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. For the two largest groups, commonlyreferred to as SF1 and SF2, a total of seven characteristic motifs has beenidentified []. These two superfamilies encompass a large number of DNA andRNA helicases from archaea, eubacteria, eukaryotes and viruses that seem to beactive as monomers or dimers. RNA and DNA helicases are considered to beenzymes that catalyze the separation of double-stranded nucleic acids in anenergy-dependent manner [].The various structures of SF1 and SF2 helicases present a common core with twoα-β RecA-like domains [, ]. Thestructural homology with the RecA recombination protein covers the fivecontiguous parallel beta strands and the tandem alpha helices. ATP binds tothe amino proximal α-β domain, where the Walker A (motif I) and WalkerB (motif II) are found. The N-terminal domain also contains motif III (S-A-T)which was proposed to participate in linking ATPase and helicase activities.The carboxy-terminal α-β domain is structurally very similar to theproximal one even though it is bereft of an ATP-binding site, suggesting thatit may have originally arisen through gene duplication of the first one.Some members of helicase superfamilies 1 and 2 are listed below:DEAD-box RNA helicases. The prototype of DEAD-boxproteins is the translation initiation factor eIF4A. The eIF4A protein isan RNA-dependent ATPase which functions together with eIF4B as an RNAhelicase [].DEAH-box RNA helicases. Mainly pre-mRNA-splicing factorATP-dependent RNA helicases [].Eukaryotic DNA repair helicase RAD3/ERCC-2, an ATP-dependent 5'-3' DNAhelicase involved in nucleotide excision repair of UV-damaged DNA.Eukaryotic TFIIH basal transcription factor complex helicase XPB subunit.An ATP-dependent 3'-5' DNA helicase which is a component of the core-TFIIHbasal transcription factor, involved in nucleotide excision repair (NER) ofDNA and, when complexed to CAK, in RNA transcription by RNA polymerase II.It acts by opening DNA either around the RNA transcription start site orthe DNA.Eukaryotic ATP-dependent DNA helicase Q. A DNA helicase that may play arole in the repair of DNA that is damaged by ultraviolet light or othermutagens.Bacterial and eukaryotic antiviral SKI2-like helicase. SKI2 has a role inthe 3'-mRNA degradation pathway, repressing dsRNA virus propagation byspecifically blocking translation of viral mRNAs, perhaps recognizing theabsence of CAP or poly(A).Bacterial DNA-damage-inducible protein G (DinG). A probable helicaseinvolved in DNA repair and perhaps also replication [].Bacterial primosomal protein N' (PriA). PriA protein is one of sevenproteins that make up the restart primosome, an apparatus that promotesassembly of replisomes at recombination intermediates and stalledreplication forks.Bacterial ATP-dependent DNA helicase recG. It has a critical role inrecombination and DNA repair, helping process Holliday junctionintermediates to mature products by catalyzing branch migration. It has aDNA unwinding activity characteristic of helicases with a 3' to 5'polarity.A variety of DNA and RNA virus helicases and transcription factorsThis entry represents the DNA-binding domain of classical SF1 and SF2 helicases. It does not recognize bacterial DinG and eukaryotic Rad3 which differ from other SF1-SF2 helicases by the presence of a large insert after the Walker A (see ).
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. For the two largest groups, commonlyreferred to as SF1 and SF2, a total of seven characteristic motifs has beenidentified []. These two superfamilies encompass a large number of DNA andRNA helicases from archaea, eubacteria, eukaryotes and viruses that seem to beactive as monomers or dimers. RNA and DNA helicases are considered to beenzymes that catalyze the separation of double-stranded nucleic acids in anenergy-dependent manner [].The various structures of SF1 and SF2 helicases present a common core with twoα-β RecA-like domains [, ]. Thestructural homology with the RecA recombination protein covers the fivecontiguous parallel beta strands and the tandem alpha helices. ATP binds tothe amino proximal α-β domain, where the Walker A (motif I) and WalkerB (motif II) are found. The N-terminal domain also contains motif III (S-A-T)which was proposed to participate in linking ATPase and helicase activities.The carboxy-terminal α-β domain is structurally very similar to theproximal one even though it is bereft of an ATP-binding site, suggesting thatit may have originally arisen through gene duplication of the first one.Some members of helicase superfamilies 1 and 2 are listed below:DEAD-box RNA helicases. The prototype ofDEAD-boxproteins is the translation initiation factor eIF4A. The eIF4A protein isan RNA-dependent ATPase which functions together with eIF4B as an RNAhelicase [].DEAH-box RNA helicases. Mainly pre-mRNA-splicing factorATP-dependent RNA helicases [].Eukaryotic DNA repair helicase RAD3/ERCC-2, an ATP-dependent 5'-3' DNAhelicase involved in nucleotide excision repair of UV-damaged DNA.Eukaryotic TFIIH basal transcription factor complex helicase XPB subunit.An ATP-dependent 3'-5' DNA helicase which is a component of the core-TFIIHbasal transcription factor, involved in nucleotide excision repair (NER) ofDNA and, when complexed to CAK, in RNA transcription by RNA polymerase II.It acts by opening DNA either around the RNA transcription start site orthe DNA.Eukaryotic ATP-dependent DNA helicase Q. A DNA helicase that may play arole in the repair of DNA that is damaged by ultraviolet light or othermutagens.Bacterial and eukaryotic antiviral SKI2-like helicase. SKI2 has a role inthe 3'-mRNA degradation pathway, repressing dsRNA virus propagation byspecifically blocking translation of viral mRNAs, perhaps recognizing theabsence of CAP or poly(A).Bacterial DNA-damage-inducible protein G (DinG). A probable helicaseinvolved in DNA repair and perhaps also replication [].Bacterial primosomal protein N' (PriA). PriA protein is one of sevenproteins that make up the restart primosome, an apparatus that promotesassembly of replisomes at recombination intermediates and stalledreplication forks.Bacterial ATP-dependent DNA helicase recG. It has a critical role inrecombination and DNA repair, helping process Holliday junctionintermediates to mature products by catalyzing branch migration. It has aDNA unwinding activity characteristic of helicases with a 3' to 5'polarity.A variety of DNA and RNA virus helicases and transcription factorsThis entry represents the ATP-binding domain found within bacterial DinG and eukaryotic Rad3 proteins, differing from other SF1 and SF2 helicases by the presence of a large insert after the Walker A motif [].
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. For the two largest groups, commonlyreferred to as SF1 and SF2, a total of seven characteristic motifs has beenidentified []. These two superfamilies encompass a large number of DNA andRNA helicases from archaea, eubacteria, eukaryotes and viruses that seem to beactive as monomers or dimers. RNA and DNA helicases are considered to beenzymes that catalyze the separation of double-stranded nucleic acids in anenergy-dependent manner [].The various structures of SF1 and SF2 helicases present a common core with twoα-β RecA-like domains [, ]. Thestructural homology with the RecA recombination protein covers the fivecontiguous parallel beta strands and the tandem alpha helices. ATP binds tothe amino proximal α-β domain, where the Walker A (motif I) and WalkerB (motif II) are found. The N-terminal domain also contains motif III (S-A-T)which was proposed to participate in linking ATPase and helicase activities.The carboxy-terminal α-β domain is structurally very similar to theproximal one even though it is bereft of an ATP-binding site, suggesting thatit may have originally arisen through gene duplication of the first one.Some members of helicase superfamilies 1 and 2 are listed below:DEAD-box RNA helicases. The prototype of DEAD-boxproteins is the translation initiation factor eIF4A. The eIF4A protein isan RNA-dependent ATPase which functions together with eIF4B as an RNAhelicase [].DEAH-box RNA helicases. Mainly pre-mRNA-splicing factorATP-dependent RNA helicases [].Eukaryotic DNA repair helicase RAD3/ERCC-2, an ATP-dependent 5'-3' DNAhelicase involved in nucleotide excision repair of UV-damaged DNA.Eukaryotic TFIIH basal transcription factor complex helicase XPB subunit.An ATP-dependent 3'-5' DNA helicase which is a component of the core-TFIIHbasal transcription factor, involved in nucleotide excision repair (NER) ofDNA and, when complexed to CAK, in RNA transcription by RNA polymerase II.It acts by opening DNA either around the RNA transcription start site orthe DNA.Eukaryotic ATP-dependent DNA helicase Q. A DNA helicase that may play arole in the repair of DNA that is damaged by ultraviolet light or othermutagens.Bacterial and eukaryotic antiviral SKI2-like helicase. SKI2 has a role inthe 3'-mRNA degradation pathway, repressing dsRNA virus propagation byspecifically blocking translation of viral mRNAs, perhaps recognizing theabsence of CAP or poly(A).Bacterial DNA-damage-inducible protein G (DinG). A probable helicaseinvolved in DNA repair and perhaps also replication [].Bacterial primosomal protein N' (PriA). PriA protein is one of sevenproteins that make up the restart primosome, an apparatus that promotesassembly of replisomes at recombination intermediates and stalledreplication forks.Bacterial ATP-dependent DNA helicase recG. It has a critical role inrecombination and DNA repair, helping process Holliday junctionintermediates to mature products by catalyzing branch migration. It has aDNA unwinding activity characteristic of helicases with a 3' to 5'polarity.A variety of DNA and RNA virus helicases and transcription factorsThis entry includes bacterial DinG and eukaryotic Rad3 proteins, differing from other SF1 and SF2 helicases by the presence of a large insert after the Walker A motif [].
