Structure of the E. coli K+ binding site in hUPP1. (A) Structural alignment of the K+ binding site of EcUPP (green) with the ligand-free structure of hUPP1 (yellow) reveals both a lack of density for K+ and loss of the coordinating glutamate residues (E49). Comparison of the equivalent serine residues (S114 in hUPP1; S73 in EcUPP) shows that rather than binding K+, the human residues are turned away from the region and spread further apart by the global inter-domain motion observed for the ligand-free enzyme. The electron density for hUPP1 is shown (blue; 2f
map contoured at 2σ) with crystallographic waters marked as red crosses. (B) Structural alignment of the K+ binding site of EcUPP (green) with the BAU-bound structure of hUPP1 (yellow), shown from the same perspective as in (A), confirms that, despite its inclusion in all crystallization solutions, K+ is not present at the dimer interface as previously observed in EcUPP.