Researchers have long speculated about the large surplus of minichromosome maintenance proteins (MCMs) that appear to be involved in managing genome duplication. Researchers in Denmark have now resolved this paradox.
Before a mother cell divides into two daughter cells, extremely complex DNA replication machinery is launched to smoothly duplicate the entire genetic material in the mother cell.
MCMs can be broadly described as the essential molecular engine required to replicate genetic information, a function necessary for the life and continuity of every species on this planet, including humans. MCMs achieve this fundamental ability after maturing to replicative helicase, a multi-protein enzyme that unwinds the DNA double helix and thus allows individual DNA strands to be copied in full and without mistakes before a mother cell gives rise to two identical daughters.
Each cell produces a tremendous quantity of MCM proteins that bind to DNA; however, from this vast amount of MCM proteins, only a small portion (about 5–10%) is used to generate replicative helicase. For several decades, scientists did not know the role of the vast majority of MCM proteins in the cell and collectively called this mystery the MCM paradox.
Researchers in Denmark have now shown that the large excess of MCMs is important for making the DNA replicate at just the right pace to avoid potentially fatal replication errors.
“DNA replication is arguably one of the key biological fundaments enabling one cell – a fertilized egg – to eventually turn into an entire organism. Given the importance of this task, understanding the regulation of this process to the tiniest molecular detail is clearly extremely important. If this regulation goes wrong, errors can occur that can derail development and also lead to severe diseases, such as cancer. Our work is an important addition to this knowledge by showing that MCMs ensure that DNA is copied at the correct speed, which minimizes the risk of errors that can damage the genome,” explains the first author, Hana Sedlackova, Postdoctoral Fellow in the research group led by Professor Jiri Lukas at the Novo Nordisk Foundation Center for Protein Research, University of Copenhagen.
The research has been published in Nature.
MCMs need to be safeguarded
Hana Sedlackova and colleagues used advanced imaging techniques and CRISPR technology to place fluorescent markers on the endogenous MCM proteins, which allowed them to follow their function in living cells. By developing this approach, the process of DNA replication literally unfolded in front of their eyes – minute after minute and hour after hour. They found that there are two types of MCM differing in age and their roles in DNA replication.
They found that, while one fraction of MCMs is recycled from the DNA replication in the mother cell (parental MCMs), the other fraction is newly created (nascent MCMs) to ensure that daughter cells inherit from their mothers the proper quantity of MCM proteins, which is important for the next round of DNA replication.
Hana Sedlackova and colleagues used microscopy to discover how and in which ratio these two fractions of MCMs are inherited by daughter cells and then what their respective functions are.
They found that the nascent MCMs – unlike their parental counterparts – do not actively participate in the DNA replication as replicative helicases but instead remain inactive and adjust the pace of the whole process, similar to speed bumps, to minimize harmful molecular collisions during DNA replication.
They also observed that, if the pace of this process is not restrained and cells try to replicate their DNA too fast, errors can occur, which can lead to fatal consequences for cells.
“We also found that nascent MCMs require interaction with a minichromosome-maintenance complex-binding protein (MCMBP) that guides them to the DNA where they perform their task. Without MCMBP safeguarding nascent MCMs, the whole process of DNA replication proceeds too fast, and the cells can die or acquire mutations that could be dangerous for the organisms they live in,” says Hana Sedlackova.
A potential target for combatting cancer
Hana Sedlackova explains that the whole DNA replication process can be compared to a highway.
In healthy cells, the DNA looks like a new highway allowing a reasonably high speed for DNA replication. However, in cancer cells, this highway is punctured by deep potholes, which can lead to serious accidents if the molecular machinery goes too fast. This means that cancer cells are more dependent on nascent MCMs and MCMBP acting as speed bumps to regulate the rate of DNA replication.
This may be a weakness in cancer cells, and Hana Sedlackova therefore suggests that MCMBP, which helps MCMs to reduce the speed of the molecular machinery, may potentially become a target for cancer treatment.
“We can envision that removing MCMBP will enable DNA to replicate at a very high speed, which can be tolerated by healthy cells. However, cancer cells will have problems and will malfunction,” says Hana Sedlackova.
This is exactly what the researchers are investigating in experiments in which they use drugs to remove MCMBP from both cancer cells and healthy cells.