Self-Assembly of the Membrane-Bound β-Stranded Peptide (KIGAKI)3 into Immobilized Amyloid Fibrils Observed by Solid-State 19F-NMR

The structure and membrane alignmentof β-stranded antimicrobial peptide KIGAKI [(KIGAKI)3-NH2] has been determined in oriented DMPC bilayers using circular dichroism and solid state 19F-NMR spectroscopy. CF3-Bpg was used as a reporter group to label the peptide, where several IIe or Ala residues were individually replaced one by one. At high peptide-to-lipid molar ratios (P/L) of 1:200 or above, the 19F dipolar coupling of all five labels exhibits the maximum possible value, which is indicative of a parallel alignment of the Cα-CF3 bond vector with respect to the bilayer normal. This finding suggests that at high concentration KIGAKI self-assembles into immobilized -sheets, which lie flat on the membrane surface. Transmission electron microscopy images reveal that the aggregated KIGAKI forms amyloid like fibrils. At low peptide concentrations, on the other hand, the dipolar couplings of the CF3-groups indicate that their time-averaged alignment is still parallel to the bilayer normal, but the mobility of the peptides increases drastically and reflects a monomeric state. Correspondingly, the flexible β-strands float on the membrane surface and undergo motional averaging in the 2-dimensional membrane plane, similar to intrinsically unstructured proteins in solution. This is the first example of concentration dependent transition of a flexible β-strand to an amyloid like fibril in membranes that is directly observed by solid state NMR.