Using a Peptide System to test the Coiled-Coil Model of Polyglutamine Aggregation

Abstract

We test a recent hypothesis from the literature that proposes a role for helical interactions in polyglutamine aggregation. We use as our model system a peptide sequence based on the GCN4-pLI parallel coiled-coil tetramer, with lysines at all e and c heptad positions, fused to a short polyglutamine of 25 residues. The coiled-coil sequence will bind preferentially to a second peptide, ecE, which contains glutamates at the same e and c heptad positions, to form a stable tetramer. using CD, FTIR, and DLS methods, we show that the lysine-containing peptide, ecK-Q25KK, when studied alone, shows a significant decrease in aggregation kinetics when compared to a control peptide, KKQ25KK. We show that ecK-Q25KK aggregates via a coiled-coil rich intermediate over a period of a week, and then transitions to a cross beta-sheet conformation after an additional week. Such a transition is not observed when ecE peptide is present, presumably due to increased stabilization of the coiled-coil conformation. We added a proline-containing linker between the coiled-coil sequence and the polyQ sequence to test whether inhibition of helix propagation would influence the kinetics of this assembly reaction. We found that the addition of the linker significantly slowed down the assembly kinetics. We also studied the morphology of the resultant fibrils, using AFM and TEM, and found that the fibrils are quite similar to those observed previously for the KKQ25KK model system. Our results are surprising since the natural N-terminal sequence from the huntingtin protein significantly accelerates beta-sheet aggregation, also in a model involving coiled-coil interactions. The difference in our results from these earlier experiments may lie in the stability of the coiled-coil interactions, with our coiled-coil sequence (28 residues) being longer than that found in the huntingtin sequence (17 residues).