Monothiol glutaredoxins and A-type proteins: partners in Fe–S cluster trafficking

Abstract

Monothiol glutaredoxins (Grxs) are proposed to function in Fe–S cluster storage and delivery, based on their ability to exist as apo monomeric forms and dimeric forms containing a subunit-bridging [Fe₂S₂]²⁺ cluster, and to accept [Fe₂S₂]²⁺ clusters from primary scaffold proteins. In addition yeast cytosolic monothiol Grxs interact with Fra2 (Fe repressor of activation-2), to form a heterodimeric complex with a bound [Fe₂S₂]²⁺ cluster that plays a key role in iron sensing and regulation of iron homeostasis. In this work, we report on in vitro UV-visible CD studies of cluster transfer between homodimeric monothiol Grxs and members of the ubiquitous A-type class of Fe–S cluster carrier proteins (^NifIscA and SufA). The results reveal rapid, unidirectional, intact and quantitative cluster transfer from the [Fe₂S₂]²⁺ cluster-bound forms of A. thaliana GrxS14, S. cerevisiae Grx3, and A. vinelandii Grx-nif homodimers to A. vinelandii^NifIscA and from A. thaliana GrxS14 to A. thaliana SufA1. Coupled with in vivo evidence for interaction between monothiol Grxs and A-type Fe–S cluster carrier proteins, the results indicate that these two classes of proteins work together in cellular Fe–S cluster trafficking. However, cluster transfer is reversed in the presence of Fra2, since the [Fe₂S₂]²⁺ cluster-bound heterodimeric Grx3–Fra2 complex can be formed by intact [Fe₂S₂]²⁺ cluster transfer from ^NifIscA. The significance of these results for Fe–S cluster biogenesis or repair and the cellular regulation of the Fe–S cluster status are discussed.