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Amyloid Formation of a Protein in the Absence of Initial Unfolding and Destabilization of the Native State

Gemma Soldi ^*, Francesco Bemporad ^*, Silvia Torrassa , Annalisa Relini , Matteo Ramazzotti ^*, Niccolò Taddei ^* and Fabrizio Chiti ^*

^* Dipartimento di Scienze Biochimiche, Università di Firenze, Firenze, Italy; and Istituto Nazionale di Fisica della Materia and Dipartimento di Fisica, Università di Genova, Genoa, Italy

Correspondence: Address reprint requests to Fabrizio Chiti, Dipartimento di Scienze Biochimiche, Università di Firenze, Viale Morgagni 50, 50134 Firenze, Italy. E-mail: fchiti{at}scibio.unifi.it.

In 5% (v/v) trifluoroethanol, pH 5.5, 25°C one of the acylphosphatases from Drosophila melanogaster (AcPDro2) forms fibrillar aggregates that bind thioflavin T and Congo red and have an extensive ß-sheet structure, as revealed by circular dichroism. Atomic force microscopy indicates that the fibrils and their constituent protofilaments have diameters compatible with those of natural amyloid fibrils. Spectroscopic and biochemical investigation, carried out using near- and far-UV circular dichroism, intrinsic and 1-anilino-8-naphthalenesulfonic acid-derived fluorescence, dynamic light scattering, and enzymatic activity assays, shows that AcPDro2 has, before aggregation, a secondary structure content packing around aromatic and hydrophobic residues, hydrodynamic diameter, and catalytic activity indistinguishable from those of the native protein. The native protein was found to have the same conformational stability under native and aggregating conditions, as determined from urea-induced unfolding. The kinetic analysis supports models in which AcPDro2 aggregates initially without need to unfold and subsequently undergoes a conformational change into amyloid-like structures. Although fully or partially unfolded states have a higher propensity to aggregate, the residual aggregation potential that proteins maintain upon complete folding can be physiologically relevant and be directly involved in the pathogenesis of some protein deposition diseases.

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