Factor VIII binds to phospholipid membranes and to von Willebrand factor (vWf) via its second C domain, which has lectin homology. The crystal structure of the C2 domain has prompted a model in which membrane binding is mediated by two hydrophobic spikes, each composed of a pair of residues displayed on a -hairpin turn, and also by net positive charge and specific interactions with phospho-L-serine. , and 91% reduction in specific activity in the activated partial thromboplastin time assay. In a phospholipidlimiting factor Xa activation assay, these mutants had a 65, 85, and 96% reduction in specific activity. Equilibrium binding of fluorescent, sonicated phospholipid vesicles to mutants immobilized on Superose beads was measured by flow cytometry. The affinities for phospholipid were reduced ϳ20-, 30-, and >35-fold for 2199/2200, 2251/2252, and 4-Ala, respectively. A dimeric form of mature vWf bound to immobilized factor VIII and the same mutants, but the affinities of the mutants were reduced ϳ5-, 10-, and >20-fold, respectively. In a competition, solution phase enzyme-linked immunosorbent assay, plasma vWf bound factor VIII and the same mutants with the affinities for the mutants reduced >5-, >5-, and >50-fold, respectively. We conclude that the two hydrophobic spikes are constituents of both the phospholipidbinding and vWf-binding motifs. In plasma, vWf apparently binds the inherently sticky membrane-binding motif, preventing nonspecific interactions.Factor VIII is a phosphatidyl-L-serine (PS) 1 binding cofactor (1, 2) for the vitamin K-dependent serine protease, factor IXa, that also binds to PS-containing membranes (3, 4). The membrane-bound factor VIIIa-factor IXa complex cleaves the zymogen, factor X, to factor Xa, which is then responsible for catalyzing prothrombin activation (5). The importance of this enzyme complex is illustrated by hemophilia, a disease in which a deficiency of either factor VIII (hemophilia A) or factor IX (hemophilia B) leads to life-threatening bleeding. Factor IXa gains more than 100,000-fold greater efficiency in activating factor X by assembling with factor VIIIa on a PS-containing membrane than when free in solution (6). We have recently found that the predominant effect of PS-containing membranes on the factor VIIIa-factor IXa complex is to increase the k cat by more than 1000-fold (7). These membranes also increase the affinity of factor IXa for factor VIIIa and for factor X. The central importance of the membrane binding function of factor VIII motivates studies to define the membrane-binding motif.Factor VIII, with M r 280,000, is homologous to another procoagulant protein, factor V, in amino acid sequence (8 -10) and in function, as a membrane-bound enzyme cofactor (5, 11-13). The proteins share a repeating domain structure of A1-A2-B-A3-C1-C2 in which the A domains are homologous with ceruloplasmin, the B domain is unique to each protein, and the C domains are homologous with discoidin I, a phospholipid-binding lectin (14), and with a murine milk fat globule membrane ...