Styelins, broad-spectrum antimicrobial peptides from the solitary tunicate, Styela clava.

Two novel phenylalanine-rich antimicrobial peptides, Styelin A and Styelin B, were purified from the hemocytes of Styela clava. The peptides had very similar masses (Styelin A, 3685.8; Styelin B, 3700.6) and amino acid compositions, and at least 17 of their first 20 N-terminal residues were identical. Both Styelins were effective against a panel of gram negative and gram positive bacterial pathogens of humans, usually acting with minimal inhibitory concentrations < 1.5 microgram/ml (< 0.5 microM), even in the presence of 100 mM NaCl. Styelins also killed marine bacteria, Psychrobacter immobilis and Planococcus citreus, in media containing 0.4 M NaCl. The presence of antimicrobial peptides (Styelins) in tunicate hemocytes is evidence that such molecules are ancient mediators of host defense within the vertebrate lineage. Peptide antibiotics from marine organisms could afford design template for the development of topical microbicides that manifest broad-spectrum antibacterial activity in the presence of physiological or elevated NaCl concentrations.