Figure 4. Proximity dependent labeling on the cell surface with HRP.

Horseradish peroxidase is not active with cells but when it is either (A) expressed on the cell surface as a fusion protein or (B) targeted to the cell surface by fusion with an antibody it can be used to label proximate proteins. By providing reactive molecules, such as arylazides or tyramines and H₂O₂, conjugated to biotin or fluorescein, the HRP generates a reactive molecule that covalently labels with proximate proteins within 300 nm. The biotin or fluorescein label enables protein identification. Methods that use this approach, including EMARS and SPPLAT, can be used to identify proximate proteins on the cell surface.