Treatise Online no. 61: Part N, Revised, Volume 1, Chapter 4: The early shell: Ontogeny, features, and evolution

Abstract

Many transcription factors and some other proteins contain glutamine repeats; their abnormal expan-sion has been linked to several dominantly inherited neuro-degenerative diseases. Having found that poly(L-glutamine) alone forms 13-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 this hypothesis, we have engi-neered a Gly-Gln1o-Gly peptide into the inhibitory loop of truncated chymotrypsin inhibitor 2 (C12), a small protein from barley seeds, by both insertion and replacement. Gel filtration resolved both mutant inhibitors into at least three fractions, which analytical ultracentrifugation identified as monomers, dimers, and trimers of the recombinant protein; the truncated wild-type C12 formed only monomers. CD difference spectra of the dimers and trimers versus wild type indicated that their glutamine repeats formed ,B-pleated sheets, while those of the monomers versus wild type were more suggestive of type I 18-turns. The CD spectra of all three