The high brightness of the x-ray beams and the experimental capabilities at Beamline 8.3.1 were critical to our success, said Berger. Another big plus was the sheer user friendliness of the ALS, which greatly facilitates data collection and imaging.
Based on the structural images he and Dong created, Berger believes that topo II employs a two-gate mechanism to carry out its tasks. The upper domain of topo II opens to admit a segment of DNA and transport it to the enzymes core where the segment is folded. A second DNA segment is then admitted and the upper domain gate closes. This closing of the upper gate triggers the cleavage of the bent DNA segment and the subsequent transport of the second DNA segment through the break. When the gate in topo IIs lower domain swings open, the second DNA segment is released and the cleaved DNA segment is reconnected.
In many ways, the enzyme works like a set of canal locks, opening and closing and certain protein interfaces, or gates, to control the passage of one DNA segment through another without accidentally letting go of the DNA and breaking the chromosome irreversibly, Berger said. Our structural studies should serve as a useful platform for future efforts to understand the chemical basis of DNA cleavage, and for efforts to understand and improve anti-topoisomerase therapeutics.
Antibacterial and anticancer drugs that target topo IIs and other topoisomerases, such as the quinolone family of antibiotics (of which the commonly-used ciprofloxacin is a member), work by preventing the enzymes from completing their tasks. In the case of topo IIs, the cleaved segments of DNA remain attached to the topo II so that th
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory