Escherichia coli HypC plays an important role in the maturation process of the pre-maturated HycE, the large subunit of hydrogenase 3. It serves as an iron transfer as well as a chaperone protein during the maturation process of pre-HycE, and interacts with both HypD and HycE. The N-terminal cysteine residue of HypC plays a key role in the protein-protein interactions. Here, we present the threedimensional structure of E coli HypC, the first solution structure of HupF/HypC family. Our result demonstrates that E. coli HypC consists of a typical OB-fold P-barrel with two C-terminal helixes. Sequence alignment and structural comparison reveal that the hydrophobic region on the surface of E coli HypC, as well as the highly flexible C-terminal helixes, may involve in the interactions of E coli HypC with other proteins. (c) 2007 Elsevier Inc. All rights reserved.