To develop novel Pro-rich model AMPs with shorter length and higher bacterial selectivity/therapeutic index (TI) than natural AMP, indolicidin, we synthesized a series of undodecapeptides derived from the sequence XXPXXPWXPXX-NH2 (X indicates Leu or Lys) with different ratios of Lys and Leu residues. Several Pro-rich model peptides (K7 WP3, K6 WL1 P3, K5 WL2 P3-1, K5 WL2 P3-2, and K4 WL3 P3) had approximate 8- to 11-fold higher bacterial selectivity/TI compared to indolicidin. These peptides selectively bind to negatively charged liposomes (EYPG/EYPG; 7:3, w/w) mimicking bacterial membranes. Their high selectivity to negatively charged phospholipids corresponds well with their high bacterial selectivity. Indolicidin showed almost complete depolarization of the cytoplasmic membrane of Staphylococcus aureus and dye-leakage from negatively charged liposomes at 10 microM, whereas all of Pro-rich model peptides had very little activity in these assays even at 80 microM, as observed in buforin 2. These results suggest that the ultimate target of our designed Pro-rich model peptides is probably the intracellular components (e.g. protein, DNA or RNA) rather than the cytoplasmic membranes. Collectively, our designed Pro-rich short model peptides appear to be excellent candidates for future development as a novel antimicrobial agent.

Caspases, Asp-specific cysteine protease, cleave proteins upon apoptosis. To identify and characterize new caspase substrate in the nucleus, the proteome of the rat liver extracts was analyzed after the treatment with caspases. One of the identified proteins was KSRP/FBP2 that is preferentially cleaved by caspase-3 and 7 at two sites after Asp102 and Asp183. The second site was cleaved only in the protein produced in cells, but not in in vitro translated protein. These results indicate that more than the primary sequence may be important for the recognition by caspases.