Expression of completely γ‐carboxylated and β‐hydroxylated recombinant human vitamin‐K‐dependent protein S with full biological activity

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Abstract
Human anticoagulant vitamin-K-dependent protein S was expressed in mouse C127 cells using a bovine papilloma virus vector system. A full-length cDNA construct was introduced into the vector in the 5′ untranslated region of the mouse metallothionein-I gene. Transfected cells expressed approximately 10 μg/ml of the recombinant protein which was purified by ion-exchange chromatography followed by affinity chromatography using Ca2+-dependent monoclonal antibodies against the region of protein S containing 4-carboxyglutamic acid. Recombinant protein S was structurally and functionally similar to protein S purified from plasma. On SDS/polyacrylamide-gel electrophoresis recombinant protein S had a slightly higher molecular mass than plasma protein S. After treatment with endoglycosidase F, the proteins comigrated suggesting the observed molecular mass difference to be due to alterations in the N-linked carbohydrate side chains. Recombinant and plasma protein S demonstrated identical amino-terminal sequences, similar amino acid composition and number of 4-carboxyglutamyl and 3-hydroxyaspartyl/asparaginyl residues. Recombinant protein S had the same affinity for Ca2+ as protein S from plasma and the two proteins had the same activated protein C cofactor activity in a functional assay. In addition, both forms of protein S formed complexes with C4b-binding protein with the same apparent Kd. Protein S is the most extensively post-translationally modified vitamin-K-dependent protein, and all the modifications were carried out in the recombinant DNA system yielding a recombinant protein S with full biological activity.