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 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 observed Golgi localization of O-glycan-deficient cargos is due to their slow Golgi export. Using a superresolution microscopy method that we previously developed, we revealed that O-glycan-deficient Tac chimeras localize at the interior of the trans-Golgi cisternae. O-Glycans were observed to be both necessary and sufficient for the efficient Golgi export of Tac chimeras. By sequentially introducing O-glycosylation sites to ST6GAL1, we demonstrated that O-glycan's effect on Golgi export is probably additive. Finally, the finding that N-glycosylated GFP substantially reduces the Golgi residence time of a Tac chimera suggests that N-glycans might have a similar effect. Therefore, both O- and N-glycans might function as a generic Golgi export signal at the trans-Golgi to promote the constitutive exocytic trafficking.