We have applied NMR spectroscopy to determine the high-resolution structure of gaegurin 4, a 37-residue antimicrobial peptide from Rana rugosa, under varying hydrophobic conditions. Even in 100% H2O, gaegurin 4 contains a nascent turn near its C-terminal Rana box. Under a more hydrophobic condition it forms two amphipathic helices, one long encompassing residues 2-23 and the other consisting of residues 25-34, similar to what has been observed in cecropin A. Functional implication of the helix-breaking kink at Gly(24) in gaegurin 4 was investigated by preparing several analogs. Based upon the current and previous results, we propose a novel seaanemone-like ion pore-forming model for gaegurin 4. (c) 2006 Elsevier Inc. All rights reserved.