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

Abstract

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 super-resolution 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.

A quantitative study of the Golgi retention of glycosyltransferases

“…Similar to constitutive secretory cargo such as Tac and VSVG, the Golgi pool of a glycosyltransferase is determined by the net effect of the following three trafficking pathways, as we previously discussed (Fig. 3A) (Sun et al, 2020): (1) the ER-to-Golgi (anterograde), (2) the Golgi-to-ER (retrograde) and (3) the Golgi-to-PM (exocytic) trafficking pathways. ER-to-Golgi transport increases the Golgi pool whereas the Golgi-to-PM and Golgi-to-ER pathways deplete it.…”

“…Here, using ST as a primary example, we demonstrate that most post-Golgi glycosyltransferases are not retrieved to the Golgi, and they shed their EDs extracellularly. Owing to the lack of the retrieval pathway, we proposed to adopt Golgi residence time (Sun et al, 2020) as a metric for the Golgi retention of glycosyltransferases. By this approach, we assessed quantitatively and systematically the contribution of the NCT, TMD and ED to the Golgi localization of ST. We found that all three regions contribute additively to efficient Golgi retention of ST and that the NCT length of ST plays a negative role.…”

Visualizing intra-Golgi localization and transport by side-averaging Golgi ministacks

“…We have recently measured the Golgi residence time of VSVG to be 12 min. Furthermore, we found that different constitutive transmembrane secretory cargos have distinct Golgi residence times, which are determined by their Golgi export signals, such as the linked glycan, the transmembrane domain, and the cytosolic tail ( Sun et al, 2021 ; Sun et al, 2020 ).…”

“…4 C ). The length of time that VSVG pauses at the trans-Golgi is indicated by its Golgi residence time, which can be measured by the half-life of the Golgi fluorescence decay after the release of 20°C block ( Sun et al, 2021 ; Sun et al, 2020 ). We have recently measured the Golgi residence time of VSVG to be 12 min.…”

Endogenous and Exogenous Regulatory Signaling in the Secretory Pathway: Role of Golgi Signaling Molecules in Cancer

“…The recent identification of new Golgi-localized oncogenes and tumor suppressors encourages the hypothesis that many other molecules of this kind have yet to be identified ( McKinnon and Mellor, 2017 ; Obata et al, 2017 ; Jensch et al, 2018 ; Sun et al, 2020 ). Unfortunately, despite the correlation between Golgi-based signaling and the maintenance of specific tumor phenotypes, a systematic analysis in a broad spectrum of tumors is still missing.…”