Identification of Residues in the 558-Loop of Factor VIIIa A2 Subunit That Interact with Factor IXa

Factor VIIIa is comprised of A1, A2, and A3C1C2 subunits. Several lines of evidence have identified the A2 558-loop as interacting with factor IXa. The contributions of individual residues within this region to inter-protein affinity and cofactor activity were assessed following alanine scanning mutagenesis of residues 555–571 that border or are contained within the loop. Variants were expressed as isolated A2 domains in Sf9 cells using a baculovirus construct and purified to >90%. Two reconstitution assays were employed to determine affinity and activity parameters. The first assay reconstituted factor Xase using varying concentrations of A2 mutant and fixed levels of A1/A3C1C2 dimer purified from wild type (WT), baby hamster kidney cell-expressed factor VIII, factor IXa, and phospholipid vesicles to determine the inter-molecular K_d for A2. The second assay determined the K_d for A2 in factor VIIIa by reconstituting various A2 and fixed levels of A1/A3C1C2. Parameter values were determined by factor Xa generation assays. WT A2 expressed in insect cells yielded similar K_d and k_cat values following reconstitution as WT A2 purified from baby hamster kidney cell-expressed factor VIII. All A2 variants exhibited modest if any increases in K_d values for factor VIIIa assembly. However, variants S558A, V559A, D560A, G563A, and I566A showed >9-fold increases in K_d for factor Xase assembly, implicating these residues in stabilizing A2 association with factor IXa. Furthermore, variants Y555A, V559A, D560A, G563A, I566A, and D569A showed >80% reduction in k_cat for factor Xa generation. These results identify residues in the 558-loop critical to interaction with factor IXa in Xase.