HUMAN THROMBOPOIETIN STRUCTURE-FUNCTION RELATIONSHIPS : IDENTIFICATION OF FUNCTIONALLY IMPORTANT RESIDUES

Thrombopoietin (TPO) is a haematopoietic growth factor responsible for megakaryocyte progenitor proliferation and differentiation. It belongs to the four-helix-bundle cytokine family and exerts its biological effects through binding to a specific receptor, c-Mpl. With the use of site-directed mutagenesis we have generated 20 TPO mutants. Each of the TPO mutants was produced in a eukaryotic expression system and the mutants9 ability to induce the proliferation of factor-dependent c-Mpl-expressing megakaryoblastic M-O7e cells was compared with that of wild-type TPO. Among the mutations studied, 10 lead to a significant decrease in TPO bioactivity. Of these ten residues, three are located in helix A of the protein (Arg 10 , Lys 14 and Arg 17 ) and four in helix D (His 133 , Gln 132 , Lys 138 and Phe 141 ), indicating that in TPO, as in other cytokines, these two helices are important for functional cytokine/receptor interactions. Surprisingly, mutant Arg 10 → Ala (R10A) lacked any proliferative activity, despite the fact that this mutation was recently reported to have no effect on TPO/c-Mpl binding in a TPO phage ELISA [Pearce, Potts, Presta, Bald, Fendly and Wells (1997) J. Biol. Chem. 272 , 20595–20602]. The lack of M-O7e proliferation is probably due to an inability of R10A mutant to promote receptor dimerization and thus receptor activation. Moreover we found that the Arg 10 and Arg 17 residues of TPO seem to be specific determinants for TPO/c-Mpl recognition. We also demonstrate that the O-glycosylation site located at position 110 of TPO is not necessary for the bioactivity of the cytokine.