SP-A-binding protein BP55 is involved in surfactant endocytosis by type II pneumocytes.

The mechanism of surfactant protein (SP)-A-mediated lipid uptake by rat type II pneumocytes was investigated. In the absence of SP-A, freshly isolated type II pneumocytes actively take up very little if any liposomes. Most of the increase with time is independent of energy or temperature but is most likely due to spontaneous exchange of labeled lipids between liposomes and cell membranes. With 5 micrograms/ml SP-A, type II cells actively take up liposomes (244 pmol dipalmitoylphosphatidylcholine.h-1.10(6) cells-1). The effect of SP-A on uptake is temperature dependent and can be abolished by ATP depletion of the cells. Coincubation with an auto-anti-idiotypic antibody against the SP-A-binding protein BP55 on the cell membrane of type II pneumocytes inhibits SP-A-mediated lipid uptake by type II cells. With increasing amounts of extracellular SP-A present, increasing amounts of liposomes are taken up and directed toward a nondegrading compartment. We suggest that SP-A-mediated surfactant lipid uptake is a receptor-mediated endocytotic process involving BP55.