Membrane-Interactive Amino Acids in the Factor VIII C1 Domain Are Cooperative with C2 Domain Epitopes for Membrane Binding and Cofactor Function.

Factor VIII functions as a cofactor in blood coagulation. When released from a non-covalent complex with von Willebrand factor (vWf), activated factor VIII assembles with factor IXa on phosphatidylserine (PS)-containing membranes to form the factor Xase complex. Binding to PS-containing membranes amplifies the activation of factor X by several orders of magnitude. Factor VIII is composed of three A domains, one B domain and two C domains (C1 and C2). The role of C2 domain, including the orientation with respect to membrane surface, vWf-binding motif, and protein-protein contact sites among Xase complex, are relatively well-documented. Recently, the position of the C domains in the factor VIII crystal structure suggested a possible role for the C1 domain in membrane binding. We recently confirmed the participation of K2092 and F2093 of the factor VIII C1 domain in membrane binding (Meems et al. Blood 2009 First edition Aug 18). This work explores the participation of additional C1 domain amino acids and the way the corresponding motif(s) cooperate with motifs of the C2 domain for membrane binding.

Four factor VIII C1 domain mutants encompassing the lower surface of the C1 domain (Arg2090/GLy2091, Lys 2092/Phe2093, Gln2042/Tyr2043, and Arg2159) had individual or paired amino acids mutated to alanine. Mutants were produced in COS-1 cells and purified by immunoaffinity chromatography. The specific activities of these mutants were assessed in a commercial PTT assay as well as phospholipid-limiting and phospholipid-saturating factor Xase assay. Their affinities to factor IXa and factor X were measured by titration experiments using different concentrations of factor IXa and factor X, respectively. Binding to plasma vWf was evaluated in a competition, solution phase enzyme-linked immunosorbent assay (ELISA). The cooperative role of C1 and C2 domains in membrane-binding for cofactor activity was carried out using C1 mutants and antibodies against established membrane-interactive C2 domain motifs, ESH4 and BO2C11.

In a competition ELISA for vWf, the affinity of Arg2159 was reduced more than 50-fold, while the other mutants were normal. All mutants had reduced specific activity (range 24-61% of wild type) in a commercial PTT assay containing excess phospholipid. All mutants had decreased apparent affinity for vesicles with limiting (4%) PS by 33, 5, 20, and 18-fold for Arg2090/GLy2091, Gln2042/Tyr2043, Arg2159, and Lys 2092/Phe20933, respectively. However, addition of excess vesicles led to near normal activity for Arg2159. Mutants Arg2090/GLy2091 and Gln2042/Tyr2043 both had 4-fold decreased apparent affinity for factor X and 77% and 84% reduction in Vmax even when phospholipid and factor X were in excess. Mutant Lys 2092/Phe2093 had normal apparent affinity for factor IXa and factor X but > 91% reduction in Vmax. These results indicate that the C1 domain affects interaction with factor X and the Vmax of the factor Xase complex aside from the effect on membrane affinity. To further explore the role of membrane-binding motif in the Xase complex, the activities of mutants were tested with the C2 domain membrane-interactive epitopes blocked by mAb's BO2C11 or ESH4. For WT factor VIII, ESH4 and B02C11 decreased apparent affinity for vesicles of 15% PS by 6-fold and 5-fold, and decreased the Vmax by 0 and 89%, respectively. BO2C11 completely inhibited the activity of Arg2090/GLy2091, Lys 2092/Phe2093, and Arg2159 while ESH4 decreased apparent affinity 2-7-fold for the three mutants. ESH4 decreased the Vmax by 2-5-fold for the mutants. Thus, the intact membrane-binding motif in C1 can independently support Xase activity although the C1 motifs and both C2 membrane-interactive epitopes are required for full activity.

Amino acids Arg2090/GLy2091, Lys2092/Phe2093 , Gln2042/Tyr2043, and Arg2159 of the factor VIII C1 domain participate in membrane binding. Our data suggest that engagement of the C1 domain through these residues, together with the ESH4 and the BO2C11 epitopes of the C2 domain, cooperatively influence alignment or an allosteric effect that alters activity for the assembled factor Xase complex.

Pipe:Baxter: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novo Nordisk: Membership on an entity's Board of Directors or advisory committees; Wyeth: Speakers Bureau; Inspiration Biopharmaceuticals: Research Funding; CSL Behring: Honoraria.