UCSF

Cell surface proteins (CSPs) are integral to cellular function and intercellular communication in complex tissues. They also comprise over 70% of drug targets, and yet robust methods for mapping them with cell-type specificity in living organisms are lacking. While proximity labeling techniques have advanced our understanding of molecular interactions within the cell, no existing tool enables selective tagging of the extracellular proteome of specific cell types in vivo. Here, we present TORCH (Tyrosinase Oxidizer for Rapid Chemical Highlighting), an engineered tyrosinase enzyme enabling rapid, genetically targeted proximity labeling of cell surface proteomes in living organisms using solely ambient oxygen as a cofactor. Developed through directed evolution and structure-guided engineering, TORCH achieves ~15-fold increased efficiency and robust mammalian expression compared with wild-type tyrosinase. Beyond cell-intrinsic profiling, TORCH's rapid kinetics enable detection of transient cell-cell interactions by labeling proteins at contact interfaces, exemplified by in vivo tagging of tumor-immune interactions. We demonstrate TORCH’s utility by profiling blood-brain barrier (BBB) endothelial cell surface proteomes directly in living mice and identify prion protein (PRNP) as a GPI-anchored brain endothelial cell target that shuttles PRNP-binding antibody across the BBB into the brain parenchyma. Overall, TORCH provides a robust and versatile platform for decoding cell type-specific surface proteomes and cell-cell interactions within native physiological contexts.