Dolichyl phosphate-mediated mannosyl transfer through liposomal membranes

A partially purified (up to 1000-fold) mannosyl transferase that catalyzed the reversible reaction GDP-Man + Dol-P in equilibrium Dol-P-Man + GDP was incorporated into liposomes consisting of soybean lecithin and dolichyl phosphate (Dol-P). The enzyme transferred the mannosyl moiety from external GDP-Man to liposome-associated Dol-P. However, when the liposomes were preloaded with GDP, mannosyl residues were also transferred to the inside, giving rise to internal GDP-Man by the reverse reaction. This transfer of an activated sugar through a membrane required the presence of Dol-P and the enzyme in the liposome. Mannosyl residues were not transferred to the inside when the liposomes were preloaded with ADP or GMP. Amphomycin completely inhibited the formation of Dol-P-Man as well as the transfer of mannose into the liposomes. The results are taken as evidence for the open postulated role of dolichols in sugar translocation through membranes. The data are discussed in relation to glycoprotein synthesis at the endoplasmic reticulum.