Spectrins bound to vesicles as a function of spectrin concentration: vesicles and spectrin from untreated cells (dashed circles). Spectrin from NEM-treated cells and vesicles from untreated cells (open circles), spectrin from untreated cells and vesicles from NEM-treated cells (filled circles). The curves are best fits for binding to a population of identical and independent sites, giving saturation binding titres of 2.8 + 0.3, 2.6 + 0.2 and 2.4 + 0.3 nM respectively, with corresponding apparent association constants of 0.6 + (0.13) × 106, 1.5 (+ 0.3) × 106, and 1.0 (+ 0.3) × 106 M−1.