The sensitive to lysis D (SlyD) protein fromEscherichia coliis related to the FK506-binding protein family, and it harbours both peptidyl-prolyl cis–transisomerase (PPIase) and chaperone-like activity, preventing aggre-gation and promoting the correct folding of other proteins. Whereas a functional role of SlyD as a protein-folding catalyst in vivo remains unclear, SlyD has been shown to be an essential component for [Ni–Fe]-hydrogenase metallocentre assembly in bacteria.