The first laminin G-type domain in the SHBG-like region of protein S contains residues essential for activation of the receptor tyrosine kinase Sky

Vitamin K-dependent protein S and the product of growth-arrest-specific gen e 6 (Gas6) both possess the ability to phosphorylate members of the AxI/Sky subfamily of receptor tyrosine kinases. However, Gas6 appears to be the bo na fide ligand for these receptors in man, as human protein S has been demo nstrated to activate murine Sky but not the human orthologue. In contrast, bovine protein S is able to stimulate human Sky despite its high degree of sequence identity with human protein S. The domain organisations of protein S and Gas6 are virtually identical and the C-terminal SHBG-like region, co ntaining two globular (G) domains, has been shown to play a crucial role in the receptor stimulation. In order to further localise the area responsibl e for the interaction, a number of protein chimeras were used to stimulate human Sky. Each chimera had one part of the human protein S SHBG-like regio n replaced by the corresponding part of bovine protein S or human Gas6. We found that human protein S may indeed activate human Sky but only above phy siological plasma concentrations. The human-bovine protein S chimeras provi ded new information implying that the first G domain contains critical resi dues for the interaction with the Sky receptor. Moreover, these residues do not seem to be clustered but rather to be distributed at various positions in the first G domain.