Characterization of the murine Mpl proto-oncogene, a member of the hematopoietic cytokine receptor family: molecular cloning, chromosomal location and evidence for a function in cell growth.

The v-mpl oncogene transduced in the myeloproliferative leukemia virus (MPLV) encodes a truncated form of a putative receptor protein that belongs to the cytokine receptor superfamily. We previously reported the cloning of complete human c-MPL cDNA. In the present report, we show that the murine Mpl proto-oncogene is located at the D-band of murine chromosome 4, in a region in synteny with human chromosome 1p34, where MPL was previously located. RNA blot analysis of murine hematopoietic tissues and cells lines indicated that Mpl is expressed in immature hematopoietic precursor cells. Molecular cloning of murine proto-oncogene c-Mpl cDNAs is also reported. Two cDNA species were isolated. One potentially encodes a transmembrane protein. The extracellular domain of this protein has two repeats of the cytokine receptor domain common to all members of this receptor family. The cytoplasmic domain has no protein kinase or phosphatase motifs, but does contain a sequence that has been shown to be essential for the transmission of a growth signal in several other members of the family. Comparison of murine and human putative proteins indicated that they shared 81% amino acid identity, the most conserved region being the cytoplasmic domain (91% identity). The other Mpl cDNA clones potentially encode a soluble form of this receptor chain. A chimeric receptor containing the extracellular domain of the granulocyte colony-stimulating factor (G-CSF) receptor fused to the transmembrane and cytoplasmic domains of Mpl was able to induce G-CSF responsiveness when transfected into the interleukin 3 (IL-3)-dependent cell line BAF/BO3. This demonstrated that the cytoplasmic Mpl domain is most probably implicated in proliferative signal transduction.