Reversible Calcium-dependent Interaction of Liposomes with Pulmonary Surfactant Protein A

Surfactant protein A (SP-A) is crucial for lung function, including tubular myelin formation and lipid uptake by type II pneumocytes. Known properties of SP-A in vitro are its Ca2+-dependent interaction with phospholipids and its role in the aggregation of liposomes. To dissect and to analyze these processes, we have immobilized SP-A and measured binding of liposomes by the resonant mirror technique. Liposome aggregation was followed separately by kinetic light scattering in suspensions. It was found that SP-A-mediated binding and aggregation of liposomes have a common K0.5 of 20 microM for free Ca2+, independent of the species (sheep, rat, or cow) and of the phospholipid composition, and that both reactions exhibit the same high cooperativity (Hill coefficients of 6-9) for Ca2+ ions. However, binding of liposomes to SP-A is >10-fold faster than aggregation. Both processes are completely reversed by low Ca2+ concentrations, but liposomes dissociate from SP-A in <0.3 s, whereas disaggregation of the liposomes takes approximately 30 s. At equilibrium, the level of aggregation depends on the concentration of free SP-A. We interpret these results to be a rapid and reversible sequence of three reactions: (i) a cooperative Ca2+-dependent conformational change in SP-A, (ii) binding of Ca2+-bound SP-A to liposomes, and (iii) aggregation of the Ca2+/SP-A-bound liposomes.