Characterization and Mechanism of Stress-induced Translocation of 78-Kilodalton Glucose-regulated Protein (GRP78) to the Cell Surface

Glucose-regulated protein (GRP78)/BiP, a major chaperone in the endoplasmic reticulum, is recently discovered to be preferably expressed on the surface of stressed cancer cells, where it regulates critical oncogenic signaling pathways and is emerging as a target for anti-cancer therapy while sparing normal organs. However, because GRP78 does not contain classical transmembrane domains, its mechanism of transport and its anchoring at the cell surface are poorly understood. Using a combination of biochemical, mutational, FACS, and single molecule super-resolution imaging approaches, we discovered that GRP78 majorly exists as a peripheral protein on plasma membrane via interaction with other cell surface proteins including glycosylphosphatidylinositol-anchored proteins. Moreover, cell surface GRP78 expression requires its substrate binding activity but is independent of ATP binding or a membrane insertion motif conserved with HSP70. Unexpectedly, different cancer cell lines rely on different mechanisms for GRP78 cell surface translocation, implying that the process is cell context-dependent. Background: ER stress induces cell surface translocation of GRP78/BiP. Results: GRP78 translocation to the cell surface requires its substrate binding domain and exists majorly as a peripheral protein. Conclusion: GRP78 anchors on the cell surface via interaction with other proteins, and the translocation mechanism is cell context-dependent. Significance: Learning how GRP78 exists on the cell surface is crucial for understanding its signaling regulatory functions.