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Coalescence of Spherical Beads of Retro-HSP12.6 into Linear and Ring-Shaped Amyloid Nanofibers

A. Shukla, M. Raje, and P. Guptasarma*

Institute of Microbial Technology, Sector 39-A, Chandigarh 160036, India; fax: (91-172) 269-5085; E-mail: pg@imtech.res.in

* To whom correspondence should be addressed.

Received July 17, 2007; Revision received October 12, 2007
The sequence-reversed form of a small heat shock protein, HSP12.6 (retro-HSP12.6), has been reported to fold and assemble into structured tetramers in aqueous solution. Upon raising the protein concentration to ~1.0-1.5 mg/ml, tetrameric retro-HSP12.6 is known to display a tendency to associate further into spherical beads of 18-20 nm in diameter containing folded protein subunits. Here we report that storage of this protein at low temperatures leads to further association of the beaded structures into linear and ring-shaped amyloid nanofibers of 18-20 nm in diameter. The electron micrographs presented in this communication provide the best visual evidence yet that amyloids can form through the association of smaller structured bead-like intermediates. The results also suggest that folded beta-sheet-rich subunits can participate in amyloid formation.
KEY WORDS: protein aggregation, amyloid-like aggregates, linear and ring-shaped amyloids, bead-amyloid transformation, amyloid filament assembly, folded protein aggregation

DOI: 10.1134/S0006297908060084