Among all amino acids repeats in eukaryotes, polyasparagine (polyN) is the second most frequent, after polyglutamine (polyQ). Expanded glutamine repeats in proteins are associated with several neurodegenerative disorders, and their aggregation is believed to be directly causative to pathology. On the other hand, albeit the widespread presence of polyN repeats in invertebrate eukaryotes, polyN is curiously rare in vertebrates. The conformational and aggregation properties of polyQ-containing peptides and proteins have been extensively studied, but virtually nothing is known about polyN-containing peptides. A comparative study of polyQ and polyN could elucidate the reasons for the paucity of N repeats in humans, and their common appearance in invertebrates. In this work, we report the synthesis of polyN peptides containing up to 24 asparagines. Disaggregation protocols developed for polyQ were not effective on polyN, suggesting that the driving force for self-association is stronger for polyN than for polyQ. We developed an improved disaggregation protocol for polyN that avoided oxidation and afforded monomeric peptide. Circular dichroism analysis revealed a greater propensity for beta-turn formation in polyN compared to polyQ, providing an explanation for the increased stability of polyN aggregates relative to polyQ. Using nanoparticle tracking (NTA), we analyzed aggregation kinetics of polyN compared to polyQ for several different repeat lengths. Our data demonstrate the utility of detailed particle-by-particle analysis to elucidate mechanisms of aggregation. Our results suggest that polyN aggregation follows a nucleation-elongation mechanism due to its higher beta-turn propensity, whereas polyQ undergoes association-rearrangement mechanism.

Asparagine-repeat peptides: Synthesis, characterization, and comparison to glutamine repeats

249th National Meeting (2015)

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