Measurement of the affinity and cooperativity of annexin V-membrane binding under conditions of low membrane occupancy.

We developed a method for measuring the binding affinity of annexin V for phospholipid vesicles and cells at very low levels of membrane occupancy. The annexin V-117 mutant was labeled with fluorescein iodoacetamide on its single N-terminal cysteine residue; binding to phospholipid vesicles containing phosphatidylserine (PS) and 2% rhodamine-phosphatidylethanolamine was measured by fluorescence quenching due to resonance energy transfer; binding to cells with exposed PS was measured by fluorometry after elution of bound protein. The equilibrium constant was calculated as a function of the midpoint of the calcium titration curve, the Hill coefficient, and the concentration of membrane binding sites. Calcium titrations at very low ratios of protein to membrane revealed Hill coefficients of approximately 8 for both vesicles and cells, far higher than previously measured, but as the protein-membrane ratio was increased above 3% of maximum membrane occupancy, the value of the Hill coefficient progressively decreased to a limiting value of about 2. High Hill coefficients were also observed for measurements performed at different ionic strengths and with membrane PS content varied over the range from 20 to 50%. This method allows the accurate determination of the affinity and cooperativity of annexin V-membrane binding and will be useful for the evaluation of modified annexin V derivatives intended for diagnostic and therapeutic applications.