Schizosaccharomyces pombe Grx4 regulates the transcriptional repressor Php4 via [2Fe–2S] cluster binding

Abstract

The fission yeast Schizosaccharomyces pombe expresses the CCAAT-binding factor Php4 in response to iron deprivation. Php4 forms a transcription complex with Php2, Php3, and Php5 to repress the expression of iron proteins as a means to economize iron usage. Previous in vivo results demonstrate that the function and location of Php4 are regulated in an iron-dependent manner by the cytosolic CGFS type glutaredoxin Grx4. In this study, we aimed to biochemically define these protein–protein and protein–metal interactions. Grx4 was found to bind a [2Fe–2S] cluster with spectroscopic features similar to other CGFS glutaredoxins. Grx4 and Php4 also copurify as a complex with a [2Fe–2S] cluster that is spectroscopically distinct from the cluster on Grx4 alone. In vitro titration experiments suggest that these Fe–S complexes may not be interconvertible in the absence of additional factors. Furthermore, conserved cysteines in Grx4 (Cys172) and Php4 (Cys221 and Cys227) are necessary for Fe–S cluster binding and stable complex formation. Together, these results show that Grx4 controls Php4 function through binding of a bridging [2Fe–2S] cluster.