Binding and calcium-induced aggregation of laminin onto lipid bilayers

Direct binding of laminin in the form of its complex with nidogen to planar lipid bilayers was demonstrated with total internal reflection fluorescence microscopy. Binding occurred equally well to zwitterionic (phosphatidylcholine) and negatively charged (phosphatidylglycerol) lipids and was enhanced by sulfatides but only at nonphysiological molar ratios higher than 30 mol %. Strong interactions with lipid bilayers were also observed for bovine serum albumin. This explains a strong inhibition of laminin binding by this protein. However, binding of laminin to sulfatide-rich bilayers was not completely inhibited. Observable by the microscopic technique was the formation of laminin clusters on the surface of the bilayer which occurred concomitantly with binding. Both processes were strongly enhanced by the presence of calcium. These results show that calcium-induced laminin self-assembly is enhanced at lipid surfaces.