Physical properties of dystrophin rod domain

We have prepared two fragments of the human dystrophin rod domain, each containing eight spectrin-like repeating units, by expression in Escherichia coli. The first corresponds to the central portion of the rod, the other to three repeats from the N-terminal end, fused to five repeats from the C-terminal end. The latter makes up the entire mutant rod, found in a patient with mild (Becker-type) muscular dystrophy. Both fragments were found to possess an ordered, stable structure, and had the form of short rod-like particles in the electron microscope. Molecular weight determinations by sedimentation equilibrium revealed that both polypeptides were monomeric in solution, suggesting that the dystrophin rod domain is incapable of forming an antiparallel homodimer. This supports the inference from sequence analyses [Winder et al., 1995: FEBS Lett. 369:27-33, 1996: Biochem. Soc. Trans. 24:2805] that the dystrophin rod domain lacks the arrangement of sites required for lateral self-association, and that dystrophin, unlike the other known proteins of the spectrin superfamily, may thus exist as a monomer.