Membrane disruptive antimicrobial activities of human β-defensin-3 analogs

Human beta defensin-3 (HβD-3) is a host-defense protein exhibiting antibacterial activity towards both Gram-negative and Gram-positive bacteria. There is considerable interest in the function of this protein due to its increased salt tolerance and activity against Gram-positive Staphylococcus aureus. In this study, analogs of HβD-3 devoid of N and C terminal regions are investigated to determine the influence of specific structural motif on antimicrobial activity and selectivity between Gram-positive and Gram-negative bacteria. Circular dichroism, fluorescence and solid-state NMR experiments have been used to investigate the conformation and mode of action of HβD3 analogs with various model membranes to mimic bacterial inner and outer membranes and also mammalian membranes. Our studies specifically focused on determining four major characteristics: (i) interaction of HβD3 analogs with phospholipid vesicles composed of zwitterionic PC or anionic PE:PG vesicles and LPS; (ii) conformation of HβD3-peptide analogs in the presence of PC or PE:PG vesicles; (iii) ability of HβD3 analogs to permeate phospholipid vesicles composed of PC or PE:PG; and (iv) activities on bacteria cells and erythrocytes. Our results infer that the linear peptide L25P and its cyclic form C25P are more active than L21P and C21P analogs. However, they are less active than the parent peptide, thus pointing towards the importance of the N terminal domain in its biological activity. The variation in the activities of L21P/C21P and L25P/C25P also suggest the importance of the positively charged residues at the C terminus in providing selectivity particularly to Gram-negative bacteria.