GOLGI LOCALIZATION IN YEAST IS MEDIATED BY THE MEMBRANE ANCHOR REGIONOF RAT-LIVER SIALYLTRANSFERASE

To investigate the function of the membrane anchor region of a mammali an glycosyltransferase in yeast we constructed a fusion gene that enco des the 34 amino terminal residues of rat liver beta-galactoside alpha -2,6-sialyltransferase (EC 2.4.99.1) (ST) fused to the mature form of yeast invertase. Transformants of Saccharomyces cerevisiae expressing the fusion gene produced an intracellular heterogeneously N-glycosylat ed fusion protein of intermediate molecular weight between the core an d fully extended N-glycosylated form of invertase, suggesting a post e ndoplasmic reticulum (ER) localization. In two types of cell fractiona tion using sucrose density gradients the ST-invertase fusion protein c ofractionated with Golgi marker proteins, whereas a minor fraction (ab out 30%) comigrated with a vacuolar marker; ST-invertase was not detec ted in other cell fractions including the ER and the plasma membrane. Consistent with Golgi localization, about 70% of the total amount of t he ST-invertase fusion was immunoprecipitated with an antibody directe d against alpha-1,6 mannose linkages. The results demonstrate that the membrane anchor region of a mammalian type II glycosyltransferase is able to target a protein to the secretory pathway and to a Golgi compa rtment of the yeast S. cerevisiae, indicating conservation of targetin g mechanisms between higher and lower eukaryotes. Since typical yeast Golgi localization signals are missing in the ST-membrane anchor regio n the results also suggest that yeast as mammalian cells utilize diver se mechanisms to direct proteins to the Golgi.