Physics. Resolving physical processes on the attosecond time scale.

1129 of the β-hairpin with the base of the A1 domain is unexpected and may account for the ability of VWD type 2B mutations to increase the binding affinity. The A1 domain contains a compact α/β fold delimited by a single disulfide loop. In the free A1 domain, the sequence extending on the amino-terminal side of the disul-fide bond lies against the α/β surface (8), and this amino-terminal extension is displaced when GpIbα binds (3). The amino-terminal extension is inhibitory: Recombi-nant A1 domains lacking this extension bind 5-to 10-fold more tightly to GpIbα (9). The structure of another high-affinity A1 domain mutant with the substitution I546V suggests that conformational changes are propagated from the mutation through the A1 domain to the GpIbα binding site at the top (10). Many VWD type 2B mutations, however, are within the amino-terminal extension or affect residues that it contacts, suggesting that the mutations mainly promote the displacement of the amino-terminal extension and facilitate binding of the GpIbα β-hairpin (3). The relative importance of these two mechanisms remains to be determined. The mutations in GpIbα that cause platelet-type pseudo-VWD also stabilize the A1-GpIbα complex, but by a different mechanism. As determined by Huizinga et al. (3) and independently by Uff et al. (11), the β-switch loop is flexible and lacks defined secondary structure in un-complexed GpIbα. Upon binding to the VWF A1 domain, the β-switch region forms a two-stranded antiparallel β sheet that aligns with the central β sheet of A1. Mutations in GpIbα that increase the affinity of this platelet glycoprotein for VWF A1 are located in the β-switch region and are predicted to stabilize the β sheet (3). Because VWD type 2B mutations and platelet-type pseudo-VWD mutations affect widely separated binding sites, one might expect them to be additive. Indeed, binding aff inity was increased two-to threefold by either type of mutation singly, and fivefold when the mutations were combined (3). The structure of the VWF A1-GpIbα complex challenges several widely held notions about how the proteins might interact. For example, the properties of chimeric GpIbα proteins suggest that LRRs 1 to 4 may be important for binding (12). Instead, the structure reveals that there is one contact with LRR 1 and multiple contacts with LRRs 5 to 8 at the opposite end of the GpIbα fragment. An anion-ic region closer to the carboxyl-terminal than the β-switch region contains three sulfated tyrosine residues …