ยฉ The Author(s) 2019Coronaviruses (Co. Vs) that cause infections such as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome phylogenetically originate from bat Co. Vs. The coronaviral nonstructural protein 3 (nsp3) has been implicated in viral replication, polyprotein cleavage, and host immune interference. We report the structure of the C domain from the SARS-Unique Domain of bat CoV HKU4. The protein has a frataxin fold, consisting of 5 antiparallel ฮฒ strands packed against 2 ฮฑ helices. Bioinformatics analyses and nuclear magnetic resonance experiments were conducted to investigate the function of HKU4 C. The results showed that HKU4 C engages in protein-protein interactions with the nearby M domain of nsp3. The HKU4 C residues involved in protein-protein interactions are conserved in group 2c Co. Vs, indicating a conserved function.