ISCU and COVID-19: Other examples for proteins with SONOS bridges include the main protease (Mpro) from SARS-CoV-2 (Fig. 3), an important drug target for fighting the current coronavirus disease 2019 (COVID-19) pandemic17,18, and the human NSF1–ISCU complex that is central to Fe–S cluster biogenesis in mitochondria19 (Extended Data Fig. 5).