CIAO3 protein forms a stable ternary complex with two key players of the human cytosolic iron-sulfur cluster assembly machinery.

The CIAO3 protein operates at a crossroad of the cytosolic iron-sulfur protein assembly (CIA) machinery. Although the functional role of CIAO3 has been recently characterized, a description of its interaction network is still not complete. Literature data suggested that CIAO3 interacts individually with CIA2A and CIAO1 protein, with the latter two interacting each other. However, no experimental data are available yet showing the formation of a possible ternary complex composed by CIAO3, CIAO1, and CIA2A. This work shows, for the first time, via size exclusion chromatography coupled with multiangle light scattering, UV-vis absorption and electron paramagnetic resonance (EPR) spectroscopies, the formation of a stable, [4Fe-4S]-bound, complex, composed by CIAO3 and the hetero-CIA2A-CIAO1 complex. Moreover, site-directed mutagenesis data suggested a structural role for the C-terminal [4Fe-4S] cluster of the CIAO3 protein. These findings can provide solid bases for further investigation of the molecular mechanisms involving these CIA machinery proteins.