Structural and functional studies of the main replication protein NS1 of human parvovirus B19

Parvovirus B19 (B19V) is a ubiquitous virus that can infect the majority of human population and cause erythema infectiosum, acute arthropathy, and many other diseases. The main replication protein NS1 plays a critical role in cell cycle arrest, transactivation of viral and host genes, and replication and package of B19V genome. Both DNA nicking and unwinding activities are required for the in vivo function of NS1, but the underlying basis is poorly understood. Here, we report extensive structural and biochemical studies of NS1, showing that NS1 can unwind various types of DNA substrates. The cryo-electron microscopy (cryo-EM) structures reveal the detailed mechanisms for ATP binding and hydrolysis, and DNA binding and unwinding by NS1. In addition to the SF3 HD domain, the C-terminal region is also required for double-stranded DNA (dsDNA) nicking by NS1. Unexpectedly, instead of enhancing, the dsDNA nicking activity of NS1 is negatively regulated by its DNA unwinding ability, suggesting that they likely function in different stages. This study advances our understanding of the structure and function of NS1 and other parvoviral replication proteins, such as the Rep proteins of adeno-associated virus.