Abstract
The Suf pathway is the most common pathway for bacterial iron-sulfur cluster assembly and uses the SufBC2D complex as a scaffold for cluster formation. In most Gram-negative bacteria, the SufB subunit of SufBC2D accepts a persulfide from the transpersulfurase, SufE, for incorporation into nascent clusters. There is no reported structure for the SufBC2D-E complex and mechanistic details concerning the coordination of persulfide delivery with other SufBC2D activities are unclear. Using the Suf pathway from Escherichia coli as a model system, we report that SufE acts as a noncompetitive inhibitor of SufBC2D ATPase activity with a Ki value of 1.8 ± 0.2 µM. This value corresponds with a KD value of 1.6 ± 0.2 µM for SufE binding to the SufBC2D complex determined by fluorescence polarization. The rate of persulfide transfer from SufE to SufBC2D is impaired in the presence of ATP, suggesting that the two reactions are mutually exclusive. An AlphaFold3 model of the SufBC2D-E complex predicts electrostatic interactions between acidic residues on SufC and basic residues on the N-terminal helix of SufE. SufE variants at the K9 and R16 positions interfere with the ability of SufE to transfer persulfide to SufBC2D and to inhibit SufBC2D ATPase activity. In vivo complementation growth assays show that these SufE variants exhibit a slow-growth phenotype under iron starvation conditions, confirming the connection between SufE and SufC as important for optimal function in the Suf pathway. The mutual exclusivity of persulfide delivery from SufE and SufBC2D ATPase activity suggests an ordered mechanism for cluster assembly.