A Novel β-Defensin Structure: Big Defensin Changes Its N-Terminal Structure To Associate with the Target Membrane

• Published in: Biochemistry (Easton) 2009-08, Vol.48 (32), p.7629-7635
• Main Authors: Kouno, Takahide, Mizuguchi, Mineyuki, Aizawa, Tomoyasu, Shinoda, Hiroyuki, Demura, Makoto, Kawabata, Shun-ichiro, Kawano, Keiichi
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; beta-Defensins - chemistry; beta-Defensins - classification; beta-Defensins - genetics; beta-Defensins - metabolism; Cell Membrane - metabolism; Molecular Sequence Data; Nuclear Magnetic Resonance, Biomolecular; Peptides - chemistry; Peptides - genetics; Peptides - metabolism; Protein Folding; Protein Structure, Secondary; Protein Structure, Tertiary
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi900756y

Big defensin is a 79-residue peptide derived from hemocytes of the Japanese horseshoe crab. The amino acid sequence of big defensin is divided into an N-terminal hydrophobic domain and a C-terminal cationic domain, which are responsible for antimicrobial activities against Gram-positive and -negative bacteria, respectively. The N-terminal domain of big defensin forms a unique globular conformation with two α-helices and a parallel β-sheet, while the C-terminal domain adopts a β-defensin-like fold. Although our previous study implied that big defensin changes its N-terminal structure in a micellar environment, due to the poor quality of the NMR spectra it remained to be resolved whether the N-terminal domain adopts any structure in the presence of micelles. In this analysis, we successfully determined the structure of the N-terminal fragment of big defensin in a micellar solution, showing that the fragment peptide forms a single α-helix structure. Moreover, NMR experiments using paramagnetic probes revealed that the N-terminal domain of big defensin penetrates into the micelle with a dipping at the N-terminal edge of the α-helix. Here, we propose a model for how big defensin associates with the target membrane.