Recent in vitro and in vivo studies suggest that destabilized
proteins with defective folding induce aggregation and toxicity in
protein-misfolding diseases. One such unstable
protein state is called
amyloid oligomer, a precursor of fully aggregated forms of
amyloid. Detection of various
amyloid oligomers with A11, an anti-
amyloid oligomer conformation-specific antibody, revealed that the
amyloid oligomer represents a generic conformation and suggested that toxic beta-aggregation processes possess a common mechanism. By using A11 antibody as a probe in combination with mass spectrometric analysis, we identified GroEL in
bacterial lysates as a
protein that may potentially have an
amyloid oligomer conformation. Surprisingly, A11 reacted not only with purified GroEL but also with several purified
heat shock proteins, including human Hsp27, 40, 70, 90; yeast Hsp104; and bovine Hsc70. The native folds of A11-reactive
proteins in purified samples were characterized by their anti-beta-aggregation activity in terms of both functionality and in contrast to the beta-aggregation promoting activity of misfolded pathogenic
amyloid oligomers. The conformation-dependent binding of A11 with natively folded Hsp27 was supported by the concurrent loss of A11 reactivity and anti-beta-aggregation activity of heat-treated Hsp27 samples. Moreover, we observed consistent anti-beta-aggregation activity not only by chaperones containing an
amyloid oligomer conformation but also by several A11-immunoreactive non-chaperone
proteins. From these results, we suggest that the
amyloid oligomer conformation is present in a group of natively folded
proteins. The inhibitory effects of A11 antibody on both GroEL/ES-assisted
luciferase refolding and Hsp70-mediated decelerated nucleation of Abeta aggregation suggested that the A11-binding sites on these chaperones might be functionally important. Finally, we employed a computational approach to uncover possible A11-binding sites on these targets. Since the beta-sheet edge was a common structural motif having the most similar physicochemical properties in the A11-reactive
proteins we analyzed, we propose that the beta-sheet edge in some natively folded
amyloid oligomers is designed positively to prevent beta aggregation.