[Show abstract][Hide abstract] ABSTRACT: The binding of the isolated alpha-subunit of human erythrocyte spectrin to calmodulin is demonstrated by partitioning in aqueous two-phase systems. The affinity of the alpha-subunit for calmodulin is slightly higher than that of the spectrin dimer, whereas the beta-subunit interacts only very weakly. The binding is in all cases calcium-dependent and is abolished on addition of chlorpromazine. At an ionic strength close to physiological conditions, about 1 microM free calcium is required to induce maximum binding of calmodulin to spectrin dimer.
[Show abstract][Hide abstract] ABSTRACT: Calmodulin is shown to interact with human spectrin dimer. The binding was highly calcium-dependent and observed in two different kinds of experiments. Firstly, affinity chromatography of calmodulin on a Sepharose 4B column with immobilized spectrin, and secondly, partition in aqueous two-phase polymer systems. In the column experiments stoichiometric amounts of calmodulin were retained on the spectrin-Sepharose column when micromolar concentrations of calcium were present. The calmodulin bound could be eluted with EGTA. The partition coefficient of calmodulin in an aqueous two-phase polymer system containing calcium was changed upon addition of spectrin, indicating an association between the two proteins. In the absence of calcium, spectrin did not cause any change in the partition behaviour of calmodulin, thus showing that the association requires calcium.