The study of membrane proteins’ residence at the Golgi

Abstract

The Golgi works as a major protein-sorting hub in the secretory pathway and is essential for the post-translational modifications such as glycosylation. However, what determines a protein’s Golgi retention or export at the molecular level remains unclear. To investigate the retention mechanisms of Golgi residents, the Golgi transmembrane protein human β-galactoside-α2,6-sialyltransferase (ST) was utilized as a reporter. Through the use of domain swapping and live-cell imaging, we observed that all three regions of ST—including the cytosolic tail, the transmembrane domain (TMD), and the luminal region—promote its Golgi retention in an additive manner. Compared to the luminal region, the cytosolic tail and the TMD make major contributions to the retention of ST at the Golgi. We found that the larger the cytosolic tail is in size, the shorter the Golgi residence time of ST. Moreover, elongating TMD length was observed to gradually decrease the Golgi retention of ST. To explore the molecular machinery that regulates the Golgi retention of Golgi enzymes, we measured the Golgi residence times of glycosylation enzymes upon the depletion of various candidate machinery components. We found that knockdown of Golgi phosphoprotein 3 (GOLPH3), conserved oligomeric Golgi (COG) complex and Golgi-associated retrograde protein (GARP) complex—but not retromer—significantly reduces the Golgi retention of ST and N-acetylglucosaminyltransferase I (MGAT1). To explore Golgi export signals in transmembrane secretory proteins, interleukin-2 receptor α subunit (IL2Rα; also known as Tac), a plasma membrane-localized type I transmembrane protein, was utilized. In contrast to wild type Tac, Tac without its extracellular domain localizes to the Golgi. Further domain truncation revealed that the stem region is essential for the Golgi export of Tac. The present research aimed to determine how the short juxtamembrane stem region contributes to the Golgi export of Tac. We discovered that mucin-type O-glycosylation at the stem region is necessary for the Golgi export of Tac by O-glycosylation null mutagenesis or the O-glycosylation inhibitor benzyl 2-acetamido-2-deoxy-α-D-galactopyranoside (GalNAc-O-Bn). Further investigations of the effect of O-glycosylation on the Golgi export of other transmembrane proteins such as the cluster of differentiation 8 (CD8a) and transferrin receptor (TfR) revealed that O-glycosylation is a general requirement for the Golgi export of transmembrane secretory cargoes. On the other hand, introducing the O-glycosylated Tac stem region into ST can compromise the Golgi retention of ST, suggesting that O-glycosylation is sufficient to promote the Golgi export of Golgi residents. We also found that the N-glycosylation can facilitate Golgi export. In summary, our study uncovered a novel cellular function of glycans, that they can function as a generic Golgi export signal of membrane proteins.