title: The Bcr kinase downregulates Ras signaling by phosphorylating AF-6 and binding to its PDZ domain.
abstract: The protein kinase Bcr is a negative regulator of cell proliferation and oncogenic transformation. We identified Bcr as a ligand for the PDZ domain of the cell junction and Ras-interacting protein AF-6. The Bcr kinase phosphorylates AF-6, which subsequently allows efficient binding of Bcr to AF-6, showing that the Bcr kinase is a regulator of the PDZ domain-ligand interaction. Bcr and AF-6 colocalize in epithelial cells at the plasma membrane. In addition, Bcr, AF-6, and Ras form a trimeric complex. Bcr increases the affinity of AF-6 to Ras, and a mutant of AF-6 that lacks a specific phosphorylation site for Bcr shows a reduced binding to Ras. Wild-type Bcr, but not Bcr mutants defective in binding to AF-6, interferes with the Ras-dependent stimulation of the Raf/MEK/ERK pathway. Since AF-6 binds to Bcr via its PDZ domain and to Ras via its Ras-binding domain, we propose that AF-6 functions as a scaffold-like protein that links Bcr and Ras to cellular junctions. We suggest that this trimeric complex is involved in downregulation of Ras-mediated signaling at sites of cell-cell contact to maintain cells in a nonproliferating state.
authors.affiliation: Institute of Medical Virology, University of Zurich, CH-8028 Zurich, Switzerland.
authors.forename: G
authors.initials: G
authors.lastname: Radziwill
authors.forename: R A
authors.initials: RA
authors.lastname: Erdmann
authors.forename: U
authors.initials: U
authors.lastname: Margelisch
authors.forename: K
authors.initials: K
authors.lastname: Moelling
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002455:Cell Division; D002460:Cell Line; D015536:Down-Regulation; D004789:Enzyme Activation; D005982:Glutathione Transferase / Q000378:metabolism; D006801:Humans; D015151:Immunoblotting; D016547:Kinesins / Q000378:metabolism*; D008024:Ligands; D008856:Microscopy, Fluorescence; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D008954:Models, Biological; D008957:Models, Genetic; D008969:Molecular Sequence Data; D009218:Myosins / Q000378:metabolism*; D015513:Oncogene Proteins / Q000378:metabolism*; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D011233:Precipitin Tests; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011505:Protein-Tyrosine Kinases; D011518:Proto-Oncogene Proteins; D051562:Proto-Oncogene Proteins c-bcr; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D014162:Transfection; D018631:ras Proteins / Q000378:metabolism*
chemical_list: C095516:AFDN protein, human; D008024:Ligands; D015513:Oncogene Proteins; D011518:Proto-Oncogene Proteins; D011993:Recombinant Fusion Proteins; D005982:Glutathione Transferase; D011505:Protein-Tyrosine Kinases; C496613:BCR protein, human; D051562:Proto-Oncogene Proteins c-bcr; D020928:Mitogen-Activated Protein Kinases; D009218:Myosins; D016547:Kinesins; D018631:ras Proteins