Structure of Haemophilus influenzae HslU protein in crystals with one-dimensional disorder twinning

The structure of the Haemophilus influenzae HslU protein, a molecular chape rone of the Clp/Hsp100 family, has been solved to 2.3 Angstrom by molecular replacement using a model of the homologous Escherichia coli protein. The crystals in which the structure was solved have an unusual twinning, or one -dimensional disorder, in which each successive crystal-packing layer is di splaced laterally relative to the one below it. A model for the twinning an d an algorithm for detwinning the data are described. It is known from othe r work that when the HslU hexamer binds its cognate protease HslV, the carb oxyterminal helices of HslU protomers distend and bind between HslV subunit s. Comparison of HslU alone with its structure in the HslUV complex reveals several conserved amino-acid residues whose side-chain interactions differ between the two structures, suggesting that they may be part of a conforma tional switch that facilitates the release of the HslU carboxyterminal heli ces when HslV binds.