Structural and dynamical changes of the bindin B18 peptide upon binding to lipid membranes.: A solid-state NMR study

Structural and dynamical features of the B 18 peptide from the sea urchin sperm bindin protein were determined in the crystalline state and in zwitterionic lipid bilayers at a peptide:lipid molar ratio of 1:12 using solid-state NMR spectroscopy. The study was focused on three C-13 and N-15 uniformly labeled leucine residues, which were introduced into three different B18 peptides at positions evenly distributed along the B18 primary structure. Isotropic C-13 and N-15 chemical shift measurements showed that while B 18 possesses a nonhelical and non-sheet-like structure in the crystalline state, the peptide adopts an oligomeric beta-sheet structure in the membrane in the presence of Zn2+ ions at high peptide:lipid ratio. Torsion angle measurements for the three leucine sites supported these results, with phi torsion angles between -80degrees and -90degrees in the crystalline state and between -110degrees and -120degrees in the membrane-bound form. These phi torsion angles determined for membrane-bound B18 are consistent with a parallel beta-sheet secondary structure. Analysis of motionally averaged dipolar coupling measurements established an increase of the mobility in the leucine side chains upon binding to the membrane, whereas the backbone mobility remained essentially unchanged, except in the binding site of Zn2+ ions. This difference in mobility was related to the H-bond network in the parallel beta-sheet structure, which involves the backbone and excludes the side chains of leucine residues. The parallel beta-sheet structure of B 18 in the membrane in the presence of Zn2+ appears to be an active state for the fusion of zwitterionic membranes in the presence of Zn2+. A fluorescence fusion assay indicated that high B 18 concentrations are required to induce fusion in these systems. Therefore, it was hypothesized that the oligomeric beta-sheet secondary structure revealed in the study represents an active state of the peptide in a membrane environment during fusion.