A cluster of basic repeats in the dystrophin rod domain binds F-actin through an electrostatic interaction

The dystrophin rod domain is composed of 24 spectrin-like repeats and was thought to act mainly as a flexible spacer between the amino-terminal actin binding domain and carboxyl-terminal membrane-associated domains. We previously demonstrated that a fragment of the dystrophin rod domain also binds F-actin. However, the nature and extent of rod domain association with F-actin is presently unclear. To begin addressing these questions, we characterized two recombinant proteins representing adjacent regions of the dystrophin rod. DYS1416 (amino acids 1416-1880) bound F-actin with a Kd of 14.2 +/- 5.2 microM and a stoichiometry of 1 mol:mol of actin. However, DYS1030 (amino acids 1030-1494) failed to bind F-actin, suggesting that not all rod domain repeats are capable of binding F-actin. Interestingly, DYS1416 corresponds to a unique region of the dystrophin rod rich in basic amino acids, whereas DYS1030 is composed mainly of acidic repeats. This observation suggested that DYS1416 may interact with acidic actin filaments through an electrostatic interaction. Supporting this hypothesis, actin binding by DYS1416 was dramatically inhibited by increasing ionic strength. We suggest that electrostatic interactions between basic spectrin-like repeats and actin filaments may contribute to the actin binding activity of other members of the actin cross-linking protein family.