Structure, calmodulin-binding, and calcium-binding properties of recombinant alpha spectrin polypeptides.

We examined the structure and the distribution of binding activities within bacterially produced fragments of Drosophila alpha spectrin. By electron microscopy, purified spectrin fragments resembled the corresponding regions of native spectrin. The contour lengths of recombinant spectrin molecules were proportional to the length of their coding sequences, which is consistent with current models of spectrin structure in which individual segments of the polypeptide contribute independently to the structure of the native molecule. We localized two sites at which calcium may regulate spectrin function. First, a site responsible for calmodulin binding to Drosophila alpha spectrin was identified near the junction of repetitive segments 14 and 15. Second, a domain of Drosophila alpha spectrin that includes two EF hand calcium-binding sequences bound 45Ca in blot overlay assays. EF hand sequences from a homologous domain of Drosophila alpha actinin did not bind calcium under the same conditions.