Conversion of a tyrosine kinase protein substrate to a high affinity ligand by ATP linkage

Protein kinases often show low affinity for their protein substrates, which makes it difficult to study kinase-substrate interactions. Here, we show using expressed protein ligation with the signaling protein Src that it is feasible to install a covalently linked ATP moiety into the tail of Src, generating a semisynthetic protein with a high affinity for its cognate tyrosine kinase, Csk. It is also established that this Src-ATP conjugate can be used to selectively pull down Csk from a complex protein mixture. This work outlines a general strategy for identifying an unknown kinase that is responsible for the phosphorylation of a protein substrate on a site of interest.