Lipopolysaccharide bound structures of the active fragments of fowlicidin-1, a cathelicidin family of antimicrobial and antiendotoxic peptide from chicken, determined by transferred nuclear Overhauser effect spectroscopy.

Cathelicidins comprise a major family of host-defense antimicrobial peptides in vertebrates. The C-terminal part of the cathelicidins is bestowed with antimicrobial and lipopolysaccharide (LPS) neutralizing activities. In this work, we repot high resolution solution structures of two nontoxic active fragments, residues 1-16 or RG16 and residues 8-26 or LK19, of fowlicidin-1, a cathelicidin family of peptide from chicken, as a complex with LPS using two-dimensional transferred nuclear Overhauser effect (Tr-NOE) spectroscopy. Both peptides are highly flexible and do not assume any preferred conformations in their free states. Upon complexation with endotoxin or LPS, peptides undergo structural transitions towards folded conformations. Structure calculations reveal that the LK19 peptide adopts a well defined helical structure with a bend at the middle. By contrast, the first seven amino acids of RG16 are found to be flexible followed by a helical conformation for the residues L8-A15. In addition, a truncated version of LK19 encompassing residues A15-K26 or AK12 displays an amphipathic helical structure in LPS. Saturation transfer difference (STD) NMR studies demonstrate that all peptides, RG16, LK19, and AK12, are in close proximity with LPS, whereby the aromatic residues showed the strongest STD effects. Fluorescence studies with fluorescein isothiocyanate (FITC) labeled LPS in the presence of full-length fowlicidin-1, LK19, RG16, and AK12 indicated that LPS-neutralization property of these peptides may result from plausible dissociation of LPS aggregates. The helical structures of peptide fragments derived from fowlicidin-1 in LPS could be utilized to develop nontoxic antiendotoxic compounds.