Ca2+-independent interaction of annexin I with phospholipid monolayers.

At pH 6.0, the interaction of annexin I, a proteolytic fragment of annexin I and annexin V, was studied with monolayers composed of dipalmitoylphosphatidylserine (DPPS), dipalmitoylphosphatidylcholine (DPPC) or DPPS/DPPC mixtures (molar ratio 1:4). The measurements reveal that only annexin I shows a significant increase in the surface pressure at constant surface area in the absence of Ca2+ ions. We interpret these pressure changes as reflecting penetration of the protein. Kinetic analyses of the annexin I/monolayer interaction at pH 6.0 in the presence and absence of Ca2+ ions show differences between the interaction mechanisms that support the occurrence of a pH-regulated process. At pH 7.4, Ca2+ ions are required for the interaction.

Distribution of the surfactant-associated protein C within a lung surfactant model film investigated by near-field optical microscopy.

Lung surfactant films at the air/water interface exhibit the particularity that surfactant molecules are expelled from the surface monolayer into a surface associated multilamellar phase during compression. They are able to re-enter the surface film during the following expansion. The underlying mechanism for this behavior is not fully understood yet. However, an important role is ascribed to the surfactant-associated protein C (SP-C). Here, we studied a model lung surfactant, consisting of dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and SP-C, by means of scanning near-field optical microscopy (SNOM). Attaching a fluorescent dye to the protein allowed the localization of its lateral distribution at various surface pressures with high resolution. At an early stage of compression, the film appears demixed into a pure lipid phase and a protein-enriched phase. Within the latter phase, protein aggregations are revealed. They show a uniform density, having three times the fluorescence intensity of their surroundings. Across the phase boundary between the lipid phase and the protein-rich phase, there is a protein density gradient rather than an abrupt border. When the film is highly compressed, we observe the formation of multilamellar structures that are fluorescent. They are often surrounded by a slightly fluorescent monolayer. The fluorescence of the multilayer stacks (i. e., the protein content per unit area) is proportional to the height of the stacks.