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Rattusin is an α-defensin-related peptide isolated from the small intestine of rats. The primary sequence of linear rattusin is composed of 31 amino acids containing five cysteines with a unique spacing pattern. It forms a homodimeric scaffold in which the primary structure occurs in an antiparallel fashion formed by five intermolecular disulfide (SS) bonds. Rattusin is a highly potent antibiotic, which not only exhibits broad-spectrum antimicrobial activity, but also maintains its antimicrobial activity at physiological salt concentrations. Therefore, to develop new antibiotics based on rattusin, structural and functional studies of rattusin should be performed. For this purpose, large amounts of linear rattusin precursor must be obtained through appropriate preparation methods. Therefore, we established a mass production technique for linear rattusin by using recombinant protein expression and purification procedures. We verified that structure and activity of the recombinant rattusin are identical to the chemically synthesized rattusin. The described method for producing recombinant rattusin provides a high yield of rattusin, which can be used to study the biochemical and functional properties of rattusin and for the development of rattusin-based peptide antibiotics.

Defensin peptides are essential for innate immunity in humans and other living systems, as they provide protection against infectious pathogens and regulate the immune response. Here, we report the solution structure of rattusin (RTSN), an α-defensin-related peptide, which revealed a novel C2-symmetric disulfide-linked dimeric structure. RTSN was synthesized by solid-phase peptide synthesis (SPPS) and refolded by air oxidation in vitro. Dimerization of the refolded RTSN (r-RTSN) resulted from five intermolecular disulfide (SS) bond exchanges formed by ten cysteines within two protomer chains. The SS bond pairings of r-RTSN were determined by mass analysis of peptide fragments cleaved by trypsin digestion. In addition to mass analysis, nuclear magnetic resonance (NMR) experiments for a C15S mutant and r-RTSN confirmed that the intermolecular SS bond structure of r-RTSN showed an I-V’, II-IV’, III-III’, IV-II’, V-I’ arrangement. The overall structure of r-RTSN exhibited a cylindrical array, similar to that of β-sandwich folds, with a highly basic surface. Furthermore, fluorescence spectroscopy results suggest that r-RTSN exerts bactericidal activity by damaging membrane integrity. Collectively, these results provide a novel structural scaffold for designing highly potent peptide-based antibiotics suitable for use under various physiological conditions.