GLYCOLIPID AND GLYCOPROTEIN TRANSPORT THROUGH THE GOLGI-COMPLEX ARE SIMILAR BIOCHEMICALLY AND KINETICALLY - RECONSTITUTION OF GLYCOLIPID TRANSPORT IN A CELL FREE SYSTEM

Glycolipid transport between compartments of the Golgi apparatus has been reconstituted in a cell free system. Transport of lactosylceramide (galactose β1-4-glucose-ceramide) was followed from a donor to an acceptor Golgi population. The major glycolipid in CHO cells is G(M3) (sialic acid α2-3 galactose β1-4-glucose-ceramide). Donor membranes were derived from a Chinese hamster ovary (CHO) cell mutant (Lec2) deficient in the Golgi CMP-sialic acid transporter, and therefore contained lactosylceramide as the predominant glycolipid. Acceptor Golgi apparatus was prepared from another mutant, Lec8, which is defective in UDP-Gal transport. Thus, glucosylceramide is the major glycolipid in Lec8 cells. Transport was measured by the incorporation of labeled sialic acid into lactosylceramide (present originally in the donor) by transport to acceptor membranes, forming G(M3). This incorporation was dependent on ATP, cytosolic components, intact membranes, and elevated temperature. Donor membranes were prepared from Lec2 cells infected with vesicular stomatitus virus (VSV). These membranes therefore contain the VSV membrane glycoprotein, G protein. Donor membranes derived from VSV-infected cells could then be used to monitor both glycolipid and glycoprotein transport. Transport of these two types of molecules between Golgi compartments was compared biochemically and kinetically. Glycolipid transport required the N-ethylmaleimide sensitive factor previously shown to act in glycoprotein transport (Glick, B.S., and J.E. Rothman. 1987. Nature [Lond.]. 326:309-312; Rothman, J.E. 1987. J. Biol. Chem. 262:12502-12510). GTPγS inhibited glycolipid and glycoprotein transport similarly. The kinetics of transport of glycolipid and glycoprotein were also compared. The kinetics of transport to the end of the pathway were similar, as were the kinetics of movement into a defined transport intermediate. It is concluded that glycolipid and glycoprotein transport through the Golgi occur by similar if not identical mechanisms.