Protein Name

Ftsk(C-domain) hexmer



Biological Context

The replication of E. coli circular DNA begins from the region called oriC. Replication forks run to both directions and they are stoped by the Tus protein (PDB:1ECR) at the region of ter which is opposite to oriC. The dimer of double-stranded DNA (dsDNA) which finished replication is recomposed in the region dif and separates into two monomers. The enzyme recognizing the dif sequence and making recombination is a complex of XerC and XerD. Furthermore, it is noted that XerCD divides the dimer into two monomers with the help of the FtsK protein. FtsK is a membrane-bound DNA translocase found in many eubacteria, such as E. coli.

The DNA translocase is an adenosine triphosphate(ATP)-dependent molecular motor which move DNA rapidly in the case of chromosome division, DNA recombination, and DNA transport. T7gp4, DnaB, SV40(PDB:1SVM), SpoIIIE, etc. belong to this type of protein. The structure of FtsK may be divided into three regions (N, linker, C). The N domain is locates on cell membrane which invaginates in cell division. The length and structure of the linker region varies depending on bacterial species. The C domain transposes DNA to the direction of a daughter cell at the rate of more than 6.7 kbp/s. At the same time, the C domain proceeds to the dif region and separates dsDNA by activating XerCD complex. FtsK expresses the function by forming a hexamer-ring. A large channel is formed in the center of each subunit and dsDNA passes through this channel.

(Fig.1) A schematic view showing the replication of circular DNA

Step A: The replication of circular DNA proceeds with two replication forks from the region of oriC to the region of ter in opposite directions.
Step B: When replication has finished at ter, XerCD complex bind to dif site located in the ter.
Step C: One replicated circlular DNA is dragged into the daughter cell through a channel in the center of the FtsK(C-domain). At the same time, this means dif move toward the FtsK(C-domain).
Step D: When dif approaches FtsK(C-domain), XerCD is acitivated by the FtsK.
Step E : The circular DNA dimer is completely resolved by the activated XerCD site-specific recombination.

(*) Although only two Ftsk is shown in the Fig.1, many FtsK exist on invaginated region of cell membrane.

Structure Description


The structures shown here are a monomer (Fig.1) and a hexamer(Fig.3, Fig.4) of the C domain of the Pseudomonas aeruginosa FtsK. The C domain is further divided into three sub-domains: α, β, and γ. The structure of the α domain is unique to FtsK, whereas the β domain has the archetypal RecA-like fold that is common to many oligomeric ATPases. The role of α domain and β domain is moving DNA by catalyzing ATP. On the other hand, γ domain is involved in the activation of XerCD (since γ domain is a glycine-rich structure and is thus highly flexible, it could not be determined). Six Ftsk monomers formed a hexamer-ring by binding to each other with in the head-head (α-α, β-β) manner. This ring contains a channel twisted clockwise in the center. This hexamer ring is formed only in the presence of dsDNA. Without dsDNA, regardless of existence of ATP and ions, FtsK exists as a monomer. The inside diameter of the channel is about 30Å. This size is larger than that of other proteins forming a hexamer ring (T7, TrwB, etc.) that acts on ssDNA (single-stranded DNA). This is due to the necessity to acommodate dsDNA(double-stranded DNA), like the beta-clamp(PDB:3BEP) and PCNA(PDB:1SXJ).

Based on the this structure, Gatti and others proposed the inchworm model which explains how Ftsk transposes DNA (Fig.2). First, DNA inserts itself from the β side of the channel and is bonded with α domain of one subunit. When β domain of the same subunit hydrolyzes ATP, structural change occurs, and the α domain pulling DNA moves by about 5.5A in the direction opposite to β domain. DNA has moved by 1.6bp the minimum by this reaction. Next, as this rebound, DNA binds with the β domain of the same subunit and separate from the α domain, and binds with the α domain of the next subunit. When each of the six subunits perfoms this step once, DNA presumably moves more than 9.6bp (1.6×6). This value is in good agreement with the pitch of dsDNA (10.5bp/1 turn). The regions which make interactions to DNA are considered to be the five loops of α-domain(300-302, 380-384) and β-domain (606-610, 633-640, 655-673) of each subunit. However, the detail of this mechanism is not yet clearly known.

1 2
(Fig.1) FtsK monomer (Fig.2) Inchworm model
3 4
(Fig.3) Ftsk hexmer-ring Top view (Fig.4) FtsK hexmer-ring Front view

Protein Data Bank (PDB)



Massey, T.H. Mercogliano, C.P. Yates, J. Sherratt, D.J. Lowe, J.; "Double-Stranded DNA Translocation: Structure and Mechanism of Hexameric Ftsk"; Molecular Cell; (2006) 23:457-469 PubMed:16916635.


author: Jun-ichi Ito

Japanese version:PDB:2IUU