The interaction of lung annexin I with phospholipid monolayers at the air/water interface

Lung annexin I (LAI), a calcium-ion-dependent phospholipid-binding protein, has been shown earlier to cause aggregation and fusion of bilayered vesicles containing phospholipids found in lung surfactant, and to be a very likely factor in the assembly of lung surfactant into the lamellar bodies stored in the Type II cell. In this study, we have measured the accumulation of LAI into spread monolayers of some major lipid components of lung surfactant, dipalmitoyl-phosphatidylcholine (DPPC), dipalmitoyl-phosphatidylglycerol (DPPG), palmitoyl-oleyoyl-phosphatidylglycerol (POPG), and selected mixtures, as a function of calcium-ion concentration and surface concentration (degree of packing) of the phospholipid monolayer. The ability of LAI to significantly penetrate such monolayers was calcium-ion-dependent and only occurred in the presence of DPPG or POPG. The relative extent of penetration into DPPG and POPG was directly related to the available free area in the monolayer, penetration being greater with POPG. Fluorescence microscopy measurements revealed that DPPC mixed with either DPPG or POPG caused a change in surface phase behavior in a manner believed to be related to certain types of bilayer fusion. A chemical breakdown product of LAI, LAI-bp, previously found not to cause aggregation and fusion of bilayers, did not exhibit comparable monolayer penetration or surface phase separation to LAI.