Nucleation of Huntingtin Aggregation Proceeds via Conformational Conversion of Pre-Formed, Sparsely-Populated...

Pathogenic huntingtin exon-1 protein (htt^ex1), 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 htt^ex1 construct with a 35-residue polyglutamine repeat, htt^ex1Q₃₅. Using exchange NMR spectroscopy, it is previously shown that the reversible formation of a sparsely-populated tetramer of the N-terminal amphiphilic domain of htt^ex1Q₃₅, 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 htt^ex1Q₃₅ 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 htt^ex1Q₃₅ aggregation.