Isolation and Characterization of St-CRPs: Cysteine-Rich Peptides from the Arctic Marine Ascidian Synoicum turgens

Ascidians are a group of marine invertebrates, most of which are sessile and soft-bodied. Their lack of an adaptive immune system makes them rely on innate immune responses to detect and eliminate invading microbes. Antimicrobial peptides (AMPs) play an essential part in this process. In this paper, we present the isolation, structure elucidation, and bioactivities of two new cysteine-rich peptides (CRPs) from the Arctic marine ascidian Synoicum turgens. The sequences and structures of the peptides were determined by Edman degradation sequencing, mass spectrometry, and NMR analysis. This revealed two novel 2 kDa peptides, St-CRP-1 and St-CRP-2, with neutral net charge and C-terminal amidation. St-CRP-1 consisted of 18 amino acids and displayed selective and moderate growth inhibition of two Gram-positive bacterial strains (Bacillus subtilis and Corynebacterium glutamicum) at 24.6 µM, whereas St-CRP-2 consisted of 19 amino acids and inhibited the growth of B. subtilis at 49.2 µM. St-CRP-1 had no effect on two mammalian cell lines or the brine shrimp Artemia salina at the highest concentration tested. Structural analysis of the St-CRPs indicated a Cys1–Cys6, Cys2–Cys4, and Cys3–Cys5 disulfide connectivity, which is also found in alpha-defensins. The results from this study show that Arctic marine ascidians are a rich source of novel bioactive peptides.