A prokaryotic protein tagging system called pupylation that is analogous to ubiquitylation in eukaryotes has recently been described. In this process, prokaryotic ubiquitin-like protein (Pup) is coupled to substrate proteins via an isopeptide bond in order to target them for degradation by the proteasome. The ligation occurs via a condensation reaction involving a carboxylate of the C-terminal glutamate of Pup (located in a conserved terminal Gly-Gly-Glu motif) and the side-chain amino group of a substrate lysine. Here we have used a combination of NMR and biochemical experiments with free lysine and a physiological protein substrate (PanB) to show that the coupling occurs through the side-chain carboxylate of the glutamate in the GGE motif rather than the carboxy terminus but that the glutamate must be located; at the C-terminal position to be coupled.