Surfaceome Profiling Identifies Basigin‐Chaperoned Protein Clients

The surface proteome or “surfaceome” is a critical mediator of cellular biology, facilitating cell‐to‐cell interactions and communication with extracellular biomolecules. Constituents of the surfaceome can serve as biomarkers for changing cell states and as targets for pharmacological intervention. While some pathways of cell surface trafficking are well characterized to allow prediction of surface localization, some non‐canonical trafficking mechanisms do not. Basigin (Bsg), a cell surface glycoprotein, has been shown to chaperone protein clients to the cell surface. However, understanding which proteins are served by Bsg is not always straightforward. To accelerate such identification, we applied a surfaceome proximity labeling method that is integrated with quantitative mass spectrometry‐based proteomics to discern changes in the surfaceome of hepatic stellate cells that occur in response to the genetic loss of Bsg. Using this strategy, we observed that the loss of Bsg leads to corresponding reductions in the cell surface expression of monocarboxylate transporters MCT1 and MCT4. We also found that these relationships were unique to Bsg and not found in neuroplastin (Nptn), a related family member. These results establish the utility of the surfaceome proximity labeling method to determine clients of cell surface chaperone proteins. A surfaceome proximity labeling method allowed for proteomic profiling of cell surface proteins. In Bsg but not in Nptn deficient cell lines, we detected the specific reduction of MCT1 and MCT4 at the cell surface, while the rest of the surfaceome remained relatively unchanged. This emphasizes Bsg's role as a specific chaperone for MCTs within the cell.