Standard calibration beads

We used flow cytometry to measure receptor and ligand site densities on platelets and BFP beads, respectively, as previously described^{36 (link)}. Platelets were incubated with a FITC-conjugated HIP-8 at 10 μg mL⁻¹ at room temperature for 30 min for α_IIbβ₃ site density (m_r) measurement, or incubated first with LM609 at 10 μg mL⁻¹ for 30 min and then PE-conjugated anti-mouse antibody for 30 min for α_Vβ₃m_r measurement. The m_r values were measured using conformation-independent mAbs, hence including all conformers. The FN and Fg coated beads were incubated first with HFN 7.1 and 1D6 (Santa Cruz Biotechnology), respectively, and then with a PE-conjugated polyclonal anti-mouse antibody. The fluorescent intensities of the cells or beads were measured by a BD LSR flow cytometer (BD Biosciences), and compared to standard calibration beads (Bangs Laboratories, Fishers, IN) to determine the number of molecules per cell/bead. The site density was calculated by dividing the total number of molecules per cell/bead to the cell/bead surface area^{36 (link)}, which was calculated from the radii measured with a customized Labview (National Instrument) program.