CHARACTERIZATION OF THE TRUNCATED THROMBOPOIETIN VARIANTS

Thrombopoietin (Tpo) is a specific cytokine which regulates megakaryoc yte differentiation and maturation. We isolated a truncated mouse Tpo cDNA, the product of which turned out to function neither as an active Tpo variant nor as an antagonist. To define the functional domains of the Tpo molecule further, various truncated and point-mutated Tpo mol ecules were prepared and their biological activity was assayed. It was found that deletion of the amino terminal side of a potential proteol ytic cleavage site, Arg-Arg motif, caused complete loss of Tpo's activ ity, and that point-mutants lacking one of four conserved cysteine res idues lost Tpo activity. We also noticed that Tpo activity was inhibit ed by the reducing agent. Thus, it was concluded that the amino termin al half of the Tpo is sufficient for Tpo activity, and that the cystei ne residues, especially the last cysteine residue located two amino ac ids away from the Arg-Arg motif, are critical for this activity. (C) 1 995 Academic Press, Inc.