title: Structural basis for Gas6-Axl signalling.
abstract: Receptor tyrosine kinases of the Axl family are activated by the vitamin K-dependent protein Gas6. Axl signalling plays important roles in cancer, spermatogenesis, immunity, and platelet function. The crystal structure at 3.3 A resolution of a minimal human Gas6/Axl complex reveals an assembly of 2:2 stoichiometry, in which the two immunoglobulin-like domains of the Axl ectodomain are crosslinked by the first laminin G-like domain of Gas6, with no direct Axl/Axl or Gas6/Gas6 contacts. There are two distinct Gas6/Axl contacts of very different size, both featuring interactions between edge beta-strands. Structure-based mutagenesis, protein binding assays and receptor activation experiments demonstrate that both the major and minor Gas6 binding sites are required for productive transmembrane signalling. Gas6-mediated Axl dimerisation is likely to occur in two steps, with a high-affinity 1:1 Gas6/Axl complex forming first. Only the minor Gas6 binding site is highly conserved in the other Axl family receptors, Sky/Tyro3 and Mer. Specificity at the major contact is suggested to result from the segregation of charged and apolar residues to opposite faces of the newly formed beta-sheet.
authors.affiliation: Max-Planck-Institut für Biochemie, Martinsried, Germany.
authors.forename: Takako
authors.initials: T
authors.lastname: Sasaki
authors.forename: Pjotr G
authors.initials: PG
authors.lastname: Knyazev
authors.forename: Naomi J
authors.initials: NJ
authors.lastname: Clout
authors.forename: Yuri
authors.initials: Y
authors.lastname: Cheburkin
authors.forename: Walter
authors.initials: W
authors.lastname: Göhring
authors.forename: Axel
authors.initials: A
authors.lastname: Ullrich
authors.forename: Rupert
authors.initials: R
authors.lastname: Timpl
authors.forename: Erhard
authors.initials: E
authors.lastname: Hohenester
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D019281:Dimerization; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000737:chemistry*; D008969:Molecular Sequence Data; D015513:Oncogene Proteins / Q000737:chemistry*; D011487:Protein Conformation; D011518:Proto-Oncogene Proteins; D020794:Receptor Protein-Tyrosine Kinases / Q000737:chemistry*; D015398:Signal Transduction; D000094963:Axl Receptor Tyrosine Kinase; D000099255:Growth Arrest-Specific Protein 6
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D015513:Oncogene Proteins; D011518:Proto-Oncogene Proteins; D020794:Receptor Protein-Tyrosine Kinases; D000094963:Axl Receptor Tyrosine Kinase; D000099255:Growth Arrest-Specific Protein 6; C000722289:AXL protein, human