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

The structure of the Haemophilus influenzae HslV protease of the HslUV `prokaryotic proteasome' has been solved by molecular replacement and refined with data to 1.9 Å resolution. The protease is a `double donut' of hexameric rings; two alternative sets of intermolecular interactions between protomers in the rings result in `quasi-equivalent' packing within the assembly. 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 bind Mg²⁺, suggesting that monovalent and divalent metal ions may influence the catalytic activity of the protease.