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.
Kramer
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A
Max-Planck-Institut für Biochemie, D-82152 Martinsried, Germany. kramer@phys.chem.etha.ch
Wintergalen
A
A
Sieber
M
M
Galla
H J
HJ
Amrein
M
M
Guckenberger
R
R
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry*; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D006801:Humans; D018625:Microscopy, Atomic Force / Q000379:methods; D008855:Microscopy, Electron, Scanning / Q000379:methods; D008969:Molecular Sequence Data; D010715:Phosphatidylglycerols / Q000737:chemistry*; D011510:Proteolipids / Q000737:chemistry* / Q000648:ultrastructure*; D011663:Pulmonary Surfactants / Q000737:chemistry* / Q000648:ultrastructure*; D011994:Recombinant Proteins / Q000737:chemistry / Q000648:ultrastructure
D010715:Phosphatidylglycerols; D011510:Proteolipids; D011663:Pulmonary Surfactants; D011994:Recombinant Proteins; D015060:1,2-Dipalmitoylphosphatidylcholine; C030345:1,2-dipalmitoylphosphatidylglycerol