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

Abstract

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. Here, we have used a combination of X-ray crystallography, solution NMR and SAXS, isothermal calorimetry, and computational chemistry methods to unravel how CsdA provides a specific recognition platform for CsdE and how their complex organizes a composite functional reaction environment. The X-ray structures of persulfurated (CsdA) and persulfurated (CsdA-CsdE) complexes reveal the crucial roles of the conserved active-site cysteine loop and additional catalytic residues in supporting the transpersulfuration reaction. A mechanistic view of sulfur transfer across protein-protein interfaces that underpins the requirement for the conserved cysteine loop to provide electrostatic stabilization for the in-transfer sulfur atom emerges from the analysis of the persulfurated (CsdA-CsdE) complex structure.