title: An amphipathic motif at the transmembrane-cytoplasmic junction prevents autonomous activation of the thrombopoietin receptor.
abstract: Ligand binding to the thrombopoietin receptor (TpoR) is thought to impose a dimeric receptor conformation(s) leading to hematopoietic stem cell renewal, megakaryocyte differentiation, and platelet formation. Unlike other cytokine receptors, such as the erythropoietin receptor, TpoR contains an amphipathic KWQFP motif at the junction between the transmembrane (TM) and cytoplasmic domains. We show here that a mutant TpoR (delta5TpoR), where this sequence was deleted, is constitutively active. In the absence of ligand, delta5TpoR activates Jak2, Tyk2, STAT5, and mitogen-activated protein (MAP) kinase, but does not appear to induce STAT3 phosphorylation. Delta5TpoR induces hematopoietic myeloid differentiation in the absence of Tpo. In the presence of Tpo, the delta5TpoR mutant appears to enhance erythroid differentiation when compared with the Tpo-activated wild-type TpoR. Strikingly, individual substitution of K507 or W508 to alanine also induces constitutive TpoR activation, indicating that the K and W residues within the amphipathic KWQFP motif are crucial for maintaining the unliganded receptor inactive. These residues may be targets for activating mutations in humans. Such a motif may exist in other receptors to prevent ligand-independent activation and to allow signaling via multiple flexible interfaces.
authors.affiliation: Ludwig Institute for Cancer Research, Université catholique de Louvain, Avenue Hippocrate 74, UCL 75-4, 1200 Brussels, Belgium.
authors.forename: Judith
authors.initials: J
authors.lastname: Staerk
authors.forename: Catherine
authors.initials: C
authors.lastname: Lacout
authors.forename: Takeshi
authors.initials: T
authors.lastname: Sato
authors.forename: Steven O
authors.initials: SO
authors.lastname: Smith
authors.forename: William
authors.initials: W
authors.lastname: Vainchenker
authors.forename: Stefan N
authors.initials: SN
authors.lastname: Constantinescu
mesh_terms: D020816:Amino Acid Motifs / Q000235:genetics; D019943:Amino Acid Substitution; D000818:Animals; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology*; D002460:Cell Line; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism; D006412:Hematopoietic Stem Cells / Q000166:cytology / Q000502:physiology*; D006801:Humans; D020935:MAP Kinase Signaling System / Q000187:drug effects / Q000502:physiology*; D008533:Megakaryocytes / Q000166:cytology / Q000502:physiology; D051379:Mice; D015513:Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D017354:Point Mutation; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D018121:Receptors, Cytokine / Q000235:genetics / Q000378:metabolism*; D053628:Receptors, Thrombopoietin; D017384:Sequence Deletion / Q000235:genetics; D034061:Thrombopoiesis / Q000187:drug effects / Q000502:physiology*; D013926:Thrombopoietin / Q000378:metabolism / Q000494:pharmacology
chemical_list: C481270:Mpl protein, mouse; D015513:Oncogene Proteins; D011518:Proto-Oncogene Proteins; D018121:Receptors, Cytokine; D053628:Receptors, Thrombopoietin; C082218:MPL protein, human; D013926:Thrombopoietin; D011505:Protein-Tyrosine Kinases