A membrane-interactive surface on the factor VIII C1 domain cooperates with the C2 domain for cofactor function.

Area of Science:

• Biochemistry
• Molecular Biology
• Hematology

Background:

• Factor VIII (FVIII) is essential for blood coagulation.
• FVIII binds to phosphatidylserine (PS)-containing membranes via its C1 and C2 domains.
• The precise role of the C1 domain in membrane interaction remains unclear.

Purpose of the Study:

• To delineate the membrane-binding characteristics of the FVIII C1 domain.
• To investigate the impact of C1 domain mutations on FVIII membrane binding and cofactor activity.
• To understand the interplay between FVIII C1 and C2 domains in the factor Xase complex.

Main Methods:

• Site-directed mutagenesis of the FVIII C1 domain to create four mutants.
• Measurement of FVIII binding to PS-containing membranes with varying PS concentrations.
• Assay of FVIII cofactor activity in the presence of PS membranes and factor X.
• Characterization of mutant FVIII activity using specific monoclonal antibodies (ESH4, BO2C11).

Main Results:

• All four C1 domain mutants exhibited significantly reduced affinity for PS membranes.
• Three mutants showed decreased apparent affinity for factor X.
• Monoclonal antibody ESH4 differentially affected wild-type and mutant FVIII membrane binding and Vmax.
• Monoclonal antibody BO2C11 demonstrated enhanced impairment of mutant FVIII activity compared to wild-type.

Conclusions:

• A specific membrane-interactive surface on the FVIII C1 domain has been identified.
• The C1 domain influences FVIII's binding to factor X.
• Cooperative interaction between the FVIII C1 and C2 domains is critical for optimal factor Xase complex function.