Utrophin lacks the rod domain actin binding activity of dystrophin

We previously identified a cluster of basic spectrin-like repeats in the dystrophin rod domain that binds F-actin through electrostatic interactions (Amann, K, J,, Renley, B, A., and Ervasti, J,M. (1998) J, Biol. Chem, 273, 28419-28423), Because of the importance of actin binding to the presumed physiological role of dystrophin, we sought to determine whether the autosomal homologue of dystrophin, utrophin, shared this rod domain actin binding activity. We therefore produced recombinant proteins representing the cluster of basic repeats of the dystrophin rod domain (DYSR11-17) or the homologous region of the utrophin rod domain (UTROR11-16). Although UTROR11-16 is 64% similar and 41% identical to DYSR11-17, UTROR11-16 (pI = 4.86) lacks the basic character of the repeats found in DYSR11-17 (pI = 7.44), By circular dichroism, gel filtration, and sedimentation velocity analysis, we determined that each purified recombinant protein had adopted a stable, predominantly ct-helical fold and existed as a highly soluble monomer, DYSR11-17 bound F-actin with an apparent Kd of 7.3 +/- 1.3 mu M and a molar stoichiometry of 1:5, Significantly, UTROR11-16 failed to bind F-actin at concentrations as high as 100 mu M. We present these findings as further support for the electrostatic nature of the interaction of the dystrophin rod domain with F-actin and suggest that utrophin interacts with the cytoskeleton in a manner distinct from dystrophin.