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Solubility versus Electrostatics: What Determines Lipid/Protein Interaction in Lung Surfactant

M. Seifert ^*, D. Breitenstein , U. Klenz ^*, M. C. Meyer ^* and H.-J. Galla ^*

^* Institute of Biochemistry and Tascon GmbH, 48149 Münster, Germany

Correspondence: Address reprint requests to H.-J. Galla, Institute of Biochemistry, 48149 Münster, Germany. Tel.: 0049-251-833200; Fax: 0049-251-8333206; E-mail: gallah{at}uni-muenster.de.

Mammalian lung surfactant is a complex lipid/protein mixture covering the alveolar interface and has the crucial function of reducing the surface tension at this boundary to minimal values. Surfactant protein SP-B plays an important role for this purpose and was the focus of many recent studies. However, the specificity of lipid/SP-B interactions is controversial. Since these investigations were accomplished at varying pH conditions (pH 5.5 and 7.0), we studied the specificity of these interactions in a dipalmitoylphosphatidylcholine (DPPC)/dipalmitoylphosphatidylglycerol (DPPG)/SP-B (4:1:0.2 mol %) model system at either pH. Mainly fluorescence microscopy and laterally resolved time-of-flight secondary ion mass spectrometry were used to reveal information about the phase behavior of the lipids and the molecular distribution of SP-B in the lipid mixture. DPPG forms separated condensed domains due to a strong hydrogen-bond network, from which the protein is mainly excluded. Considering the protein as an impurity of the lipid mixture leads to the principle of the zone melting process: an impurity is highly more soluble in a liquid phase than in a solid phase. The phase behavior effect of the lipids mainly outperforms the electrostatic interactions between DPPG and SP-B, leading to a more passively achieved colocalization of DPPC and SP-B.

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