s. Fluorescence emitted by each Vβ-immobilized bead due to toxin binding, was plotted on bar graphs shown. The error-bars represent standard deviations from two independent experiments. Similar data (but with higher background) were obtained in other experiments where unoccupied sites on biotin-Vβ immobilized-beads were not blocked with excess biotin ([S0 Fig](#pone.0000000.s000){ref-type="supplementary-material"}).](pone.0000000.g000){#pone.0000000.g000}  