Ligand mediated inter-domain motion of hUPP1. While the structural conformations of monomers of hUPP1 vary only subtly between BAU-bound and ligand-free forms, there is substantial movement between the two domains upon inhibitor binding, as illustrated here. In the presence of BAU, phosphate coordination is also promoted within the substrate pocket, despite its absence in purification or crystallization solutions. The presence of both molecules within the ligand pocket causes the enzyme to dramatically close the active site, resulting in backbone carbon motion of 3–5 Å and individual residue repositioning of over twice that magnitude. The open state seen for the ligand-free structure of hUPP1 has not been previously observed in bacterial homologues, possibly because the hexameric assembly of these microbial enzymes restricts their range of inter-domain motion.