Nucleation of Huntingtin Aggregation Proceeds via Conformational Conversion of Pre‐Formed, Sparsely‐Populated Tetramers

Abstract

AbstractPathogenic huntingtin exon‐1 protein (httex1), characterized by an expanded polyglutamine tract located between the N‐terminal amphiphilic region and a C‐terminal polyproline‐rich domain, forms fibrils that accumulate in neuronal inclusion bodies, and is associated with a fatal, autosomal dominant neurodegenerative condition known as Huntington's disease. Here a complete kinetic model is described for aggregation/fibril formation of a httex1 construct with a 35‐residue polyglutamine repeat, httex1Q35. Using exchange NMR spectroscopy, it is previously shown that the reversible formation of a sparsely‐populated tetramer of the N‐terminal amphiphilic domain of httex1Q35, comprising a D2 symmetric four‐helix bundle, occurs on the microsecond time‐scale and is a prerequisite for subsequent nucleation and fibril formation on a time scale that is many orders of magnitude slower (hours). Here a unified kinetic model of httex1Q35 aggregation is developed in which fast, reversible tetramerization is directly linked to slow irreversible fibril formation via conversion of pre‐equilibrated tetrameric species to “active”, chain elongation‐capable nuclei by conformational re‐arrangement with a finite, monomer‐independent rate. The unified model permits global quantitative analysis of reversible tetramerization and irreversible fibril formation from a time series of 1H‐15N correlation spectra recorded during the course of httex1Q35 aggregation.