| Type: | Domain | Name: | Helicase, superfamily 3, single-stranded RNA virus |
| Description: | 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 helicasesencoded mainly by small DNA viruses and some large nucleocytoplasmic DNA viruses [, ]. Small viruses are very dependent on the host-cell machinery toreplicate. SF3 helicase in small viruses is associated with an origin-binding domain. By pairing a domain that recognises the ori with a helicase, the viruscan 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.In SF3 helicases the Walker A and Walker B motifs are separated by spacers of rather uniform, and relatively short, length. In addition to the A and Bmotifs this family is characterised by a third motif (C) which resides between the B motif and the C terminus of the conserved region. This motif consists ofan Asn residue preceded by a run of hydrophobic residues [].Several structures of SF3 helicases have been solved []. Theyall possess the same core alpha/beta fold, consisting of a five-stranded parallel beta sheet flanked on both sides by several alpha helices. Incontrast to SF1 and SF2 helicases, which have RecA-like core folds, the strand connectivity within the alpha/beta core domain is that of AAA+ proteins [].The SF3 helicase proteins assemble into a hexameric ring.Some proteins known to contain an SF3 helicase domain are listed below: Polyomavirus large T antigen. It initiates DNA unwinding and replication via interactions with the viral origin of replication.Papillomavirus E1 protein. An ATP-dependent DNA helicase required forinitiation of viral DNA replication.Parvovirus Rep/NS1 protein, which is also required for the initiation of viral replication.Poxviridae and other large DNA viruses D5 protein.Bacteriophage DNA primase/helicase protein.Bacterial prophage DNA primase/helicase protein.The entry represents the core alpha/beta fold of the SF3 helicase domain from predominantly single-stranded RNA viruses. | Short Name: | Helicase_SF3_ssRNA_vir |
