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Factor VIII is a 330 kd molecule with the domain organization A1-a1-A2-a2-B-a3-A3-C1-C2. The crystallographic structure of isolated factor VIII’s C2 domain was solved by Pratt et al (1999). Based upon homology, a model of the C1 domain has been constructed (Leu et al, 2000). Binding of C domains to VWF and vitamin K dependent factors is considerably stronger to C1C2 than to C2 (Liu& Thompson, 2001).
Fig. 1: Model of FVIIIa based on 2D electron densitic
(Stoilova-McPhie, 2002)
Fig. 2: Model of FVIIIa based on crystal structure of factor Va
(Autin etal, 2004)
These data indicate that C1 either participates directly in binding or influences the conformation of binding sites on C2. To date, there are two models have been proposed for C1C2: First, the membrane-bound factor VIIIa structure was observed in low resolution cryo-electron microscopy by Stoilova-McPhie et al (2002). Here, C2 is slightly inclined to the membrane (PL) and C1 forms almost a right angle with C2, its long axis nearly parallel to the membrane (Fig 1). Second, a somewhat different model (Fig 2), the orientation of C1C2 in factor VIIIa, is suggested from the crystal structure of a partial bovine factor Va preparation (Adams et al, PNAS, 2004) as modeled by Autin et al (2004 Blood Abstract). In this model, both C1 and C2 may provide binding to a lipid surface.
To address these issues, our primary aim is to isolate C1C2 and characterize its structure and binding properties. |
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