INTRAMOLECULAR DISULFIDE BONDS ENHANCE THE ANTIMICROBIAL AND LYTIC ACTIVITIES OF PROTEGRINS AT PHYSIOLOGICAL SODIUM-CHLORIDE CONCENTRATIONS

Protegrins are 2-kDa antimicrobial peptides that contain 16-18 amino a cid residues and two intramolecular disulfide bonds. We studied the co ntribution of these disulfide bonds to the bactericidal activity of pr otegrins in physiological concentrations of NaCl by comparing protegri n PG-1 with variants that lacked one or both cysteine disulfides. Wher eas the bactericidal and liposome-lytic properties of protegrin PG-1 w ere enhanced by adding 100 mM NaCl to the phosphate-buffered medium, N aCl addition strongly inhibited the effects of its linearized, disulfi de free variant, [A6, A8, A13, A15]protegrin-1. Whereas protegrin PG-1 manifested beta-sheet structure by CD (circular dichroism) and ATR-FT IR ated-total-reflectance-Fourier-transform-infrared) spectroscopy in buffer or membrane-mimetic environments, [A6, A8, A13, A15]protegrin-1 manifested disordered structure in phosphate buffer and alpha-helical characteristics in membrane-mimetic environments. Both single-disulfi de protegrin variants, [A8, A13]protegrin-1 and [A6, A15]protegrin-1, assumed beta-sheet conformations with liposomes that simulated bacteri al membranes, and both retained substantial bactericidal activity when 100 mM NaCl was present. These findings demonstrate that the intramol ecular disulfide bonds of protegrins are required for their antiparall el beta-sheet conformation in membrane-mimetic environments and for th eir potent antimicrobial activity in media containing NaCl concentrati ons comparable to those found In serum and extracellular fluids.