A study of the CDGSH protein family: biophysical and bioinformatic analysis of the [2FE-2S] cluster protein mitoneet

Abstract

Iron-sulfur clusters, an important class of redox active cofactors, are ligated by protein-based Cys ligands in a variety of nuclearities. Traditionally, these clusters serve as one-electron transfer units, though many clusters are capable of catalytic activity and sensing functions. Recently, a greater number of iron-sulfur clusters with non-Cys ligation have been identified, wherein one or more of the Cys ligands are replaced by an alternative amino acid residue such as His or Asp. In most cases the role of this ligand substitution is unknown. Some hypotheses are that non-Cys ligation may modify reduction potential, allow for proton-coupled electron transfer, or modulate cluster stability. The human mitoNEET protein contains a 1-His, 3-Cys ligated [2Fe-2S] cluster, identified by the presence of a CDGSH peptide motif. MitoNEET is a binding target for the type II-diabetes drug, pioglitazone, and is implicated in controlling mitochondrial iron levels. How exactly mitoNEET functions in the cell is unknown, as is the role its uniquely ligated FeS cluster may play. This thesis uses mitoNEET as a model for the study of non-Cys ligated FeS clusters and their biological function. Protein film voltammetry was used to examine the pH-dependent electrochemical properties of the mitoNEET cluster, indicating that multiple as yet unidentified protonations control redox potential and that drug binding impacts cluster reduction and protonation. Additionally, the effect of reduction and protonation on cluster and protein structure instability was examined through absorbance and circular dichroism measurements, suggesting an important role for cluster lability in protein function. The CDGSH-motif family of [2Fe-2S] cluster-binding proteins was examined using protein similarity networks. This technique highlights the evolutionary relationship among these proteins, and has led to further work examining the DUF1271 domain containing proteins E. coli YjdI and A. vinosum Alvin0680 (a CDGSH-DUF1271 fusion). This work furthers the scientific knowledge of non-Cys ligated Fe-S clusters by improving our understanding of how the mitoNEET His-ligand contributes to proton-coupled electron transfer and cluster instability, and how the broader class of CDGSH-motif proteins is organized.