UW-Madison researchers have developed an enzyme-substrate pair that can be used as a tool for C-terminal thioesterification of proteins and peptides. Their tool is based on E. coli MccB, an E1-like enzyme whose native function is formation of a C-terminal phosphoramidate linkage in biosynthesis of microcin C7. Their research showed that MccB has a latent capacity to catalyze C-terminal O-AMPylation on non-native substrates, resulting in a C-terminal electrophile that can react with a variety of exogenous nucleophiles, including hydrazines, alkoxyamines, amines, and thiols. The result of which is a diverse set of C-terminal functional groups. From these initial results, the inventors engineered a sequence tag, which they coined Thioesterification C-terminal Handle (TeCH-tag), that enables MccB-catalyzed, ATP-dependent thioesterification of protein C-termini.