Page 40 - Read Online

Page 150 Lotz et al. Cancer Drug Resist 2020;3:149-60 I http://dx.doi.org/10.20517/cdr.2019.114

A

B

Figure 1. Domain organization of Type 2 DNA topoisomerase and catalytic cycle. A: domain organization of the human Top2 enzymes.
The Top2 enzymes are composed of three dimeric interfaces. The N-gate (in purple) is composed of a GHKL domain binding ATP
and a transducer domain. The DNA-gate (in grey) comprises the WHD with the conserved catalytic tyrosine (Y805 in red) and the
Toprim domain, that contains three acidic residues binding magnesium ions. The C-gate forms a dimeric interface where the DNA
exits the enzyme at the end of the reaction. The CTD (dashed lines) differs between species and between the human isoforms Top2a and
Top2β. Residue numbering is that of the Top2a isoform. B: catalytic cycle of Top2 relaxation activity. (1) The Top2 enzyme binds a DNA
G-segment (in brown) at the dimeric interface at the DNA-gate. (2) Upon ATP binding (red stars), a second DNA fragment called
T-segment (in blue) is trapped in the ATPase domain. (3) Introduction of a reversible DNA double strand break in the G-segment is
coupled to the hydrolysis of an ATP molecule, leading to the release of ADP (yellow sphere) and an inorganic phosphate, and results
in the transport of the T-segment through the break. Catalytic inhibitors such as bispiperazine (ICRF derivatives) trap a closed clamp
intermediate, affecting ATP-driven conformational changes of the allosteric assembly. (4) Hydrolysis of a second ATP molecule triggers
the release of the T-segment through the C-gate and resets the enzyme. Top2 poisons such as etoposide and doxorubicin prevent
G-segment religation. Top2: type 2 DNA topoisomerase; CTD: C-terminal domain; WHD: winged helix domain; G: gated; T: transported;
GHKL: Gyrase, Hsp90, Histidine Kinase, MutL domain; TOPRIM: topoisomerase-primase domain

[3]
mitosis and is essential for cell division . Its main function is to regulate topological entanglements in
[4]
DNA that can compromise cell division or gene transcription .
Top2 are large multidomain enzymes that fold into a homodimer forming three-protein interfaces called
[5]
“gates” [Figure 1A]. The C-terminal domain (CTD) whose 3D structure is unknown is less conserved
[6]
and seems to play a role in protein-DNA and protein-protein interactions . To remove topological stress,
Top2 introduce a reversible double strand break in a DNA molecule and transport another DNA duplex
[5]
through the break [Figure 1B]. This elaborate mechanism can be targeted by chemicals that affect the
catalytic sites. In particular, the Top2a isoform is a major target for antineoplastic agents that are widely used
[7]
in cancer chemotherapy . While catalytic inhibitors affect ATP hydrolysis, topoisomerase “poisons” stabilize
[8,9]
the cleavage complex (Top2cc), leading to accumulation of lethal DNA double-strand breaks [Figure 1B].
However, cancer cells may develop resistance that can be attributed to Top2 single point mutations,
[10]
alteration of gene expression, or regulation of post-translational modifications (PTM) .