SlyD is a putative folding helper protein from the Escherichia coli cytosol, which consists of an N-terminal prolyl isomerase domain of the FKBP type and a presumably unstructured C-terminal tail. It is involved in the biosynthesis of the metal cluster in the [NiFe]-hydrogenase enzymes, and exhibits several activities including that of a peptidyl-prolyl isomerase. Recombinant E.Coli slyD was expressed in E.coli and purified by conventional chromatography.