Relation between Serum Amyloid A Truncated Peptides and Their Suprastructure Chirality

摘要

Amyloids are pathological fibrillar aggregates of proteins related to over 20 diseases. Amyloid fibers are characterized by the cross-β motif, which is minimally defined as a series of β-strands extended perpendicular to the fiber axis, joined by hydrogen bonds parallel to the fiber direction. Several structures, all in agreement with the cross-β definition, have been proposed for specific amyloids. We study the correlation among the suprastructural chirality, molecular structure, and molecular chirality of amyloids. Here we investigate the suprastructure chirality of different (all-S) serum amyloid A (SAA) truncated peptides. We found that the suprastructure chirality of amyloid fibers from segments SAA2−6, SAA1−11 and the majority of those from SAA2−9 is left-handed, which is consistent with the β-sheet protofilament model. In contrast, SAA1−12 and SAA2−12 as well as SAA1−12, where the C-terminal aspartic acid was point mutated to either leucine or alanine, form right-handed helical amyloid fibers. Such a suprastructure switch indicates a molecular change in the protofilament structure. This is supported by the behavior observed in the FTIR spectra, where the amide I peak of all of the right-handed fibers is red shifted relative to the left-handed amyloid fibers. This work is a case study where isolated short fragments of SAA containing the same amyloidogenic core sequence fold into different amyloid structures. We show that core sequences, supposed to start the misfolding aggregation of the full-length amyloid peptides, may have structures different from those assumed by the isolated segments.