EndoU, an endoribonuclease, has diverse functions. In eukaryotes, its catalytic domain requires divalent metal ions like calcium for activation, potentially through an N-terminal extension near the catalytic core [3]. It is involved in multiple biological processes, including antiviral responses, translation regulation, and cellular stress responses. In the context of viruses, such as coronaviruses, it plays a crucial role in innate immune antagonism [2].

In coronavirus-infected cells, EndoU targets viral RNA, preferentially cleaving the 3' side of pyrimidines, especially at U↓A and C↓A sequences. This cleavage, detected across the viral RNA from 5' NTR to 3' NTR, including transcriptional regulatory sequences, may suppress negative-strand RNA synthesis and dsRNA accumulation, thereby evading dsRNA-activated host responses [1]. In SARS-CoV-2, inactivation of the EndoU-encoding nsp15 leads to increased dsRNA accumulation, enhanced activation of IFN signaling and PKR pathways, and attenuation of viral replication in lung-derived epithelial cell lines and primary nasal epithelial air-liquid interface cultures [2]. In addition, a catalytically inactivated EndoU mutant of SARS-CoV-2 shows attenuated viral replication in vitro and in vivo, and loss of its activity disrupts RNA recombination, reducing viral sub-genomic message but increasing recombination events contributing to defective viral genomes [4].

In summary, EndoU is a multifunctional endoribonuclease. In the context of coronaviruses, it significantly impacts viral replication, RNA recombination, and innate immune evasion. The study of EndoU using gene-modified models like inactivated mutants in coronaviruses has provided crucial insights into viral-host interactions and viral pathogenesis, which may offer potential targets for antiviral therapies.