Gauthier, N.P. et al. Nat. Methods 10, 768–773 (2013).

Tape, C.J. et al. Mol. Cell. Proteomics doi:10.1074/mcp.O113.037119 (12 May 2014).

To understand cell-cell communication, methods for detecting cell-specific protein signaling in coculture are needed. Gauthier et al. solved this in part with their cell type–specific labeling with amino acid precursors (CTAP) method, in which non-native amino acid biosynthesis enzymes (diaminopimelate decarboxylase (DDC) and lysine racemase (Lyr)) are expressed in cocultured, auxotrophic cells to produce differentially isotope-labeled L-lysine from their differentially labeled precursors. This enables the proteome cell of origin to be determined using mass spectrometry. Tape et al. have now improved the CTAP method by engineering optimized DDC and Lyr enzymes to address specific limitations of each. The optimized DDC allows the CTAP method to be expanded to more cell types, and both optimized DDC and Lyr contain retention sequences to anchor them to the endoplasmic reticulum to prevent their secretion. The new developments enable continuous cell-cell communication studies.