Science & Technology (Commonwealth Union) – The great understanding of every minute detail of the functions of a cells always gives researchers the edge in developing new treatments and innovating new biological products.
Scientists from the MRC Laboratory of Medical Sciences (LMS), Imperial College London and other participants have marked a key mechanism that makes it possible for cells to control when DNA replication begins. The discovery provides new insight into one of biology’s most essential processes: the accurate duplication of genetic material.
Whenever a cell divides, it needs to faithfully reproduce its entire genome. To prepare for replication, cells place a DNA-copying complex called the MCM2-7 helicase, made up of six protein subunits, onto the DNA. However, this machinery remains inactive until the appropriate stage of the cell cycle, helping to ensure that DNA replication does not begin prematurely.
Although this process has been investigated for many years, exactly how cells activate the replication machinery has remained unclear.
The new study, led by first authors Dr Yasunori Noguchi and Dr Almutasem Saleh and senior author Professor Christian Speck of the MRC Laboratory of Medical Sciences and Imperial College London, has now uncovered the structural changes that trigger the start of DNA replication.
The research appearing in Nature Communications, tells us the way a specialised protein pair, Sld3 and Sld7, are marked when the MCM2-7 helicase has been “switched on”. This enables the proteins to recruit Cdc45, an essential component required to activate the helicase and move DNA replication into its next stage.
To uncover this mechanism, the researchers first needed to determine how the helicase is primed for activation. Earlier studies by other research groups had shown that a flexible region of the Mcm4 subunit within the MCM2-7 helicase functions like a molecular “safety catch”. It physically shields important areas of Mcm4, keeping the helicase inactive until the appropriate time.
Christian’s team demonstrated for the first time that this flexible region also conceals surfaces on the neighbouring Mcm6 subunit. An enzyme known as DDK adds a chemical tag through a process called phosphorylation, which releases the safety catch. This exposes the surfaces required for the subsequent stages of DNA replication.
The finding provides a structural explanation for how phosphorylation transforms the helicase from an inactive state into a form that is prepared for activation.
The study’s central discovery was that Sld3 functions as a molecular sensor, with its partner Sld7 helping position it correctly.
After the safety mechanism is released, Sld3 detects the newly accessible regions of Mcm4 and Mcm6 and attaches to them. In this way, it effectively determines whether the replication machinery has been properly activated and moves forward only when the process has been correctly initiated.
This finding helps explain how cells carefully regulate DNA replication so that it starts at the correct location and at the appropriate stage.
One of the study’s most unexpected findings was the way Sld3 transports Cdc45, a vital component that eventually becomes part of the active CMG helicase — the molecular machine responsible for separating the two strands of the DNA double helix.
The scientists noted that Sld3 plays a role as a molecular adaptor. It initially attaches to the Mcm2 section of the helicase, detects that activation has taken place and then shifts position across the helicase to bring Cdc45 to another binding site, located where Mcm2 and Mcm5 meet.
Professor Speck indicated that their cells have to replicate billions of DNA letters precisely each time they divide, hence the machinery that commences this process has to be handled with careful accuracy. He further pointed out that they are now capable of seeing the way a phosphorylation signal releases a molecular safety catch, the way Sld3 identifies that signal and the way it then sends Cdc45 to put together the DNA-unwinding motor.
“Understanding this sequence gives us a much clearer picture of the intricate regulation that protects the stability of cellular genomes,” said Professor Speck.
