Reversible supramolecular assembly of the anti-microbial peptide plectasin into helical non-amyloid fibrils

Self-assembly and fibril formation play important roles in protein behavior. Amyloid fibrils formation is well-studied due to its role in neurodegenerative diseases and characterized by refolding of the protein into predominant {beta}-sheet form. However, much less is known about the assembly of proteins into other types of supramolecular structures. Using cryo-electron microscopy at a resolution of 1.97 [A], we show that a triple-mutant of the anti-microbial peptide plectasin assembles reversibly into helical non-amyloid fibrils. Plectasin contains a cysteine-stabilized -helix-{beta}-sheets structure, which remains intact upon fibril formation. Two fibrils form a right-handed superstructure with each fibril consisting of double helical, left-handed structures. The fibril formation is reversible and follows sigmoidal kinetics with a pH-dependent equilibrium between soluble monomer and protein fibril. The anti-microbial activity does not appear compromised by fibril formation. This is the first high-resolution structure of this type of /{beta} protein fibrils.