Formation of membrane lattice structures and their specific interactions with surfactant protein A

Biological membranes exist in many forms, one of which is known as tubular myelin (TM). This pulmonary surfactant membranous structure contains elonga ted tubes that form square lattices. To understand the interaction of surfa ctant protein (SP) A and various lipids commonly found in TRI, we undertook a series of transmission-electron-microscopic studies using purified SP-A and lipid vesicles made in vitro and also native surfactant from bovine lun g. Specimens from in vitro experiments were negatively stained with 2% uran yl acetate, whereas fixed native surfactant was delipidated, embedded, and sectioned. We found that dipalmitoylphosphatidylcholine-egg phosphatidylcho line (1:1 wt/wt) bilayers formed corrugations, folds, and predominantly 47- nm-square latticelike structures. SP-A specifically interacted with these l ipid bilayers and folds. We visualized other proteolipid structures that co uld act as intermediates for reorganizing lipids and SP-As. Such a reorgani zation could lead to the localization of SP-A in the lattice corners and co uld explain, in part, the formation of TM-like structures in vivo.