Glycans function as a Golgi export signal to promote the constitutive exocytic trafficking

Most proteins in the secretory pathway are glycosylated. However, the role of glycans in the membrane trafficking is still unclear. Here, we discovered that transmembrane secretory cargos, such as interleukin 2 receptor α subunit or Tac, transferrin receptor and cluster of differentiation 8a, unexpectedly displayed substantial Golgi localization when their O-glycosylation was compromised. By quantitatively measuring their Golgi residence times, we found that the apparent Golgi localization of these O-glycan deficient cargos is due to their slow Golgi export. The super-resolution microscopy method that we previously developed revealed that O-glycan deficient Tac chimeras localize at the interior of the trans -Golgi cisternae. The O-glycan was observed to be both necessary and sufficient for the efficient Golgi export of Tac chimeras. By sequentially introducing O-glycosylation sites to β-galactoside α-2,6-sialyltransferase1, we demonstrated that the O-glycan’s effect on the Golgi export is probably additive. Finally, the finding that N-glycosylated GFP substantially reduces the Golgi residence time of Tac chimera suggests that the N-glycan might have a similar effect. Therefore, both O- and N-glycan might function as a generic Golgi export signal at the trans -Golgi to promote the constitutive exocytic trafficking.