Conformational studies on amyloid beta peptide (A beta) in aqueous solution are complicated by its tendency to aggregate. In this study, we determined the atomic-level structure of A beta(28-42) in an aqueous environment. We fused fragments of A beta, residues 10-24 (A beta(10-24)) or 28-42 (A beta(28-42)), to three positions in the C-terminal region of ribonuclease HII from a hyperthermophile, Thermococcus kodakaraensis (Tk-RNase HII). We then examined the structural properties in an aqueous environment. The host protein, Tk-RNase HII, is highly stable and the C-terminal region has relatively little interaction with other parts. CD spectroscopy and thermal denaturation experiments demonstrated that the guest amyloidogenic sequences did not affect the overall structure of the Tk-RNase HII. Crystal structure analysis of Tk-RNase HII1-197-A beta(28-42) revealed that A beta(28-42) forms a beta conformation, whereas the original structure in Tk-RNase HII1-213 was alpha helix, suggesting beta-structure formation of A beta(28-42) within full-length A beta in aqueous solution. A beta(28-42) enhanced aggregation of the host protein more strongly than A beta(10-24). These results and other reports suggest that after proteolytic cleavage, the C-terminal region of A beta adopts a beta conformation in an aqueous environment and induces aggregation, and that the central region of A beta plays a critical role in fibril formation. This study also indicates that this fusion technique is useful for obtaining structural information with atomic resolution for amyloidogenic peptides in aqueous environments.
