title: Gas6 is complexed to the soluble tyrosine kinase receptor Axl in human blood.
abstract: BACKGROUND
  The vitamin K-dependent Gas6 protein (product of growth arrest specific gene 6) binds to, and activates the TAM receptor tyrosine kinases Tyro3, Axl and Mer. It has been suggested that Gas6 and the TAM receptors are important for primary platelet functions, but Gas6 cannot be found in human platelets. However, Gas6 is present in human plasma at a concentration of around 0.2 nM, which is a thousand-fold lower than that of the homologous protein S. The Axl and Mer receptors can be cleaved close to the cell membrane, yielding soluble molecules consisting of the extracellular parts of the receptors.
  
  
  OBJECTIVE
  To investigate if soluble Axl (sAxl) is present in human serum and plasma and if Gas6 circulates in complex with sAxl.
  
  
  METHODS
  We expressed recombinant sAxl, raised antibodies and developed and validated an ELISA for Axl. Serum and plasma were analyzed using ELISAs for Gas6, Axl and sAxl-Gas6 complexes. Serum was gel filtered and fractions analyzed by the different ELISAs to determine if Gas6 in serum is free or complexed. Immunoprecipitation was used to investigate binding between Gas6 and sAxl in serum.
  
  
  RESULTS
  sAxl is present in serum and plasma at around 0.6 nM and all Gas6 is bound to sAxl. No complexes between Gas6 and the soluble forms of Mer and Tyro3 could be detected, indicating that sAxl is the physiological binder of Gas6 in human serum.
  
  
  CONCLUSIONS
  Gas6 in circulation is bound to sAxl suggesting circulating Gas6 to be inhibited and incapable of stimulating the TAM receptors.
authors.affiliation: Department of Laboratory Medicine, Division of Clinical Chemistry, University of Lund, Malmö University Hospital, Wallenberg Laboratory, Malmö, Sweden.
authors.forename: C
authors.initials: C
authors.lastname: Ekman
authors.forename: J
authors.initials: J
authors.lastname: Stenhoff
authors.forename: B
authors.initials: B
authors.lastname: Dahlbäck
mesh_terms: D002460:Cell Line; D003470:Culture Media / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D006801:Humans; D047468:Immunoprecipitation; D036341:Intercellular Signaling Peptides and Proteins / Q000097:blood*; D008297:Male; D011485:Protein Binding; D011518:Proto-Oncogene Proteins / Q000097:blood* / Q000235:genetics; D020794:Receptor Protein-Tyrosine Kinases / Q000097:blood* / Q000235:genetics; D011994:Recombinant Proteins / Q000097:blood; D014162:Transfection; D000094963:Axl Receptor Tyrosine Kinase; D000099255:Growth Arrest-Specific Protein 6
chemical_list: D003470:Culture Media; D036341:Intercellular Signaling Peptides and Proteins; D011518:Proto-Oncogene Proteins; D020794:Receptor Protein-Tyrosine Kinases; D011994:Recombinant Proteins; D000094963:Axl Receptor Tyrosine Kinase; D000099255:Growth Arrest-Specific Protein 6; C000722289:AXL protein, human