Coassembly of Peptides Derived from β-Sheet Regions of β-Amyloid

In this paper, we investigate the coassembly of peptides derived from the central and C-terminal regions of the beta-amyloid peptide (A beta). In the preceding paper, J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b06000, we established that peptides containing residues 17-23 (LVFFAED) from the central region of A beta and residues 30-36 (AIIGLMV) from the C-terminal region of A beta assemble to form homotetramers consisting of two hydrogen-bonded dimers. Here, we mix these tetramer-forming peptides and determine how they coassemble. Incorporation of a single N-15 isotopic label into each peptide provides a spectroscopic probe with which to elucidate the coassembly of the peptides by H-1,N-15 HSQC. Jobs method of continuous variation and nonlinear least-squares fitting reveal that the peptides form a mixture of heterotetramers in 3:1, 2:2, and 1:3 stoichiometries, in addition to the homotetramers. These studies also establish the relative stability of each tetramer and show that the 2:2 heterotetramer predominates. N-15-Edited NOESY shows the 2:2 heterotetramer comprises two different homodimers, rather than two heterodimers. The peptides within the heterotetramer segregate in forming the homodimer subunits, but the two homodimers coassemble in forming the heterotetramer. These studies show that the central and C-terminal regions of A beta can preferentially segregate within beta-sheets and that the resulting segregated beta-sheets can further coassemble.