Minimum folding unit of dystrophin rod domain

Fragments of the rod domain of dystrophin, which consists of spectrin-like repeating sequences, have been prepared by expression in Escherichia coli. The phasing established earlier for the dystrophin rod, as well as for Drosophila spectrin and smooth muscle alpha-actinin, suggested a length of less than 113 residues for the dystrophin repeat that we have chosen. Fragments with a common N-terminus and lengths between 113 and 119 residues were prepared. The formation of the stable native tertiary fold could be recognized by resistance to proteolysis, the circular dichroism spectrum in the regions of both peptide and aromatic absorption bands and the resolution of the long-wavelength component in the tryptophan absorption spectrum. It was found that the critical length for folding was 117 residues: shortening the chain by 1 further residue resulted in loss of the capacity to form a defined tertiary structure. Residue 117 is a glutamine; replacement of this by a methionine residue did not impair the ability of the chain to enter the folded conformation, implying that it is the length of the C-terminal alpha-helix, rather than any specific side-chain interaction, that is critical in determining the stability of the native structure. The fragment of 119 residues forms a significantly more stable structure than that of 117. It appears that the minimum unit capable of forming the native fold extends some residues into the adjoining sequence repeat.