The iron-sulfur cluster (ISC) machinery, located in mitochondria, exports an unknown sulfur-containing compound (X-S) via the Atm1 transporter to the cytosol. Using this sulfur atom and an undefined source of iron, a 4Fe-4S cluster that contains four iron atoms and four sulfur atoms (show as red and yellow circles) is assembled on a scaffold complex consisting of the proteins Cfd1 and Nbp35. The assembled cluster is transferred to Nar1 and the CIA targeting complex (Mms19, Cia1, and Cia2) for subsequent insertion into target proteins. This changes the target from its ‘apo’ (without prosthetic group) form to a ‘holo’ form (with prosthetic group). Paul et al. found that cluster assembly on Rli1 requires the CIA targeting complex and two Rli1-specific adaptor proteins, Yae1 and Lto1. In a unique binding mechanism, a conserved tryptophan residue (shown as a yellow ‘W’) in the Lto1 protein interacts with the CIA targeting complex, while Yae1 recruits Rli1. In addition, conserved deca-GX3 motifs in Yae1 and Lto1 are required to form the CIA targeting complex.