Natural mutations in the EGF4 domain of protein S produce extended conformational changes and alter phospholipid and C4bBP interactions

Protein S (PS), through its anticoagulant activities, plays a crucial role in the control of coagulation. Rare natural PS mutations responsible for dysfunctional molecules in plasma (type II deficiency) represent unique models to relate plasma phenotype with recombinant PS (rPS) expression, and to elucidate the functional role of PS domains. We characterized the first type II PS deficiency affecting the EGF4 domain, a module with a poorly defined functional role. The proband suffered from recurrent deep vein thrombosis and showed reduced PS anticoagulant activity (31%), and total, free PS antigen and C4bBP levels in the normal range. The patient was heterozygous for the IVSg-2A/T mutation that, by activating a cryptic splice site, causes the deletion of codons I203D204. Recombinant rPSwt and rPSDelI203D204 variants were expressed in BHK21 cells and immunopurified from conditioned medium. The rPSDelI203D204 showed reduced anticoagulant (