Besides key roles in prey capture and predator defense, scorpion venom also functions as internal immune agents protecting the venom gland from infection and external immune agents cleaning saprophytic microbes from their own body surfaces. However, antimicrobials (typically antimicrobial peptides, AMPs) in the venom often exist in low abundance that might exclude their immune role alone, leaving an open question with regard to their in vivo biological function. Here, we report the bactericidal activity of seven peptides isolated from the scorpion Mesobuthus eupeus venom, including one classical α-helical AMP and five ion channel-targeted neurotoxins. This AMP of 49 amino acids (named Meucin-49) is a multifunctional molecule that displays a wide-spectrum and highly potent activity against Gram-positive and Gram-negative bacteria with strong hemotoxicity on scorpion's predators (i.e., mammals, lizards, and birds) and high insecticidal activity. Although the neurotoxins targeting voltage-gated sodium (Nav) and/or large conductance calcium-activated potassium (BK) channels showed only marginal activity towards several species of bacteria, they were capable of significantly potentiating the bactericidal potency of Meucin-49. This observation highlights, for the first time, the venom's antibacterial immune function mediated by a joint action between neurotoxins and AMPs. The findings that traditionally defined neurotoxins possess (synergistic) bactericidal activity, while the classical AMPs play predatory and defensive roles, provide new evidence in favor of a general and intrinsic multifunctionality of scorpion venom components.
Keywords: Cytotoxic peptide; Hemotoxin; Innate immunity; Mesobuthus eupeus; Moonlighting protein.