Department of Biology, Graduate School of Science, Osaka University^* Department of Biochemistry, Faculty of Science, Okayama University of Science, Japan^**
○Norika Sumi^* Kei Wada^* Shintaro Kitaoka^* Kei Suzuki^* Yuko Hasegawa^* Yoshiko Minami^** Yasuhiro Takahashi^* Keiichi Fukuyama^*

Iron-sulfur (Fe-S) clusters are cofactors of Fe-S proteins that are required for a wide variety of biological processes, such as electron transfer, redox and non-redox catalysis, and the regulation of gene expression. The cellular assembly of the Fe-S clusters requires multicomponent systems, of which the SUF machinery, encoded in E.coli by the sufABCDSE operon, is conserved in the three domains of life. Among the SUF components involved, SufS (cysteine desulfurase) and SufE work in concert as a sulfur donor for the Fe-S clusters. The precise roles of the remaining components have yet to be elucidated, though SufC has a weak ATPase activity and forms a soluble complex with SufB and SufD.
Here we report the crystal structures of the SufC monomer, the SufD dimer and the SufCD complex. SufC shows a structure similar to the ABC-ATPases. Interestingly, SufC has a unique salt-bridge that binds one catalytic residue (Glu171) to another domain, suggesting a regulatory mechanism of the ATPase activity. The SufD protein has three domains, the N-terminal helical domain, the core-domain comprising a right-handed parallel β-helix, and the C-terminal helical domain. The dimer interface of the SufD is held primarily by hydrogen bonds that form anti-parallel β-sheets. The crystal structure of the SufCD complex, consisting of two SufC monomers and a SufD dimer (SufC₂D₂), revealed the interaction between the Q-loop of SufC and the C-terminal helical domain of SufD. Of interest are the structural rearrangements of SufC including the cleavage of the key salt-bridge, which may occur upon association with SufD and allow interaction between the SufC monomers for ATP binding and hydrolysis. Possible interactions among SufB, C and D during the Fe-S cluster formation will be discussed.