The Golgi localization of Arabidopsis thaliana β1,2-xylosyltransferase in plant cells is dependent on its cytoplasmic and transmembrane sequences

To investigate the targeting of proteins to the plant Golgi we studied Arabidopsis thaliana beta1,2-xylosyltransferase (XylT), a glycosyltransferase which is unique to plants and some invertebrates. Different deletion constructs of the putative cytoplasmic (C)-transmembrane (T)-stem (S) region of the enzyme were transiently expressed in the tobacco-related model plant species Nicotiana benthamiana. Subcellular localization of fusion proteins between CTS, CT, T, or C domains and the reporter molecule green fluorescent protein by fluorescence microcopy and density-gradient centrifugation revealed that the CT region alone is sufficient to sustain Golgi retention of XylT without the contribution of any luminal sequences. The finding of an incomplete retention by the T region alone suggests an important auxiliary role of the C domain in Golgi retention of the protein. However, the C segment did not confer any Golgi retention by itself, as the respective fusion protein was found exclusively in the cytoplasm. These results provide evidence that plant and mammalian cells rely on similar mechanisms to deliver glycosyltransferases to the Golgi apparatus.