IscS from Archaeoglobus fulgidus has no desulfurase activity but may provide a cysteine ligand for [Fe2S2] cluster assembly.

Iron sulfur ([Fe–S]) clusters are essential prosthetic groups involved in fundamental cell processes such as gene expression regulation, electron transfer and Lewis acid base chemistry. Central components of their biogenesis are pyridoxal-5′-phosphate (PLP) dependent l -cysteine desulfurases, which provide the necessary S atoms for [Fe–S] cluster assembly. The archaeon Archaeoglobus fulgidus ( Af ) has two ORFs, which although annotated as l -cysteine desulfurases of the ISC type (IscS), lack the essential Lys residue (K199 in Af ) that forms a Schiff base with PLP. We have previously determined the structure of an Af (IscU–D35A–IscS) 2 complex heterologously expressed in Escherichia coli and found it to contain a [Fe 2 S 2 ] cluster. In order to understand the origin of sulfide in that structure we have performed a series of functional tests using wild type and mutated forms of Af IscS. In addition, we have determined the crystal structure of an Af IscS–D199K mutant. From these studies we conclude that: i) Af IscS has no desulfurase activity; ii) in our in vitro [Fe 2 S 2 ] cluster assembly experiments, sulfide ions are non-enzymatically generated by a mixture of iron, l -cysteine and PLP and iii) the physiological role of Af IscS may be to provide a cysteine ligand to the nascent cluster as observed in the [Fe 2 S 2 ]– Af (IscU–D35A–IscS) 2 complex. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases. [ABSTRACT FROM AUTHOR]