The escalating crisis of antimicrobial resistance in aquaculture, driven by the indiscriminate use of antibiotics, underscores the urgent need to develop novel anti-infective agents. This study addresses this requirement by investigating cysteine-rich antimicrobial peptides (AMPs) in understudied crustacean species. A cysteine-rich AMP, designated PpRcys1, was identified and characterized from the genome of Pollicipes pollicipes. PpRcys1 comprises 104 amino acids, with 85 residues forming the mature peptide region, and exhibits random coils, a CSαβ-fold, and one β-sheet. Our findings demonstrated that recombinant PpRcys1 (rPpRcys1) possesses broad-spectrum antimicrobial activity against three Gram-positive bacteria (Staphylococcus aureus, Bacillus sp. T2, and Streptococcus agalactiae) and four Gram-negative bacteria (Aeromonas hydrophila, Escherichia coli, Vibrio alginolyticus, and Acinetobacter sp. L3), with minimum inhibitory concentrations ranging from 8 to 32 μM. It exerts antimicrobial effects by inducing membrane disruption without impacting bacterial protease activity, DNA migration, or respiratory chain reductase activity. Further investigation is warranted to determine whether it can target and interfere with intracellular bacterial processes. Our discovery and characterization of this novel AMP provide a promising foundation for its development as an alternative to antibiotics.

The excessive use of conventional antibiotics in aquaculture has created significant challenges, making it essential to explore and develop effective alternatives. Antimicrobial peptides (AMPs) have gained attention as potential therapeutic agents owing to their wide-ranging antibacterial effects and their ability to address pathogens resistant to conventional drugs. PpRcys1 is an antimicrobial peptide that mainly targets bacterial cell membranes, exhibiting a minimum inhibitory concentration of 8–32 μM. Its antibacterial activity should be further optimized. Before such optimization, however, it is crucial to identify the key amino acid residues that determine its functional activity. In this study, molecular dynamics simulations indicated that arginine 40 (ARG40), lysine 55 (LYS55), lysine 90 (LYS90), and lysine 93 (LYS93) play critical roles in the interaction between PpRcys1 and bacterial membranes. To investigate this further, these residues were mutated to serine, producing the mutant peptide PpRcys1_RMRK. Compared with PpRcys1, the mutant peptide PpRcys1_RMRK showed a significant reduction in antibacterial activity. Results from molecular dynamics simulations, Western blot, and ELISA demonstrated a marked decrease in its ability to bind to bacterial cell membranes. Membrane permeation assays, cell membrane depolarization experiments, and scanning electron microscopy revealed that PpRcys1 could not compromise the integrity of the bacterial membrane after losing ARG40, LYS55, LYS90 and LYS93. These findings highlight the critical roles of ARG40, LYS55, LYS90, and LYS93 in sustaining the antibacterial activity of PpRcys1. This study provides important initial insights into the structure–activity relationship of PpRcys1 and establishes a theoretical foundation for its future optimization.