SAAP-148 oligomerizes into a hexamer forming a hydrophobic inner core.

Abstract

Human cathelicidin LL-37 is a widely studied antimicrobial 37-mer peptide with various ascribed functions, that serves as a template for designing novel peptides. Its derivative, the 24-mer SAAP-148, is highly effective in vitro in eradicating multidrug-resistant bacteria, persistent cells, and biofilms, without inducing resistance. SAAP-148 is characterized by a high cationic charge (+11) and 46% hydrophobicity, which, once the peptide folds into an alpha helix, forms a wide hydrophobic face. This highly amphipathic nature facilitates on the one hand its insertion into the membrane's fatty acyl chain region and on the other hand it´s interaction with anionic membrane components, which contributes to its mode of action in killing bacteria. However, the contributions of the secondary and quaternary structures remain underexplored. To address this, we conducted a study using anionic and zwitterionic membrane models, applying circular dichroism, NMR spectroscopy, X-ray scattering, AlphaFold 3 and molecular dynamics simulations. Our results reveal that SAAP-148 adopts a stable hexameric bundle composed of three parallel dimers, creating a hydrophobic core of aromatic residues. This structure is retained at the membrane interface, where MD simulations suggest the formation of a fiber-like structure on anionic membranes, most likely driven by exposed cationic side chains.