L.N.C.I.B. Laboratorio Nazionale Consorzio Interuniversitario Biotecnologie, AREA Science Park, Padriciano, Trieste, Italy.
The product of gas6 (Gas6) is a growth factor with high level of similarity to protein S and was identified as the ligand for Axl family of tyrosine kinase receptors. Gas6 contains an N-terminal gamma-carboxylated domain (Gla), four epidermal growth factor like domains and a large C-terminal D region. An alternative Gas6 spliced form (Gas6SV) having an additional 43 amino acids between fourth EFG like and D domain was characterised. Here we show data indicating that Gas6SV is specifically cleaved within the inserted sequence, thereby splitting the D domain from the remaining part of the protein. The resulting two proteolytic products of 36 kDa and 50 kDa were separated and the 50 kDa fragment corresponding to region D was shown to be responsible for Axl receptor activation. Furthermore a deletion mutant of Gas6 containing only the D domain was shown to similarly activate Axl receptor phosphorylation unequivocally demonstrating that D domain can act as a signalling molecule. The possible roles of the proteolytic processing of Gas6SV in the regulation of growth factor availability are discussed.