Directed evolution of LaccID for cell surface proximity labeling and electron microscopy.

Enzymes that oxidize aromatic substrates have been harnessed for cell-based technologies including proximity labeling and electron microscopy; however, they are associated with drawbacks such as the need for toxic HO. Here, we explore multicopper oxidases (laccases) as a new enzyme class for proximity labeling and electron microscopy in mammalian cells. LaccID was generated through 11 rounds of directed evolution from an ancestral fungal laccase and catalyzes one-electron oxidation of diverse aromatic substrates using O instead of toxic HO. Surprisingly, we found that LaccID is selectively active at the surface plasma membrane of both living and fixed cells. We use LaccID proximity labeling and mass spectrometry to map the changing surface proteome of T cells that engage with tumor cells through antigen-specific T cell receptors. In addition, we use LaccID as a genetically encodable tag for EM visualization of cell surface features in mammalian cell culture and in the fly brain. Our study paves the way for future cell-based applications of LaccID.