The role of Ca2+ binding in the self-aggregation of laminin-nidogen complexes

Laminin-nidogen complexes were found to aggregate in the presence of divalent cations in a manner dependent on ion concentration. This effect shows a selectivity for Ca2+, as half-maximal aggregation is achieved already at about 10 microM Ca2+, while Mg2+ induces aggregation at 10-fold higher ion concentrations and always to a lesser extent. When binding of Ca2+ to laminin-nidogen complexes was measured by equilibrium dialysis, a total of about 16 binding sites with dissociation constants in the range of 5-300 microM could be identified. At 50 microM Ca2+, where the aggregation is maximal, only two to three Ca2+ ions are bound to laminin-nidogen complexes, indicating that the aggregation reaction is induced by the binding of Ca2+ to a small number of sites and possibly to a single distinct site. Analysis of Ca2+ binding to various proteolytic fragments of laminin allowed the tentative localization of a high affinity binding site to a large fragment comprising two of the short arms connected by the central part of the laminin molecule.