title: Evidence that downregulation of the M-CSF receptor is not dependent upon receptor kinase activity.
abstract: The downregulation of tyrosine kinase receptors attenuates signalling and is thought to be dependent upon intrinsic receptor kinase activity, largely because down-regulation is inhibited by a kinase-inactivating mutation of an invariant lysine residue of the receptors for EGF, insulin, M-CSF and PDGF. We confirmed that this mutation inhibited the degradation of the M-CSF receptor. However, two different kinase inactivating mutations of the invariant amino acids Gly 591 and Glu 633 did not prevent M-CSF-induced receptor degradation, so demonstrating that receptor kinase activity is not essential for this process. Three other kinase-inactivating mutations were found to cause constitutive receptor degradation in the absence of M-CSF, most probably by disrupting the structure of the activating loop of the kinase domain. It is known that extensive movement of the A-loop is necessary for kinase activation and is normally induced by ligand-binding. It is therefore suggested that some aspect or consequence of the change in structure of the A-loop caused by ligand binding also activates receptor downregulation, so ensuring that downregulation is coupled to but is not necessarily dependent upon receptor kinase activity.
authors.affiliation: Cell Signalling Unit, Division of Paediatrics, Obstetrics and Gynaecology, Imperial College School of Medicine, Hammersmith Hospital, London, UK.
authors.forename: M
authors.initials: M
authors.lastname: Uden
authors.forename: G M
authors.initials: GM
authors.lastname: Morley
authors.forename: N J
authors.initials: NJ
authors.lastname: Dibb
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D002460:Cell Line; D015536:Down-Regulation; D005347:Fibroblasts / Q000201:enzymology; D006801:Humans; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D051381:Rats; D020794:Receptor Protein-Tyrosine Kinases / Q000096:biosynthesis* / Q000235:genetics; D011972:Receptor, Insulin / Q000737:chemistry / Q000378:metabolism; D016186:Receptor, Macrophage Colony-Stimulating Factor / Q000096:biosynthesis* / Q000235:genetics; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology*; D013329:Structure-Activity Relationship
chemical_list: D020794:Receptor Protein-Tyrosine Kinases; D011972:Receptor, Insulin; D016186:Receptor, Macrophage Colony-Stimulating Factor