Human anamorsin binds [2Fe-2S] clusters with unique electronic properties

The eukaryotic anamorsin protein family, which has recently been proposed to be part of an electron transfer chain functioning in the early steps of cytosolic iron-sulfur (Fe/S) protein biogenesis, is characterized by a largely unstructured domain (CIAPIN1) containing two conserved cysteine-rich motifs (CX8CX2CXC and CX2CX7CX2C) whose Fe/S binding properties and electronic structures are not well defined. Here, we found that (1) each motif in human anamorsin is able to bind independently a [2Fe-2S] cluster through its four cysteine residues, the binding of one cluster mutually excluding the binding of the second, (2) the reduced [2Fe-2S](+) clusters exhibit a unique electronic structure with considerable anisotropy in their coordination environment, different from that observed in reduced, plant-type and vertebrate-type [2Fe-2S] ferredoxin centers, (3) the reduced cluster bound to the CX2CX7CX2C motif reveals an unprecedented valence localization-to-delocalization transition as a function of temperature, and (4) only the [2Fe-2S] cluster bound to the CX8CX2CXC motif is involved in the electron transfer with its physiological protein partner Ndor1. The unique electronic properties of both [2Fe-2S] centers can be interpreted by considering that both cysteine-rich motifs are located in a highly unstructured and flexible protein region, whose local conformational heterogeneity can induce anisotropy in metal coordination. This study contributes to the understanding of the functional role of the CIAPIN1 domain in the anamorsin family, suggesting that only the [2Fe-2S] cluster bound to the CX8CX2CXC motif is indispensable in the electron transfer chain assembling cytosolic Fe/S proteins.