Journal
BLOOD
Volume 117, Issue 24, Pages 6685-6693Publisher
AMER SOC HEMATOLOGY
DOI: 10.1182/blood-2010-11-317099
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Funding
- British Heart Foundation [PG/09/105]
- Novartis
- Swedish Research Council [07143]
- British Heart Foundation [PG/09/105/28138] Funding Source: researchfish
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Protein S has an important anticoagulant function by acting as a cofactor for activated protein C (APC). We recently reported that the EGF1 domain residue Asp95 is critical for APC cofactor function. In the present study, we examined whether additional interaction sites within the Gla domain of protein S might contribute to its APC cofactor function. We examined 4 residues, composing the previously reported Face1 (N33S/P35T/E36A/Y39V) variant, as single point substitutions. Of these protein S variants, protein S E36A was found to be almost completely inactive using calibrated automated thrombography. In factor Va inactivation assays, protein S E36A had 89% reduced cofactor activity compared with wild-type protein S and was almost completely inactive in factor VIIIa inactivation; phospholipid binding was, however, normal. Glu36 lies outside the omega-loop that mediates Ca(2+)-dependent phospholipid binding. Using mass spectrometry, it was nevertheless confirmed that Glu36 is gamma-carboxylated. Our finding that Gla36 is important for APC cofactor function, but not for phospholipid binding, defines a novel function (other than Ca(2+) coordination/phospholipid binding) for a Gla residue in vitamin K-dependent proteins. It also suggests that residues within the Gla and EGF1 domains of protein S act cooperatively for its APC cofactor function. (Blood. 2011;117(24):6685-6693)
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