, suggesting in volvement of the Ras signal transduction pathway. Thus, at least two d istinct regions are involved in signal transduction by the c-Mpl.