Review
Structure of pulmonary surfactant membranes and films: The role of proteins and lipid–protein interactions

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Abstract

The pulmonary surfactant system constitutes an excellent example of how dynamic membrane polymorphism governs some biological functions through specific lipid–lipid, lipid–protein and protein–protein interactions assembled in highly differentiated cells. Lipid–protein surfactant complexes are assembled in alveolar pneumocytes in the form of tightly packed membranes, which are stored in specialized organelles called lamellar bodies (LB). Upon secretion of LBs, surfactant develops a membrane-based network that covers rapidly and efficiently the whole respiratory surface. This membrane-based surface layer is organized in a way that permits efficient gas exchange while optimizing the encounter of many different molecules and cells at the epithelial surface, in a cross-talk essential to keep the whole organism safe from potential pathogenic invaders.

The present review summarizes what is known about the structure of the different forms of surfactant, with special emphasis on current models of the molecular organization of surfactant membrane components. The architecture and the behaviour shown by surfactant structures in vivo are interpreted, to some extent, from the interactions and the properties exhibited by different surfactant models as they have been studied in vitro, particularly addressing the possible role played by surfactant proteins. However, the limitations in structural complexity and biophysical performance of surfactant preparations reconstituted in vitro will be highlighted in particular, to allow for a proper evaluation of the significance of the experimental model systems used so far to study structure–function relationships in surfactant, and to define future challenges in the design and production of more efficient clinical surfactants.

Keywords

Lamellar body
Tubular myelin
Collectin
Saposin
Membrane domain
Raft
Lipid phase
Monolayer
Membrane fusion
SP-A
SP-B
SP-C
Air–liquid interface

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