Characterization of the binding of calmodulin to non-erythroid spectrin

Both brain and erythrocyte spectrin bound calmodulin in a calcium-dependent manner when immobilized on a polyvinylidene difluoride (PVDF) membrane, though the affinity of the non-erythroid spectrin was much greater than that of the erythroid isoform. The interaction was characterized further using equilibrium partition. In the presence of calcium, the partition behavior of calmodulin was affected by both spectrins, though brain spectrin caused a much larger change in partition. However, in both cases it was evident that the observed partition behavior of calmodulin was due to complex formation with spectrin. Analysis of the equilibrium partition data indicated the presence of a high-affinity site characterized by a dissociation constant of about 0.3 microM and probably one or more much weaker sites (> 0.3 mM). The presence of at least two distinct binding sites was substantiated by the observation that truncated recombinant spectrin fusion proteins comprising either the middle part or the C-terminal of non-erythroid alpha-spectrin bound calmodulin.