Expression of a cytoplasmically epitope-tagged human Golgi glycosyltransferase in homologous cells results in mislocalization of multiple Golgi proteins.

We have compared the effect of mislocalization of a Golgi glycosyltransferase in heterologous and homologous cell systems on the distribution of other Golgi-associated proteins. Mycspacer-human N-acetylglucosaminyltransferase I (NAGT-I), an N-terminally epitope-tagged NAGT-I, in which the first added negatively charged amino acid is in position 13, localizes to the endoplasmic reticulum (ER) by immunofluorescence when expressed in monkey (Vero) or human (HeLa) cells. When myc-spacer-human NAGT-I was expressed in Vero cells, the distribution of the Golgi-associated coat protein, beta-COP, was concentrated juxtanuclearly and undisturbed relative to control. When myc-spacer-human NAGT-I was expressed in HeLa cells, however, both endogenous beta-COP and GalT were no longer concentrated in a juxtanuclear manner but were rather cytoplasmically distributed as was the myc-tagged human NAGT-I. Based on these observation, we suggest that extensive interactions between proteins that normally show overlapping distributions between the medial Golgi stack and trans Golgi/TGN are possible. Moreover, we suggest that small differences in sequence may play a large role in potentiating interactions of Golgi complex proteins.