Figure 1. Overview of Cell Type specific labeling with Amino acid Precursors (CTAP).

(a) The CTAP methodology takes advantage of vertebrate cells’ inability to produce essential amino acids, resulting in the requirement that these molecules be supplemented in culture media or diet for cell growth. We focus on one of these amino acids, L-lysine, and the enzymes used to produce it from precursor molecules. By expressing exogenous L-lysine biosynthesis enzymes, transgenic cells produce their own supply of L-lysine and (b) can be labeled selectively by supplementing the media with heavy isotope-labeled forms of the precursors. Expressing distinct L-lysine biosynthesis enzymes in different cell types enables continuous cell-selective proteome labeling with differentially-labeled precursors when grown in media lacking L-lysine. (c) CTAP can be used to investigate direct contact or secreted factor mediated cell-cell communication, relevant for a range of biological phenomena.