Studies on Glycolipid Transfer Protein

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Glycolipid transfer protein (GL-TP) purified from pig brain contains the major component (80%) of apparent molecular weight of 22K and the minor component (15%) of molecular weight of 21K. The major component has three free sulfhydryl groups per molecule and the minor component has one free sulfhydryl group per molecule. One sulfhydryl group common to both components locates on the surface of the molecule and can be modified with n-ethylmaleimide. The minor component is formed from the major component when an intramolecular disulfide bond is formed from the other two sulfhydryl groups. Results so far obtained indicate that the GL-TP with an intramolecular disulfide bond has twice more activity than the reduced form of GL-TP. Much larger partion of GL-TP was found to form GL-TP. Glycolipid complex, an intermediate of the transfer reaction, when GL-TP contained an intramolecular disulfide bond.Purified GL-TP was subjected to the cleavage with cyanogen bromide. Partial amino acid sequence was determined from the peptides formed by the cleavage. A cDNA cloning is in progress based on the sequence obtained.Specific antibody against the purified GL-TP was prepared. A protein of molecular weight about 20K, immunoreactive with the antibody, and with glycolipid transfer activity has been found in PK(15) cells, a porcine epithelial cell line, ret brain, human kidney, mouse neuroblastoma cell lines, and PC12 cells, a mouse pheochromocytoma line. GL-TP seems to be present in most animal cells. A study on the subcellular localization of GL-TP in PK(15) cells indicates that GL-TP is a cytoplasmic protein. GL-TP seems to participate in the transloctaion of glucosylceramide, synthesized on the cytoplasmic side of cis Golgi membrane, to middle and trans Golgi cisternae, where further glycosylation of glucosylceramide takes place on the luminal face of the Golgi apparatus.