Viral Helicases

Helicases are motor proteins that use the energy gained from ATP hydrolysis to separate nucleic acid strands. In viruses with double stranded nucleic acid intermediate, the double helix must be separated for the polymerase to function. Helicases are also required for the transcription of viral mRNAs, translation, the disruption of RNA-protein complexes and the packaging of nucleic acids into virions

Depending on their Baltimore classification, viruses have different requirements for the helicase associated with double-stranded nucleic acid intermediates that must be separated for transcription/replication:

dsDNA viruses either encode their own helicase or utilise cellular activity when replicating in the nucleus.
ssDNA viruses Rep protein show SF3 helicase activity to initiate the rolling circle.
Reverse transcription viruses do not encode a helicase because RNAseH breaks the DNA /RNA duplex during reverse transcription. dsRNA viruses encode several unknown helicases for transcription of dsRNA templates.
ssRNA+ viruses encode SF1, SF2 and SF3 to unwind their dsRNA replication template.
ssRNA- viruses do not encode a helicase as their genome is always enveloped by a nucleocapsid and does not produce dsRNA.

PMID: 16611118

Enzymatic reaction:

Domain:

Class	Interpro		Viruses
SF1	PF01443	Monomer	SSRNA+ viruses: Pisuviricota and kitriniviricota
SF2	IPR001650	Monomer	ssRNA+, hepacivirus; dsDNA viruses, Duplodnaviria, varidnaviria
	IPR034711	Monomer	dsDNA Herpesvirales
SF3	IPR014759	Ring hexamer	ssRNA+ viruses: Pisuviricota
SF3	see Rep	Homo-oligomer	ssDNA viruses
DnaB-like/SF4	IPR007694	Ring hexamer	dsDNA caudoviruses
Unknown	IPR054176	Pentamers	dsRNA viruses
Unknown	IPR011181	Tetramers	dsRNA viruses

Mechanism : Helicases are characterized by an ATPase that provides the energy to unwind ds nucleic acids. A conserved motif binds a magnesium ion. The reaction begins with the deprotonation of a water molecule, which attacks the phosphate bond as a nucleophile and finally releases a free phosphate.