Abstract
The binding of Ca 2+ to spectrin from human erythrocytes was investigated by equilibrium dialysis, and the binding of Mn 2+ 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 · 10 4 M −1 for Mn 2+ is observed. These may be divalent cation binding sites on the actin molecules. Weak interactions of Ca 2+ and Mg 2+ 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 Mg 2+ activity thought to correspond to that in the cytoplasm.
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