Amyloid diseases are caused by the aberrant assembly of a protein in the extracellular space. Folded proteins are not amyloidogenic; however, the native state is generally in equilibrium with a minor population of unfolded or partially folded aggregation-competent conformers outside of the cell. Understanding how the partially unfolded conformers kinetically partition between the competing refolding and aggregation pathways provides insight into how misfolding, which occurs continuously, becomes pathogenic. Towards this end, we have previously studied the amyloidogenicity of transthyretin (TTR), a human beta-sheet-rich homotetrameric protein that must undergo rate-limiting tetramer dissociation and partial monomer unfolding to misassemble into amyloid and other aggregates. We demonstrate herein that TTR homotetramers reassemble by an unusual monomer-dimer-trimer-tetramer (MDRT) pathway. Therefore, the rate of every step in the reassembly pathway is dependent on the concentration of folded TTR monomer. Partitioning soluble TTR monomers between the reassembly pathway and the aggregation pathway should therefore depend on the relative concentrations of aggregates and assembly intermediates. Aggregate clearance is envisioned to play an important role in the partitioning of protein in vivo, where partitioning to the aggregation pathway becomes increasingly favorable under conditions where the concentration of aggregates is increased because aggregate clearance is slow relative to the rate of aggregation. This shift from efficient to inefficient aggregate clearance could occur with aging, offering an explanation for the age-associated nature of these neurodegenerative diseases.
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