The 78 kDa glucose-regulated protein (GRP78) is an endoplasmic reticulum chaperone, whose function is generally thought to be restricted to controlling the structural maturation of nascent glycoproteins. However, GRP78 also is expressed on the cell surface where it functions as a receptor for a wide variety of ligands, behaving as an autoantigen for several classes of autoantibodies. GRP78 is a signaling receptor for activated alpha2-macroglobulin, plasminogen kringle 5, and microplasminogen, and it plays a critical role in viral entry of coxsackie B, and dengue fever viruses. GRP78 is also implicated in the regulation of tissue factor procoagulant activity and functions as a receptor for angiogenic peptides via a mechanism independent of the VEGF receptor. Cell surface GRP78 is found associated with such diverse proteins as the voltage-dependent anion channel (VDAC), the major histocompatibility complex class I (MHC-I), the teratocarcinoma-derived growth factor I (Cripto), and the DnaJ-like protein MTJ-1. These associations suggest a unique GRP78 cell surface topography, which appears to be compartmentalized to respond differently to agonists that bind to its N- or C-terminal domains. Here, we discuss the significance of these associations, and the possible mechanisms involved in the transportation of GRP78 from the cytosol to the cell surface.

Circulating autoantibodies against the glucose-regulated protein of 78 kDa (GRP78) are present at high levels in prostate cancer patients and are a biomarker of aggressive tumor behavior. We purified the anti-GRP78 IgGs and examined their effect on 1-LN, PC-3, DU145, and LnCap human prostate cancer cells. We also evaluated its effects on the breast cancer MDA-MB231 and melanoma DM413 cell lines. The anti-GRP78 antibody binds only to cells expressing GRP78 on the surface, to a site also recognized by its physiologic agonist, activated A 2 -macroglobulin (A 2 M*). This antibody is completely specific for a peptide, including the primary amino acid sequence CNVKSDKSC, which contains a tertiary structural motif mimicking an epitope in GRP78. Tertiary structual analysis suggested the linear GRP78 primary amino acid sequence LIGRTWNDPSVQQDIKFL (Leu 98 -Leu 115 ) as the putative binding site, containing the tertiary structual arrangement described above, which was confirmed experimentally. The anti-GRP78 antibodies from prostate cancer patients recognize almost exclusively this epitope. We produced animal antibodies against both these peptides, and they are able to mimic the effects of the human antibody. Our experiments also suggest this epitope as highly immunogenic, thereby explaining the specificity of the immune response against this epitope in GRP78, observed in humans. Using 1-LN cells as a model, we show that anti-GRP78 IgG purified from the sera of these patients mimics the proproliferative effects induced by A 2 M* via the common receptor, GRP78. Furthermore, increasing concentrations of human anti-GRP78 IgG show a dose-dependent protective effect on apoptosis induced by tumor necrosis factor A.

MTJ-1 associates with a glucose-regulated protein of Mr ∼78,000(GRP78) in the endoplasmic reticulum and modulates GRP78 activity as a chaperone. GRP78 also exists on the cell surface membrane, where it is associated with a number of functions. MHC class I Ags on the cell surface are complexed to GRP78. GRP78 also serves as the receptor for α2-macroglobulin-dependent signaling and for uptake of certain pathogenic viruses. The means by which GRP78, lacking a transmembrane domain, can fulfill such functions is unclear. In this study we have examined the question of whether MTJ-1, a transmembrane protein, is involved in the translocation of GRP78 to the cell surface. MTJ-1 and GRP78 coimmunoprecipitated from macrophage plasma membrane lysates. Silencing of MTJ-1 gene expression greatly reduced MTJ-1 mRNA and protein levels, but also abolished cell surface localization of GRP78. Consequently, binding of the activated and receptor-recognized form of α2-macroglobulin to macrophages was greatly reduced, and activated and receptor-recognized form of α2-macroglobulin-induced calcium signaling was abolished in these cells. In conclusion, we show that in addition to assisting the chaperone GRP78 in protein quality control in the endoplasmic reticulum, MTJ-1 is essential for transport of GRP78 to the cell surface, which serves a number of functions in immune regulation and signal transduction.