SP-D, a recently described surfactant apoprotein structurally closely related to SP-A and other members of the collectin protein family, has been implicated in the host defense of the lung. Its potential role in alveolar surfactant homeostasis is unknown, however. SP-D interacts with and aggregates phosphatidylinositol (PI)-containing liposomes (Biochemistry, 31:12183-89, 1992). We previously reported the cloning and heterologous expression of mouse SP-D cDNA (rSP-D; Pediatric Research, 37:346A, 1995) and showed that it assembles as oligomers of trimers, binds the sugar maltose and aggregates PI-containing liposomes in a calcium-dependent manner.
In the present experiment, we measured rSP-D surface activity in a captive bubble surfactometer apparatus and examined the ultrastructure of reconstituted mixtures of dipalmitoylphosphatidylcholine (DPPC), PI, and dog SP-B in the presence and absence of rSP-D by thin-section electron microscopy. Minimal surface tension obtained upon first compression of PI-containing phospholipids adsorbed at the air-water-interface of the captive bubble was significantly lower in the presence of rSP-D and dog SP-B than in the presence of dog SP-B alone. The speed of phospholipid adsorption to the air water interface was not significantly altered by the presence of rSP-D. Reconstituted mixtures of DPPC-PI (7:3;wt:wt), dog SP-B and rSP-D showed striking tubular aggregates similar to those obtained with human SP-A but of different geometry (polygonal rather than square) and inter-membranous spacing(about twice that seen in the presence of SP-A). No tubular structure was seen when phosphatidylglycerol was substituted for PI.
