T7 DNA helicase explained

Symbol:	4
Organism:	Enterobacteria phage T7
DNA primase/helicase
Uniprot:	P03692

T7 DNA helicase (gp4) is a hexameric motor protein encoded by T7 phages that uses energy from dTTP hydrolysis to process unidirectionally along single stranded DNA, separating (helicase) the two strands as it progresses. It is also a primase, making short stretches of RNA that initiates DNA synthesis.^[1] It forms a complex with T7 DNA polymerase. Its homologs are found in mitochondria (as Twinkle) and chloroplasts.^{[2] [3]}

Crystal structure

The crystal structure was solved to 3.0 Å resolution in 2000, as shown in the figure in the reference.^[4] In (A), notice that the separate subunits appear to be anchored through interactions between an alpha helix and an adjacent subunit. In (B), there are six sets of three loops. The red loop, known as loop II, contains three lysine residues and is thought to be involved in binding the ssDNA that is fed through the center of the enzyme.

Mechanism of sequential dTTP hydrolysis

Crampton et al. have proposed a mechanism for the ssDNA-dependent hydrolysis of dTTP by T7 DNA helicase as shown in the figure below.^[5] In their model, protein loops located on each hexameric subunit, each of which contain three lysine residues, sequentially interact with the negatively charged phosphate backbone of ssDNA. This interaction presumably causes a conformational change in the actively bound subunit, providing for the efficient release of dTDP from its dTTP binding site. In the process of dTDP release, the ssDNA is transferred to the neighboring subunit, which undergoes a similar process. Previous studies have already suggested that ssDNA is able to bind to two hexameric subunits simultaneously.^[6]

See also

• Helicase

Notes and References

1. Lee SJ, Richardson CC . Choreography of bacteriophage T7 DNA replication . Current Opinion in Chemical Biology . 15 . 5 . 580–586 . October 2011 . 21907611 . 3195405 . 10.1016/j.cbpa.2011.07.024 .
2. Spelbrink JN, Li FY, Tiranti V, Nikali K, Yuan QP, Tariq M, Wanrooij S, Garrido N, Comi G, Morandi L, Santoro L, Toscano A, Fabrizi GM, Somer H, Croxen R, Beeson D, Poulton J, Suomalainen A, Jacobs HT, Zeviani M, Larsson C . 6 . Human mitochondrial DNA deletions associated with mutations in the gene encoding Twinkle, a phage T7 gene 4-like protein localized in mitochondria . Nature Genetics . 28 . 3 . 223–231 . July 2001 . 11431692 . 10.1038/90058 . 22237030 .
3. Diray-Arce J, Liu B, Cupp JD, Hunt T, Nielsen BL . The Arabidopsis At1g30680 gene encodes a homologue to the phage T7 gp4 protein that has both DNA primase and DNA helicase activities . BMC Plant Biology . 13 . 36 . March 2013 . 23452619 . 3610141 . 10.1186/1471-2229-13-36 . free .
4. Singleton MR, Sawaya MR, Ellenberger T, Wigley DB . Crystal structure of T7 gene 4 ring helicase indicates a mechanism for sequential hydrolysis of nucleotides . Cell . 101 . 6 . 589–600 . June 2000 . 10892646 . 10.1016/S0092-8674(00)80871-5 . free .
5. Crampton DJ, Mukherjee S, Richardson CC . DNA-induced switch from independent to sequential dTTP hydrolysis in the bacteriophage T7 DNA helicase . Molecular Cell . 21 . 2 . 165–174 . January 2006 . 16427007 . 10.1016/j.molcel.2005.11.027 . free .
6. Yu X, Hingorani MM, Patel SS, Egelman EH . DNA is bound within the central hole to one or two of the six subunits of the T7 DNA helicase . Nature Structural Biology . 3 . 9 . 740–743 . September 1996 . 8784344 . 10.1038/nsb0996-740 . 12425998 .