Biophys J, January 2000, p. 458-465, Vol. 78, No. 1

Distribution of the Surfactant-Associated Protein C within a Lung Surfactant Model Film Investigated by Near-Field Optical Microscopy

A. Kramer,^* A. Wintergalen, M. Sieber, H.J. Galla, M. Amrein, and R. Guckenberger^*

 ^*Max-Planck-Institut für Biochemie, D-82152 Martinsried, Germany;  Institut für Biochemie and  Institut für Medizinische Physik und Biophysik, Westfälische Wilhelms-Universität, D-48149 Münster, Germany

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.

Biophys J, January 2000, p. 458-465, Vol. 78, No. 1
© 2000 by the Biophysical Society   0006-3495/00/01/458/08  $2.00

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