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The Transforming Receptor Tyrosine Kinase, Axl, Is Post-translationally Regulated by Proteolytic Cleavage (*)

Several receptor tyrosine kinases generate soluble ligand binding domains either by differential splicing resulting in a truncated RNA transcript, or by proteolytic cleavage. Although the exact role in vivo of these soluble extracellular domains is unclear, proteolysis may function to down-regulate the receptor, and soluble extracellular domains (ECD) may compete with the intact receptor binding to ligand. Axl is a member of a new class of receptor tyrosine kinases characterized by an ECD resembling cell adhesion molecules and unique sequences in the kinase domain. In addition, Axl is transforming in both fibroblast and hematopoietic cells, and appears to be involved in mesenchymal development. We now find that Axl is post-translationally processed by cleavage in a 14 amino acid region immediately NH2-terminal to the transmembrane domain resulting in a soluble ECD and a membrane bound kinase domain. The sequence of this putative cleavage site shares no homology with recognition sites of known proteases. Characterization of this proteolytic processing shows that it does not require protein synthesis or transport but is augmented by phorbol ester treatment. Since the cleavage of Axl enhances turnover of the kinase on the cell surface, we suggest that proteolytic processing down-regulates Axl kinase activity.