The GP Ibα AP1 peptide binds VWF, VWP A1, and AP1, but it does not inhibit GP Ib-mediated cell rolling on VWF. GP Ibα peptide encompassing Leu214 to Val229 binds immobilized VWF, VWF A1 domain, and AP1 (A) and affects the rolling of GP Ibα-expressing cells on immobilized VWF (B). (A) Increasing amounts of the GP Ibα Leu214 to Val229 peptide were incubated with immobilized VWF, VWF A1 domain, or AP1. Detection of peptide binding is described in “Materials and methods.” In the inhibition assay, free AP1 antibody and 60 μM peptide were added simultaneously. This figure is representative of 3 individual experiments. (B) Cells expressing wild-type GP Ibα were introduced into a parallel-platelet flow chamber in the presence of 250 μM peptide for 1 minute and allowed to settle on the surface of immobilized VWF (coated with 5 or 10 μg/mL VWF). The chamber was then perfused with TBS at flow rates that generated wall shear stresses of 20 dynes/cm². The rolling velocities of cells in the absence of peptide were measured and compared with that in the presence of peptide. Error bars indicate SEM.