(672e) Investigating the Structure and Dynamics of Human ? Defensin Type 3 Interacting with Chemokine Receptor CXCR4 in Lipid Bilayers

Human β defensin type 3 (hBD-3) is a cationic cysteine rich peptide. It belongs to the human innate immune system, mainly secreted from human epithelial tissues and mucosa. It has broad-spectrum of bactericidal activities and also chemotactic activity by directly binding with chemokine receptor CXCR4. It has six cysteine residues that are capable of forming three pairs of intramolecular disulfide bonds which can help restrain its structure (called wildtype). These disulfide bonds are known to break in a particular order in a reducing environment, converting hBD-3 into the linear analog form. The disulfide bonding status of hBD-3 may influence its chemotactic activities comparing the naturally occurring structure (wildtype) with three disulfide bonds connected with the linear-analog structure (analog). hBD-3 may also form a dimer at high concentrations in solution.

CXCR4 is a membrane-bound G-protein coupled receptor (GPCR) involved with chemotaxis in lymphocytes. Because CXCR4 is involved in inflammatory diseases and in the replication of HIV-1, it has become a target of interest for therapeutic control. This study focuses on structural changes that can modify the chemotactic activities of hBD-3. A clear understanding of the function of hBD-3 on CXCR4 is important for development of a novel endogenous treatment. All-atom molecular dynamics simulations have been performed on hBD-3 monomer and dimer in both wildtype and analog forms interacting with the CXCR4 receptor embedded inside lipid bilayers. To consider the lipid membrane effect on the structure and dynamics of the binding, two kinds of membranes were worked on, one is the negatively charged POPG bilayer to represent bacterial lipid membrane and the other is the neutrally charged POPC bilayer to represent normal red-blood cell membrane. The protein-in-membrane system was set up, solvated and ionized using VMD program. The simulations were performed at 310.15 K and 1 atm running NAMD simulations for at least 240 ns. Select simulations were extended for up to 4.8 microseconds on ANTON2 machine. The binding structure and dynamics of hBD-3 with the CXCR4 receptor were studied, including RMSD, RMSF, residue pair distance map, radius of gyration, dynamic network analysis, and dihedral angle correlation matrix. Results up to now support a monomer structure binding with the second extracellular loop of CXCR4. Dimers of hBD-3 binding to CXCR4 have proven to be mostly unstable in microsecond scale simulations, and replacing POPC with POPG lipid membrane can influence the binding stability of hBD-3 with CXCR4.