INCORPORATION OF GLUTAMINE REPEATS MAKES PROTEIN OLIGOMERIZE - IMPLICATIONS FOR NEURODEGENERATIVE DISEASES

Many transcription factors and some other proteins contain glutamine r epeats; their abnormal expansion has been linked to several dominantly inherited neurodegenerative diseases, Having found that poly(L-glutam ine) alone forms beta-strands held together by hydrogen bonds between their amide groups, we surmised that glutamine repeats may form polar zippers, an unusual motif for protein-protein interactions. To test th is hypothesis, we have engineered a Gly-Gln(10)-Gly peptide into the i nhibitory loop of truncated chymotrypsin inhibitor 2 (CI2), a small pr otein from barley seeds, by both insertion and replacement. Gel filtra tion resolved both mutant inhibitors into at least three fractions, wh ich analytical ultracentrifugation identified as monomers, dimers, and trimers of the recombinant protein; the truncated wild-type CI2 forme d only monomers, CD difference spectra of the dimers and trimers versu s wild type indicated that their glutamine repeats formed beta-pleated sheets, while those of the monomers versus wild type were more sugges tive of type I beta-turns. The CD spectra of all three fractions remai ned unchanged even after incubation at 70 degrees C; neither the dimer s nor the trimers dissociated at this temperature. We argue that the s tability of all three fractions is due to the multiplicity of hydrogen bonds between extended Strands of glutamine repeats in the oligomers or within a beta-hairpin formed by the single glutamine repeat of each monomer. Pathological effects may arise when expanded glutamine repea ts cause proteins to acquire excessively high affinities for each othe r or for other proteins with glutamine repeats.