This family includes RNA helicases thought to be involved in duplex unwinding during viral RNA replication. Members of this family are found in a variety of single stranded RNA viruses.
Helicases have been classified in 5 superfamilies (SF1-SF5). All of the proteins bind ATP and, consequently, all of them carry the classical Walker A (phosphate-binding loop or P-loop) and Walker B (Mg2+-binding aspartic acid) motifs. Superfamily 3 consists of helicases encoded mainly by small DNA viruses and some large nucleocytoplasmic DNA viruses [PUBMED:11689653, PUBMED:15037234]. Small viruses are very dependent on the host-cell machinery to replicate. SF3 helicase in small viruses is associated with an origin-binding domain. By pairing a domain that recognises the ori with a helicase, the virus can bypass the host-cell-based regulation pathway and initiate its own replication. The protein binds to the viral ori leading to origin unwinding. Cellular replication proteins are then recruited to the ori and the viral DNA is replicated.