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

The structure of the Haemophilus influenzae HslU protein, a molecular chaperone of the Clp/Hsp100 family, has been solved to 2.3 A 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 displaced laterally relative to the one below it. A model for the twinning and an algorithm for detwinning the data are described. It is known from other work that when the HslU hexamer binds its cognate protease HslV, the carboxy-terminal helices of HslU protomers distend and bind between HslV subunits. 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 conformational switch that facilitates the release of the HslU carboxy-terminal helices when HslV binds.