The Cryo-EM structures of two amphibian antimicrobial cross-β amyloid fibrils

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• The amyloid-antimicrobial link hypothesis proposes shared properties between human amyloids and antimicrobial peptides (AMPs).
• Human amyloids are implicated in neurodegenerative and systemic diseases, while AMPs possess antimicrobial functions.
• Amyloidal structural features are observed in AMPs, suggesting a potential link.

Purpose of the Study:

• To determine the fibril structure of two amphibian AMPs, uperin 3.5 and aurein 3.3, using cryogenic electron microscopy (cryo-EM).
• To provide atomic-resolution structural data supporting the amyloid-antimicrobial link hypothesis.
• To investigate the secondary structure mechanisms of these AMPs.

Main Methods:

• Cryogenic electron microscopy (cryo-EM) was employed to determine the fibril structures of uperin 3.5 and aurein 3.3 at atomic resolution.
• Structural analysis focused on identifying amyloid cross-β fibril conformations and peptide arrangements.
• Comparison with previously determined crystallographic structures (for uperin 3.5) was performed.

Main Results:

• Both uperin 3.5 and aurein 3.3 formed amyloid cross-β fibrils composed of mated β-sheets.
• Uperin 3.5 adopted a 3-blade symmetrical propeller structure with nine peptides per layer, complementing its known cross-α conformation and indicating a secondary structure switch.
• Aurein 3.3 formed fibrils with six peptides per layer, featuring kinked β-sheets that contribute to a compact structure, similar to human functional amyloids (LARKS).

Conclusions:

• The study provides atomic-resolution cryo-EM structures of uperin 3.5 and aurein 3.3 amyloid fibrils.
• These findings support the amyloid-antimicrobial link hypothesis by demonstrating amyloid fibril formation in AMPs.
• The structural insights reveal mechanisms of secondary structure switching and functional amyloid-like features in antimicrobial peptides.