This domain is found near the N-terminus of a variety of bacterial surface proteins and pili. This domain contains an unusual covalent ester bond between a conserved cysteine and glutamine residue [1].

1: A highly unusual thioester bond in a pilus adhesin is required for efficient host cell interaction. Pointon JA, Smith WD, Saalbach G, Crow A, Kehoe MA, Banfield MJ; J Biol Chem. 2010;285:33858-33866. PMID:20729215