CHARACTERIZATION OF THE BINDING OF CALMODULIN TO NONERYTHROID SPECTRIN

Both brain and erythrocyte spectrin bound calmodulin in a calcium-depe ndent manner when immobilized on a polyvinylidene difluoride (PVDF) me mbrane, though the affinity of the non-erythroid spectrin was much gre ater than that of the erythroid isoform. The interaction was character ized further using equilibrium partition. In the presence of calcium, the partition behavior of calmodulin was affected by both spectrins, t hough 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 th e equilibrium partition data indicated the presence of a high-affinity site characterized by a dissociation constant of about 0.3 mu M and p robably one or more much weaker sites (> 0.3 mM). The presence of at l east two distinct binding sites was substantiated by the observation t hat truncated recombinant spectrin fusion proteins comprising either t he middle part or the C-terminal of non-erythroid alpha-spectrin bound calmodulin.