Binding and Interaction of Human Beta Defensin Type 3 with Mixed PIP2 Lipid Membranes

Abstract

Defensins are cationic cysteine-rich small molecules with molecular masses ranging from 3 to 5 kDa. Human β defensin type 3 (hBD-3) belongs to the human innate immune system, and is mainly secreted from the epithelial cells. It has a broad-spectrum of antimicrobial activities on virus, fungi, and both Gram-positive and Gram-negative bacteria. hBD-3 can bind to the negatively charged and even zwitterionic (but charge-neutral) lipid membrane, disrupt and then translocate through the lipid membrane. In order to understand its dynamics and functional mechanism in molecular level, all-atom molecular dynamics simulations on hBD-3 binding with and crossing mixed lipid membranes were investigated. Since PIP2 is the key minor component in cell membranes, mixed PIP2 lipid membrane represented by neutrally charged POPC lipids mixed with 10% of PIP2 lipids were studied. Simulations on hBD-3 placed inside the mixed lipid bilayer, and on the surface of the lipid bilayer were conducted. In order to consider the influence of the initial binding orientation on the hBD-3 binding structure, in total five different initial binding orientations of hBD-3 on the lipid bilayer were studied. Since hBD-3 can form a dimer, the binding of hBD-3 in both monomer and dimer forms with lipid membranes were explored. The structure of hBD-3 on lipid membranes and the key residues on the binding interface were predicted and compared. During the micro-second long simulations, the assembly of PIP2 lipids towards hBD-3 was observed in both hBD-3 on the lipid membrane surface systems and inside the lipid membrane systems. hBD-3 staying inside the mixed lipid bilayer can behave as the channel to facilitate water molecules to cross the mixed lipid bilayer. The result emphasized the importance of PIP2 in hBD-3's disruption capability with the mixed cell membrane.