The 78 kDa glucose-regulated protein (GRP78) is the eukaryotic orthologue to the prokaryotic heat shock 70 kDa protein 5 (HSPA5). GRP78 is also sometimes referred to as BiP. GRP78 is a member of the HSP70 family and plays dynamic roles in protein regulation within the endoplasmic reticulum. GRP78 is the most abundant chaperone in the ER and plays an important role in regulating the unfolded protein response (UPR) (1). GRP78 forms a multiprotein chaperone complex with DNAJB11, HSP90B1, HYOU, PDIA2, PDIA4, PDIA6, PPIB, SDF2L1, UGT1A1, and ERP29. It assists with protein translocation, folding, assembly, and initiation of the UPR. In an unstressed cell, GRP78 also forms a stable complex with IREI, ATF6, and PERK to keep these ER stress sensors in an inactive state. Misfolded proteins relieve the interaction of GRP78 with these proteins, freeing them to initiate the UPR. GRP78 was previously thought to be localized only to the endoplasmic reticulum; however, more recent evidence suggests dynamic roles for GRP78 in the mitochondria, nucleus, and cell surface. GRP78 on the cell surface has been identified as a both a downstream target and upstream regulator of the PI3K/AKT pathway (2). GRP78 outside of the ER plays a role in many cellular processes such as proliferation, migration, apoptosis, inflammation, and immunity. GRP78 promotes tumorigenesis and metastasis and thus has become a novel target for therapeutic manipulation.
Zhang et. al. used the GRP78 antibody to elucidate a mechanism for the translocation of GRP78 from the ER to the cell surface (3). While it had become widely accepted that GRP78 could be localized to the cell surface, a mechanism for its trafficking was unknown. The group used a variety of techniques including mutagenesis, overexpression, and localization studies to begin to understand how GRP78 translocated to the plasma membrane. The group used the GRP78 antibody for fluorescence-activated cell sorting (FACS) of cells transfected with FLAG-GRP78-His. This GRP78 antibody recognizes the middle domain of GRP78, between amino acids 250-300. The group identified the FLAG and His-tag on the cell surface indicating both the N and C-termini are exposed. The GRP78 antibody also recognized the middle domain of GRP78 on the surface of the cells, indicating that all 3 domains are exposed to the cell surface. These results led the group to question if GRP78 could be stably bound to the extracellular surface of the cell instead of being anchored by a transmembrane domain. Further investigation with the GRP78 antibody led the group to conclude that GRP78 does not stably associate with the extracellular surface of the cell. Understanding the mechanism behind GRP78 translocation and its biological activity at the cell surface will help guide new therapeutic interventions.
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