Antimicrobial peptides are evolutionarily ancient weapons distributed throughout the animal and plant kingdoms. Unlike the conventional antibiotics that target protein receptors, antimicrobial peptides act on the lipid matrix of cell membranes, a previously under-appreciated design feature that distinguishes broad species of microbes from multicellular plants and animals. Despite their ancient lineage, antimicrobial peptides have remained effective without known bacterial resistance. The goal of this application is to understand the peptide-membrane interactions in general and the design principles of antimicrobial peptides in particular. As our need for new antibiotics becomes more pressing, such principles might facilitate development of new classes of anti-infective drugs. Although peptide-lipid interactions have often been described as non-specific, studies of peptides from diverse sources have revealed a general mode of action. Peptides bind to lipid bilayers as monomers initially. As the density of bound peptides increases, pores are formed in the membrane that cause leakage and cell death. While the process prior to the pore formation has been analyzed by various techniques, it has been difficult to study the pore structures. A new technique has stabilized the pores in a lattice; thereby X-ray diffraction analysis of the pore structures has become possible. Heavy atom labeling methods of protein crystallography have also been extended to crystals of amorphous unit cells such as transmembrane pores. The techniques are also applicable to a related membrane structure formed during membrane fusion that is mediated by an insertion of fusion peptides. 3 specific aims of the application are: (1) Use the labeling methods of X-ray diffraction to analyze the structures of membrane pores induced by antimicrobial peptides. (2) Use the same methods to analyze the intermediate states of membrane fusion. The result will help us to understand the mechanism of viral infection. (3) Compare the wide type peptides with less active point-mutated analogues so as to understand the structural requirements for peptides' activity.