Cross-α/β polymorphism of PSMα3 fibrils

Abstract

The formation of ordered cross-β amyloid protein aggregates is associated with a variety of human disorders. While conventional infrared methods serve as sensitive reporters of the presence of these amyloids, the recently discovered amyloid secondary structure of cross-α fibrils presents new questions and challenges. Herein, we report results using Fourier transform infrared spectroscopy and two-dimensional infrared spectroscopy to monitor the aggregation of one such cross-α–forming peptide, phenol soluble modulin alpha 3 (PSMα3). Phenol soluble modulins (PSMs) are involved in the formation and stabilization of Staphylococcus aureus biofilms, making sensitive methods of detecting and characterizing these fibrils a pressing need. Our experimental data coupled with spectroscopic simulations reveals the simultaneous presence of cross-α and cross-β polymorphs within samples of PSMα3 fibrils. We also report a new spectroscopic feature indicative of cross-α fibrils.

Natural Antimicrobial Peptides Self-assemble as α/β Chameleon Amyloids

“…Specifically, for PSMα3, although the major population was α-helical, a minor population with β-rich content was observed. This observation was recently validated by two-dimensional infrared (2DIR) studies, which identified a peak related to cross-β structures, appearing after four days of incubation, in addition to the main peak attributed to α-helical content 55 . This suggests that the chameleon secondary structure switch is a widespread phenomenon, inherent to many self-assembling polymorphic sequences forming toxic amyloid fibrils.…”

“…Peaks in the region of 1637–1645 cm -1 indicate disordered species, partially overlapping with peaks at 1630–1643 cm -1 correlating with small and disordered β-rich amyloid fibrils with absorbance typical of bent β-sheets in native proteins 48,52–54 . Peaks in the region of 1645–1662 cm -1 indicate the existence of α-helices 48,52–54 , but may overlap with random coil structures, especially for broad peaks 55,56 ; therefore, both options are indicated (“R/α”) in Table 1. In case of a broad peak that covered both α, β and disordered ranges, the secondary structure is denoted as “mixed” in Table 1.…”

“…In the right panels, the spectra are of the peptides incubated in D2O for 2 h (orange curve), 24 h (green curve) or 72 h (blue curve). Peaks in the region of 1611–1630 cm -1 as well as ∼1685–1695 cm -1 are indicative of rigid cross-β fibrils, 1637–1645 cm -1 indicate disordered species, partially overlapping with peaks at 1630–1643 cm -1 correlating with small and disordered β-rich amyloid fibrils with absorbance which is typical of bent β-sheets in native proteins, and 1645–1662 cm -1 indicate the existence of α-helices 17–20 , which overlap with random coil structures, especially for broad peaks 21,22 (both are therefore indicated in Table 1). The peaks are indicated based on the second derivative calculated by OPUS software.…”

Natural Antimicrobial Peptides Self-assemble as α/β Chameleon Amyloids

“…Specifically, for PSMα3, although the major population was α-helical, a minor population with β-rich content was observed. This observation was recently validated by two-dimensional infrared studies, which identified a peak related to cross-β structures, appearing after 4 days of incubation, in addition to the main peak attributed to α-helical content . This suggests that the chameleon secondary structure switch is a widespread phenomenon, inherent to many self-assembling polymorphic sequences forming toxic amyloid fibrils.…”

“…Peaks in the region of 1611–1630 cm –1 and ∼1685–1695 cm –1 are indicative of rigid cross-β fibrils. Peaks in the region of 1637–1645 cm –1 indicate disordered species, partially overlapping with peaks at 1630–1643 cm –1 correlating with small and disordered β-rich amyloid fibrils with absorbance typical of bent β-sheets in native proteins. ,,, Peaks in the region of 1645–1662 cm –1 indicate the existence of α-helices ,,, but may overlap with random coil structures, especially for broad peaks; , therefore, both options are indicated (“R/α”) in Table . In the case of a broad peak that covered both α, β, and disordered ranges, the secondary structure is denoted as “mixed” in Table .…”