Summary
Protein S functions as a cofactor to activated protein C (APC) in the degradation
of FVa and FVIIIa. In protein S, the thrombin sensitive region (TSR) and the first
EGF-like domain are important for expression of the APC cofactor activity. A naturally
occurring Thr103Asn (T103N) mutation in the first EGF-like domain of protein S has
been associated with functional (type II) protein S deficiency. To elucidate the functional
consequences of the T103N mutation, recombinant protein S mutant was expressed in
mammalian cells and functionally characterised. The expression level of protein S
T103N from transiently transfected COS 1 cells was equal to that of wild type protein
S. The mutant protein S and wild type protein S were also expressed in 293 cells after
stable transfection, and the recombinant proteins purified. In APTT-and PT-based coagulation
assays, the mutant protein demonstrated approximately 50% lower anticoagulant activity
as compared to wild type protein S. The functional defect was further investigated
in FVa-and FVIIIa-degradation assays. The functional defect of mutant protein S was
attenuated at increasing concentrations of APC. The results demonstrate the region
around residue 103 of protein S to be of functional importance, possibly through a
direct interaction with APC.
Key words
Anticoagulant protein S - functional deficiency - natural mutation -
in vitro reproduction