Protegrin 1 (PG-1) is a broad-spectrum antimicrobial peptide that contains 18 amino acid residues (RG GRLCYCRRRFCVCVGR) and has two intramolecular cystine disulfide bonds. To determine the minimal structure responsible for protegrin-mediated activity against Neisseria gonorrhoeae, we synthesized 15 protegrin variants and tested them against two well-characterized gonococcal strains. The MICs of PG-1 were 0.61 microM (1.31 microg/ml) for the serum-sensitive strain F 62 and 0.98 microM (2.11 microg/ml) for the serum-resistant strain FA 19. Six amino acid residues (Arg1, Gly2, Gly3, Arg4, Gly17, and Arg18) and either disulfide bond could be deleted from PG-1 without impairing its potency against strain F 62. In contrast, only Gly17 and Arg18 could be removed without decreasing its activity against FA 19. Protegrin congener 64a (PC-64a; LTYCRRRFCVTV), a variant of PG-1 with 12 amino acid residues and one disulfide bond, displayed MICs of 0.45 microM (0.68 microg/ml) for strain F 62 and 1.37 microM (2.07 microg/ml) for strain FA 19, which approximated those of intact PG-1. Serum-sensitive sac-1+ and sac-3+ transformants of N. gonorrhoeae FA 19 and two FA 19 derivatives with truncated lipooligosaccharide structures were more susceptible to PG-1 and variants with altered disulfide structures. These data suggest that structurally simpler protegrin variants, such as PC-64a, could be used as topical microbicides for N. gonorrhoeae. They also suggest that the cystine-stabilized antiparallel beta-sheet formed by PG-1 residues 5 to 16 is principally responsible for its activity against gonococci.