Structure of Haemophilus influenzae HslV protein at 1.9 A 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 A 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 Mg(2+), suggesting that monovalent and divalent metal ions may influence the catalytic activity of the protease.