The cytoplasmic tail of alpha 1,3-galactosyltransferase inhibits Golgi localization of the full-length enzyme.

It is currently under debate whether the mechanism of Golgi retention of different glycosyltransferases is determined by sequences in the transmembrane, luminal, or cytoplasmic domains or a combination of these domains. We have shown that the cytoplasmic domains of alpha1,3-galactosyltransferase (GT) and alpha1,2-fucosyltransferase (FT) are involved in Golgi localization. Here we show that the cytoplasmic tails of GT and FT are sufficient to confer specific Golgi localization. Further, we show that the expression of only the cytoplasmic tail of GT can lead to displacement or inhibition of binding of the whole transferase and that cells expressing the cytoplasmic tail of GT were not able to express full-length GT or its product, Galalpha1,3Gal. Thus, the presence of the cytoplasmic tail prevented the localization and function of full-length GT, suggesting a possible specific Golgi binding site for GT. The effect was not altered by the inclusion of the transmembrane domain. Although the transmembrane domain may act as an anchor, these data show that, for GT, only the cytoplasmic tail is involved in specific localization to the Golgi.