Structural Plasticity of NFU1 Upon Interaction with Binding Partners: Insights into the Mitochondrial [4Fe-4S] Cluster Pathway

In humans, the biosynthesis and trafficking of mitochondrial 4Fe-4S]²⁺ clusters is a highly coordinated process that requires a complex protein machinery. In a mitochondrial pathway among various proposed to biosynthesize nascent 4Fe-4S]²⁺ clusters, two 2Fe-2S]²⁺ clusters are converted into a 4Fe-4S]²⁺ cluster on a ISCA1-ISCA2 complex. Along this pathway, this cluster is then mobilized from this complex to mitochondrial apo recipient proteins with the assistance of accessory proteins. NFU1 is the accessory protein that first receives the 4Fe-4S]²⁺ cluster from ISCA1-ISCA2 complex. A structural view of the protein–protein recognition events occurring along the 4Fe-4S]²⁺ cluster trafficking as well as how the globular N-terminal and C-terminal domains of NFU1 act in such process is, however, still elusive. Here, we applied small-angle X-ray scattering coupled with on-line size-exclusion chromatography and paramagnetic NMR to disclose structural snapshots of ISCA1-, ISCA2- and NFU1-containing apo complexes as well as the coordination of 4Fe-4S]²⁺ cluster bound to the ISCA1-NFU1 complex, which is the terminal stable species of the 4Fe-4S]²⁺ cluster transfer pathway involving ISCA1-, ISCA2- and NFU1 proteins. The structural modelling of ISCA1-ISCA2, ISCA1-ISCA2-NFU1 and ISCA1-NFU1 apo complexes, here reported, reveals that the structural plasticity of NFU1 domains is crucial to drive protein partner recognition and modulate 4Fe-4S]²⁺ cluster transfer from the cluster-assembly site in the ISCA1-ISCA2 complex to a cluster-binding site in the ISCA1-NFU1 complex. These structures allowed us to provide a first rational for the molecular function of the N-domain of NFU1, which can act as a modulator in the 4Fe-4S]²⁺ cluster transfer.