An amphipathic motif at the transmembrane-cytoplasmic junction prevents autonomous activation of the thrombopoietin receptor

• Published in: Blood 2006-03, Vol.107 (5), p.1864-1871
• Main Authors: Staerk, Judith, Lacout, Catherine, Sato, Takeshi, Smith, Steven O., Vainchenker, William, Constantinescu, Stefan N.
• Format: Article
• Language: English
• Subjects: Amino Acid Motifs - genetics; Amino Acid Substitution; Animals; Biological and medical sciences; Cell Differentiation - drug effects; Cell Differentiation - physiology; Cell differentiation, maturation, development, hematopoiesis; Cell Line; Cell Membrane - genetics; Cell Membrane - metabolism; Cell physiology; Fundamental and applied biological sciences. Psychology; Hematopoiesis; Hematopoietic Stem Cells - cytology; Hematopoietic Stem Cells - physiology; Humans; MAP Kinase Signaling System - drug effects; MAP Kinase Signaling System - physiology; Megakaryocytes - cytology; Megakaryocytes - physiology; Mice; Molecular and cellular biology; Oncogene Proteins - genetics; Oncogene Proteins - metabolism; Point Mutation; Protein-Tyrosine Kinases - metabolism; Proto-Oncogene Proteins - genetics; Proto-Oncogene Proteins - metabolism; Receptors, Cytokine - genetics; Receptors, Cytokine - metabolism; Receptors, Thrombopoietin; Sequence Deletion - genetics; Thrombopoiesis - drug effects; Thrombopoiesis - physiology; Thrombopoietin - metabolism; Thrombopoietin - pharmacology
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2005-06-2600

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 (Δ5TpoR), where this sequence was deleted, is constitutively active. In the absence of ligand, Δ5TpoR activates Jak2, Tyk2, STAT5, and mitogen-activated protein (MAP) kinase, but does not appear to induce STAT3 phosphorylation. Δ5TpoR induces hematopoietic myeloid differentiation in the absence of Tpo. In the presence of Tpo, the Δ5TpoR 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.