Coronaviruses (CoVs) that cause infections such as severe acute respiratory syndrome (SARS) and Middle East respiratorysyndrome phylogenetically originate from bat CoVs. The coronaviral nonstructural protein 3 (nsp3) has been implicated inviral replication, polyprotein cleavage, and host immune interference. We report the structure of the C domain from theSARS-Unique Domain of bat CoV HKU4. The protein has a frataxin fold, consisting of 5 antiparallel β strands packed against2 α helices. Bioinformatics analyses and nuclear magnetic resonance experiments were conducted to investigate the functionof 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 CoVs, indicating a conservedfunction