Characterization of the membrane-associating domain of the sperm adhesive protein, bindin

Bindin is an adhesive protein that mediates the binding of sea urchin sperm to the egg during fertilization. Bindin selectively associates with gel-phase phospholipid vesicles in a peripheral fashion. Bindin interacts specifically with sulfated fucan on the egg's surface, and directly with the phospholipid bilayer of the sperm. Analysis of a series of deletion mutants of recombinant bindin was undertaken to define the membrane associating domain of bindin. Recombinant and sperm bindin display nearly identical binding kinetics to gel-phase phospholipids and have equivalent saturation points of approx. 250 lipid molecules per molecule of bindin. Deletion mutants of bindin which contain residues 75-130 retained specific membrane binding activity. Synthetic peptides corresponding to residues 69-130, and 92-130 also display gel-phase specific membrane association. This region is highly conserved within four different species of bindin molecules. Circular dichroism spectroscopy of synthetic peptides corresponding to residues 92-130 and 69-130 suggests that a distinct change in conformation takes place upon binding liposomes. Taken together, these data indicate that the membrane binding activity of bindin residues within this highly conserved region of the bindin molecule.