Mechanism of sulfur transfer across protein-protein interfaces: The cysteine desulfurase model system

CsdA cysteine desulfurase (the sulfur donor) and the CsdE sulfur acceptor are involved in biological sulfur trafficking and in iron-sulfur cluster assembly in the model bacterium Escherichia coli. CsdA and CsdE form a stable complex through a polar interface that includes CsdA Cys328 and CsdE Cys61, the two residues known to be involved in the sulfur transfer reaction. Although mechanisms for the transfer of a sulfur moiety across protein-protein interfaces have been proposed based on the IscS-IscU and IscS-TusA structures, the flexibility of the catalytic cysteine loops involved has precluded a high resolution view of the active-site geometry and chemical environment for sulfur transfer. H…