beta-Hydroxyasparagine in domains homologous to the epidermal growth factor precursor in vitamin K-dependent protein S

Vitamin K-dependent protein S is involved in the regulation of blood coagulation. It is a 75-kDa single chain protein with an NH2-terminal gamma-carboxyglutamic acid-containing domain followed by a thrombin-sensitive region and four domains arranged in tandem, each of which is homologous to the epidermal growth factor (EGF) precursor. The NH2-terminal EGF-like domain contains beta-hydroxyaspartic acid, which has been identified in vitamin K-dependent proteins. The following EGF-like repeat has a very pronounced sequence homology (10 consecutive residues identical) to one of the EGF-like units in the EGF precursor. We now show that, in protein S, this EGF-like repeat has one beta-hydroxyasparagine residue formed by hydroxylation of asparagine. The two COOH-terminal EGF-like repeats also contain beta-hydroxyasparagine, an amino acid not previously found in proteins. Sequence comparisons have enabled us to identify a consensus sequence that seems to be required by the hydroxylase(s).