Isolation, characterization and anti-cancer activity of SK84, a novel glycine-rich antimicrobial peptide from Drosophila virilis

We report herein the isolation and characterization of a novel glycine-rich antimicrobial peptide purified from the larvae of Drosophila virilis. A range of chromatographic methods was used for isolation and its antibacterial activity against Bacillus subtilis was employed to screen for the most active fractions. The peptide, termed SK84 due to its N-terminal serine, C-terminal lysine and a total of 84 residues, was completed sequenced using RT-PCR cDNA cloning. SK84 contains a high level of glycine (15.5%) and a hexaglycine cluster motif in the N-terminal part. SK84 displayed antibacterial activity against the tested Gram-positive bacteria (B. subtilis, Bacillus thuringiensis and Staphylococcus aureus), but had no effect on Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli) and fungi (Saccharomyces cerevisiae, Candida albicans). SK84 had specific inhibitory effects on the proliferation of several cancer cell lines (Human leukemia THP-1, liver cancer HepG2, and breast cancer MCF-7 cells), but no hemolytic activity. The results from scanning electron microscopy observations revealed that SK84 killed THP-1 cells by destroying the cell membranes. Alignment results show that SK84 is a mature protein processed from the pseudoprotein GJ19999 from D. virilis, and is very similar to several pseudoproteins from different Drosophila species. Our results show that SK84 represents a novel glycine-rich peptide family in Drosophila species with antimicrobial and anti-cancer cell activities.