Glucosylceramide uptake protein from spleen cytosol

Incubation of rat red cells with liposomes containing radioactive glucosylceramide, together with unlabeled phosphatidylcholine and cholesterol, produced cells labeled with the glycolipid. Part of the bound cerebroside could be removed by further incubation with unlabeled liposomes. Both processes, uptake and release, were stimulated by adding a preparation obtained from bovine spleen. The active material was purified partially by ammonium sulfate precipitation and gel permeation with a Sephadex G-75 column. The latter step indicated a molecular weight of about 22,000. The material lost much of its activity when treated with trypsin or N-ethylmaleimide, indicating that it is a protein. The "cerebroside uptake protein" was quite stable if stored in dithioerythritol at 4 degrees C. Tests with liposomes containing labeled phosphatidylcholine or cholesterol showed that the red cells took up these lipids as well, but the transfer protein exerted little effect on this. Ghosts prepared from rat red cells also took up the three lipids from the liposomes and here, as well, the uptake protein showed high specificity in facilitating the uptake of cerebroside. This appears to be the first demonstration of the existence of a glycolipid transfer protein. Possible physiological roles for the uptake protein in red cell metabolism and in the modulation of the glycosphingolipid composition of cell membranes are discussed.