POINT MUTATIONS WITHIN A DIMER INTERFACE HOMOLOGY DOMAIN OF C-MPL INDUCE CONSTITUTIVE RECEPTOR ACTIVITY AND TUMORIGENICITY

c-Mpl, a receptor for thrombopoietin (TPO), belongs to the haemopoieti n/cytokine receptor superfamily, a group of cell surface molecules cha racterized by conserved sequence motifs within their ligand binding do mains, A recurring mechanism for the activation of haemopoietin recept ors is the formation of functional complexes by receptor subunit oligo merization, Within the growth hormone receptor, a cluster of extracell ular amino acids forms a dimer interface domain that stabilizes ligand -induced homodimers. This domain appears to be functionally conserved in the erythropoietin (EPO) receptor because substitution of cysteines for residues in the analogous region causes EPO-independent receptor activation via disulfide-linked homodimerization. This report identifi es an homologous domain within the c-Mpl receptor. The substitution of cysteine residues for specific amino acids in the dimer interface hom ology regions of c-Mpl induced constitutive receptor activity. Factor- dependent FDC-P1 and Ba/F3 cells expressing the active receptor mutant s no longer required exogenous factors and proliferated autonomously. The results imply that the normal process of TPO-stimulated Mpl activa tion occurs through receptor homodimerization and is mediated by a con served haemopoietin receptor dimer interface domain, Moreover, cells e xpressing activated mutant Mpl receptors were tumorigenic in transplan ted mice, Thus, like v-mpl, its viral counterpart, mutated forms of th e cellular mpl gene also have oncogenic potential.