Subcellular localization of ERGIC-53 under endoplasmic reticulum stress condition.

Newly synthesized glycoproteins destined for secretion are transported from the endoplasmic reticulum (ER), through the Golgi and toward the cell surface. In this secretion pathway, several intracellular ER- or Golgi-resident transmembrane proteins serve as cargo receptors. ER-Golgi intermediate compartment (ERGIC)-53, VIP36 and VIPL, which have an L-type lectin domain within the luminal portion, participate in the vectorial transport of glycoproteins via sugar-protein interactions. To understand the nature of these receptors, monoclonal antibodies were generated against human ERGIC-53, VIP36 and VIPL using 293T cells expressing these receptors on cell surfaces. These cells were used to immunize rats and for screening antibody-producing clones. Flow cytometric analysis and immunoprecipitation studies showed that the obtained monoclonal antibodies bound specifically to the corresponding cargo receptors. Immunostaining of HeLa cells using the monoclonal antibodies showed that the localization of ERGIC-53 changed from relatively broad distribution in both the ER and the Golgi under normal conditions to a compact distribution in the Golgi under ER stress conditions. This redistribution was also observed by the overexpression of ERGIC-53 and abrogated by co-expression with VIPL but not VIP36. Real-time polymerase chain reaction revealed that ERGIC-53 along with several chaperone proteins was up-regulated after tunicamycin treatment; however, the expression of VIPL was unchanged. Furthermore, ERGIC-53 co-precipitated with VIPL but not VIP36, indicating that ERGIC-53 may interact with VIPL in the ER, which may regulate the localization of ERGIC-53 inside cells. Taken together, these observations provide new insights into the regulation of these cargo receptors and the quality control of glycoproteins within cells.