Interaction of divalent cations with human red cell cytoskeletons.

The binding of Ca2+ to spectrin from human erythrocytes was investigated by equilibrium dialysis, and the binding of Mn2+ by electron paramagnetic resonance. The results led to the conclusion that no binding sites of high affinity (greater than about 10(4) M-1) are present. In the cytoskeletal protein complex isolated from erythrocytes, which (like crude spectrin) contains actin and some other proteins, a set of sites with an association constant of 4 x 10(4) M-1 for Mn2+ is observed. These may be divalent cation binding sites on the actin molecules. Weak interactions of Ca2+ and Mg2+ with spectrin are reflected by self-association of the spectrin heterodimers, which can be followed in the analytical ultracentrifuge and by light-scattering. This self-association is affected by the state of the protein thiol groups. Conditions in which self-association of spectrin occurs have been defined. No aggregation is observed at the Mg2+ activity thought to correspond to that in the cytoplasm.