Solid-state NMR could complement X-ray crystallography and cryo-EM: A novel α-helix to β-sheet structural transition for the SAF-p1/p2a designer nanofiber-forming peptide system

Anant Paravastu, Evan K. Roberts, Kong M. Wong

 

Geogia Institute of Technology School of Chemical and Biomolecular Engineering

 

Self-assembled fiber (SAF) peptides are binary systems of peptide variants that were rationally designed to assemble into α-helical coiled-coil nanofibers.  Secondary structure is promoted by “heptad repeat” patterning of hydrophobic and polar sidechains in the primary structure.  Coiled-coil assembly between complementary pairs of peptides is promoted through hydrophobic patches on α-helix surfaces and coulombic interactions between oppositely charged sidechains.  While nanofibers themselves are not compatible with X-ray crystallography, a crystal structure has been solved for an SAF variant with charge sidechains modified to promote a coiled-coil dimer.    Furthermore, cryo-electron microscopy has been employed to measure arrangements of α-helices within the nanofibers.  Our group applied solid-state NMR spectroscopy to detect alignments between hydrophobic and charged sidechains within the SAF-p1/p2a nanofiber variant.  Surprisingly, our NMR results revealed that the α-helical coiled-coil structure is unstable at temperatures between 25°C to 40°C: the system undergoes a post-assembly structural transformation to β-sheets.  This structural transformation occurs without disassembly of the peptide nanofiber, and shows evidence of reversibility at 10°C.  We hypothesize that the β-sheet structure is novel, since is it not formed through an amyloid-like assembly mechanism.