The Cia1 and Cia2 subunits of the CTC mediate recognition of apo-FeS proteins with a C-terminal targeting complex recognition motif
The cytosolic iron-sulfur cluster assembly (CIA) targeting complex is essential for the maturation of over 30 cytosolic and nuclear iron-sulfur (Fe-S) proteins, which are critical for key cellular processes such as DNA replication and repair. Approximately 25% of these client proteins possess a targeting complex recognition (TCR) motif, a conserved C-terminal tripeptide that directs the CIA targeting complex (CTC) to the client for Fe-S cluster insertion.
In this study, we combined computational, biochemical, and biophysical methods to demonstrate that TCR-containing client proteins bind specifically at the interface between the Cia1 and Cia2 subunits of the CTC. Consequently, mutations that destabilize the Cia1–Cia2 interaction impair the recognition of TCR-bearing clients, highlighting the functional importance of this subunit interface in client recruitment.
Our work also uncovers a broader role for the relatively understudied human Cia2 paralog, CIAO2A. Previously thought to serve primarily as a targeting factor for iron regulatory protein 1 (IRP1), we show that CIAO2A can also interact with clients harboring the TCR motif. This suggests a more general role for CIAO2A in Fe-S protein maturation than previously understood.
Together, these findings provide new insights into the molecular mechanisms underlying client recognition by the CIA targeting complex and expand our understanding of how Fe-S protein assembly contributes to human health and disease.