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Terminal amine isotopic labeling of substrates is a method in quantitative proteomics that identifies the protein content of samples based on N-terminal fragments of each protein and detects differences in protein abundance among samples.
Like other methods based on N-terminal peptides, this assay uses trypsin to break proteins into fragments and separates the N-terminal peptides from the other fragments. TAILS isolates the N-terminal peptides by identifying and removing the internal tryptic peptides. This negative selection allows the TAILS method to detect all N-termini in the given samples. Alternative methods that rely on the free amino group of the N-terminus to identify the N-terminal peptides cannot detect some N-termini because they are "naturally blocked".
The TAILS method has a number of applications including the identification of new substrates and proteases and as a way to define the termini of proteins that enables protein annotation. TAILS can also be used to link proteases with a variety of defined biological pathways in diseases such as cancer, in order to gain a clearer understanding of the substrates and proteases involved in the disease state.