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 Escheric hia coli. The phasing established earlier for the dystrophin rod, as w ell as for Drosophila spectrin and smooth muscle alpha-actinin, sugges ted 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 nativ e tertiary fold could be recognized by resistance to proteolysis, the circular dichroism spectrum in the regions of both peptide and aromati c 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 furthe r residue resulted in loss of the capacity to form a defined tertiary structure. Residue 117 is a glutamine; replacement of this by a methio nine residue did not impair the ability of the chain to enter the fold ed conformation, implying that it is the length of the C-terminal alph a-helix, rather than any specific side-chain interaction, that is crit ical in determining the stability of the native structure. The fragmen t of 119 residues forms a significantly more stable structure than tha t of 117. It appears that the minimum unit capable of forming the nati ve fold extends some residues into the adjoining sequence repeat.