The calmodulin-binding site in alpha-fodrin is near the calcium-dependent protease-I cleavage site

Fodrin (brain spectrin) binds calmodulin and is susceptible to proteolysis by calcium-dependent protease I (CDP-I, calcium-activated neutral protease I, or calpain I). Both events involve the central region of the alpha-fodrin subunit, and calmodulin binding enhances the sensitivity of fodrin to CDP-I mediated proteolysis. Fragments of fodrin, generated chemically or proteolytically, which retain calmodulin binding activity have been identified and analyzed by two-dimensional peptide mapping and by direct protein sequencing. Both CDP-I and calmodulin interact with the terminal portion of the eleventh repetitive unit in fodrin, which is at the center of the molecule. CDP-I cleavage occurs between Tyr104 and Gly105 and preserves the calmodulin binding activity of the carboxyl-terminal fragment. In contrast, chymotryptic cleavage at Trp120 reduces the ability of this fragment to bind calmodulin, and tryptic cleavage beyond Trp120 completely eliminates calmodulin binding activity. It is concluded that Ser-Lys-Thr-Ala-Ser-Pro-Trp-Lys-Ser-Ala-Arg-Leu-Met-Val-His-Thr-Val-Ala- Thr- Phe-Asn-Ser-Ile-Lys, a 24-residue peptide which bridges repeats 11 and 12 of brain alpha spectrin contains the high affinity calmodulin binding domain.