Unwinding mechanism of SARS-CoV helicase (nsp13) in the presence of Ca(2+), elucidated by biochemical and single-molecular studies

The recent outbreak of COVID-19 has created a serious health crisis with fatFal infectious viral diseases, such as Severe Acute Respiratory Syndrome (SARS). The nsp13, a helicase of coronaviruses is an essential element for viral replication that unwinds secondary structures of DNA and RNA, and is thus considered a major therapeutic target for treatment. The replication of coronaviruses and other retroviruses occurs in the cytoplasm of infected cells, in association with viral replication organelles, called virus-induced cytosolic double-membrane vesicles (DMVs). In addition, an increase in cytosolic Ca(2+) concentration accelerates viral replication. However, the molecular mechanism of nsp13 in the presence of Ca(2+) is not well understood. In this study, we applied biochemical methods and single-molecule techniques to demonstrate how nsp13 achieves its unwinding activity while performing ATP hydrolysis in the presence of Ca(2+). Our study found that nsp13 could efficiently unwind double stranded (ds) DNA under physiological concentration of Ca(2+) of cytosolic DMVs. These findings provide new insights into the properties of nsp13 in the range of calcium in cytosolic DMVs.