The transmembrane domain of N-acetylglucosaminyltransferase I is the key determinant for its Golgi subcompartmentation

Golgi-resident type-II membrane proteins are asymmetrically distributed across the Golgi stack. The intrinsic features of the protein that determine its subcompartment-specific concentration are still largely unknown. Here, we used a series of chimeric proteins to investigate the contribution of the cytoplasmic, transmembrane and stem region of Nicotiana benthamiana N-acetylglucosaminyltransferase I (GnTI) for its cis/medial-Golgi localization and for protein-protein interaction in the Golgi. The individual GnTI protein domains were replaced with those from the well-known trans-Golgi enzyme 2,6-sialyltransferase (ST) and transiently expressed in Nicotiana benthamiana. Using co-localization analysis and N-glycan profiling, we show that the transmembrane domain of GnTI is the major determinant for its cis/medial-Golgi localization. By contrast, the stem region of GnTI contributes predominately to homomeric and heteromeric protein complex formation. Importantly, in transgenic Arabidopsis thaliana, a chimeric GnTI variant with altered sub-Golgi localization was not able to complement the GnTI-dependent glycosylation defect. Our results suggest that sequence-specific features in the transmembrane domain of GnTI account for its steady-state distribution in the cis/medial-Golgi in plants, which is a prerequisite for efficient N-glycan processing in vivo.