Filaments of surfactant protein A specifically interact with corrugated surfaces of phospholipid membranes

Pulmonary surfactant, a mixture of lipids and surfactant proteins (SPs), pl ays an important role in respiration and gas exchange. SP-A, the major SP, exists as an octadecamer that can self-associate to form elongated protein filaments in vitro. We have studied here the association of purified bovine SP-A with lipid vesicle bilayers in vitro with negative staining with uran yl acetate and transmission electron microscopy. Native bovine surfactant w as also examined by transmission electron microscopy of thinly sectioned em bedded material. Lipid vesicles made from dipalmitoylphosphatidylcholine an d egg phosphatidylcholine (1:1 wt/wt) generally showed a smooth surface mor phology, but some large vesicles showed a corrugated one. On the smooth-sur faced vesicles, SP-As primarily interacted in the form of separate octadeca mers or as multidirectional protein networks. On the surfaces of the striat ed vesicles, SP-As primarily formed regularly spaced unidirectional filamen ts. The mean spacing between adjacent striations and between adjacent filam ents was 49 nm. The striated surfaces were not essential for the formation of filaments but appeared to stabilize them. In native surfactant preparati ons, SP-A was detected in the dense layers. This latter arrangement of the lipid bilayer-associated SP-As supported the potential relevance of the in vitro structures to the in vivo situation.