Glycoprotein recycling to the galactosyltransferase compartment of the Golgi complex

The recycling of plasma membrane glycoproteins to the Golgi complex is well established, but it is not clear which Golgi subcompartments receive this traffic. To date, recycling into the trans-Golgi compartment that contains sialyltransferase and the early Golgi region that contains alpha-mannosidase I has been demonstrated. However, transport into other Golgi compartments has not been reported. In this study we tested the return of cell surface glycoproteins to the Golgi galactosyltransferase compartment using the ldlD mutant of Chinese hamster ovary cells. The cation-independent mannose 6-phosphate/insulin-like growth factor-II receptor recycled through this Golgi region with a half-time of 4 h and was transported to the sialyltransferase compartment as well. Because galactosyltransferase and sialyltransferases are probably located in different trans-Golgi regions in Chinese hamster ovary cells, these results suggest that the two compartments each receive recycling traffic or that recycling glycoproteins enter one region and are then transported to the other. The extent of cell surface protein recycling through the galactosyltransferase compartment was also studied. At least 10 different glycoproteins were transported from the cell surface to this Golgi region. Moreover, our results suggest that recycling glycoproteins make up 12-25% of the flux of cell surface glycoproteins through the Golgi galactosyltransferase compartment; the balance is comprised of newly made glycoproteins.