The interaction of calmodulin with human and avian spectrin.

An air-driven ultracentrifuge was used to investigate the calcium-dependent interaction of 125I-calmodulin with human and avian spectrins. The equilibrium constants (Ka) for the interaction between calmodulin and human spectrin dimer and tetramer under non-denaturing conditions were estimated to be 4.6 X 10(4) M-1 and 7.3 X 10(4) M-1, respectively. The denaturation of human spectrin by urea (5 M) increased the Ka for calmodulin to 4.6 X 10(5) M-1. The value of Ka for the interaction of calmodulin with avian spectrin dimer under non-denaturing conditions was 5.1 X 10(5) M-1. A bifunctional reagent cross-linked both avian spectrin and human spectrin to calmodulin in a calcium-dependent manner.