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

摘要

Big defensin is a 79-residue peptide derived from hemocytes of the Japanese horseshoe crab. Thenamino acid sequence of big defensin is divided into an N-terminal hydrophobic domain and a C-terminalncationic domain, which are responsible for antimicrobial activities against Gram-positive and -negativenbacteria, respectively. TheN-terminal domain of big defensin forms a unique globular conformation with twonR-helices and a parallel β-sheet, while the C-terminal domain adopts a β-defensin-like fold. Although ournprevious study implied that big defensin changes itsN-terminal structure in amicellar environment, due to thenpoor quality of the NMR spectra it remained to be resolved whether the N-terminal domain adopts anynstructure in the presence of micelles. In this analysis, we successfully determined the structure of thenN-terminal fragment of big defensin in a micellar solution, showing that the fragment peptide forms a singlenR-helix structure. Moreover, NMR experiments using paramagnetic probes revealed that the N-terminalndomain of big defensin penetrates into the micelle with a dipping at the N-terminal edge of the R-helix. Here,nwe propose a model for how big defensin associates with the target membrane.