Intra- and Intermolecular β-Pleated Sheet Formation in Glutamine-repeat Inserted Myoglobin as a Model for Polyglutamine Diseases*

An aberrant structure of the expanded polyglutamine might be involved in the formation of aggregates in CAG repeat diseases. To elucidate structural properties of the expanded polyglutamine, we prepared sperm whale myoglobin (Mb) mutants, in which 12, 28, 35, and 50 repeats of glutamine were inserted at the corner between the C and D helices (Gln12, Gln28, Gln35, and Gln50, respectively). Circular dichroism and IR spectroscopies showed that the expanded polyglutamine, which was recognized by the monoclonal antibody 1C2 in Gln28, Gln35, and Gln50 Mb forms an antiparallel β-pleated sheet structure. Gln50 Mb aggregates were found to comprise an intermolecular antiparallel β-pleated sheet. Fluorescence together with 1H NMR spectra revealed partial unfolding of the protein surface in Gln35 and Gln50 Mb, although the structural changes in the protein core were rather small. The present results indicate that the fluctuating β-pleated sheet of the expanded polyglutamine exposed on the protein surface facilitates the formation of aggregates through intermolecular interactions. The present study has first established and characterized structural properties of a molecular model for polyglutamine diseases in which various lengths of polyglutamine including a pathologically expanded glutamine repeat were inserted into a structurally known protein.

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