Factor IX consists of a gamma-carboxyglutamic acid-rich domain followed by two epidermal growth factor (EGF)-like domains and the C-terminal protease domain. To delineate the function of EGF1 domain in factor IX, we constructed three mutants: an EGF1 domain-deleted mutant (IX delta EGF1), a point mutant (IXQ50P) with a Gln-50-->Pro change, and a replacement mutant (IXPCEGF1) in which the EGF1 domain of factor IX was replaced by that of protein C. These mutants and wild-type (WT) factor IX (IXWT) were expressed in 293 kidney cells by using pRc/CMV vector. The purified proteins had the same gamma-carboxyglutamic acid content as the normal plasma factor IX (IXNP) and were activated normally by factor XIa-Ca2+. In contrast, IX delta EGF1 could not be activated by factor VIIa-tissue factor-Ca2+, and the activation of IXPCEGF1 in this system was markedly slow; however, IXQ50P was activated at a normal rate. In additional studies, both IXWT and IX delta EGF1 were rapidly converted to their respective IX alpha forms by factor Xa-phospholipid-Ca2+. Since this reaction has an absolute requirement for phospholipid, it indicates that the mutants under study are not impaired in their interactions with phospholipid. Relative coagulant activities of factor XIa-activated proteins were IXNP, 100%; IXWT, 75-85%; IX delta EGF1, < or = 1%; IXPCEGF1, < or = 2%; and IXQ50P, 6-10%. We conclude that the EGF1 domain of factor IX is required for its activation by factor VIIa-tissue factor and that the Gln-50 residue is not critical for this activation. Further, the EGF1 domain of factor IX is not essential for phospholipid binding and for its activation by factor XIa. In addition, the low coagulant activities of the activated mutants indicate that the EGF1 domain is also important in factor X activation by factor IXa-factor VIIIa-Ca(2+)-phospholipid complex
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