Golgi enzymes do not cycle through the endoplasmic reticulum during protein secretion or mitosis

Villeneuvea, Julien; Duran, Joan; Scarpa, Margherita; Bassaganyas Bars, Laia, 1985-; van Galen, Josse; Malhotra, Vivek

doi:http://dx.doi.org/10.1091/mbc.e16-08-0560

Golgi enzymes do not cycle through the endoplasmic reticulum during protein secretion or mitosis

License

Citation

Villeneuve J, Duran J, Scarpa M, Bassaganyas L, Van Galen J, Malhotra V. Golgi enzymes do not cycle through the endoplasmic reticulum during protein secretion or mitosis. MBoC. 2017 Jan;28(1):141-51. DOI: 10.1091/mbc.e16-08-0560

Abstract

Golgi-specific sialyltransferase (ST) expressed as a chimera with the rapamycin-binding domain of mTOR, FRB, relocates to the endoplasmic reticulum (ER) in cells exposed to rapamycin that also express invariant chain (Ii)-FKBP in the ER. This result has been taken to indicate that Golgi-resident enzymes cycle to the ER constitutively. We show that ST-FRB is trapped in the ER even without Ii-FKBP upon rapamycin addition. This is because ER-Golgi–cycling FKBP proteins contain a C-terminal KDEL-like sequence, bind ST-FRB in the Golgi, and are transported together back to the ER by KDEL receptor–mediated retrograde transport. Moreover, depletion of KDEL receptor prevents trapping of ST-FRB in the ER by rapamycin. Thus ST-FRB cycles artificially by binding to FKBP domain–containing proteins. In addition, Golgi-specific O-linked glycosylation of a resident ER protein occurs only upon artificial fusion of Golgi membranes with ER. Together these findings support the consensus view that there is no appreciable mixing of Golgi-resident enzymes with ER under normal conditions.