Structure of Haemophilus influenzae HslV protein at 1.9 angstrom resolution, revealing a cation-binding site near the catalytic site

The structure of the Haemophilus influenzae HslV protease of the HslUV 'pro karyotic proteasome' has been solved by molecular replacement and refined w ith data to 1.9 Angstrom resolution. The protease is a 'double donut' of he xameric rings; two alternative sets of intermolecular interactions between protomers in the rings result in 'quasi-equivalent' packing within the asse mbly. Anomalous scattering data from crystals with potassium present in the mother liquor reveal a K+ ion bound with octahedral coordination near the active-site Thr1 residue. The site also binds Na+ ions and is likely to bin d Mg2+, suggesting that monovalent and divalent metal ions may influence th e catalytic activity of the protease.