By:
Santina Russo
Very little was known till now about DNA repair by
homologous recombination, which is fundamental for human health. Now an
ETH research group has for the first time isolated and studied all the
key proteins involved in this process, laying the foundation for
investigating many diseases.
Which proteins are
essential for cell division? The biochemist Philipp Wild (left) and his
colleagues Ilaria Piazza and Christian Dörig examine the results from
the mass spectrometer. (Photograph: ETH Zurich / Adrian Henggeler)
Within our body, the process of cell division is constantly
creating new cells to replace old or damaged ones. The genetic
information is also duplicated and passed on to the new cells. Complex
interaction of many different proteins ensures a smooth process. This is
because these proteins immediately repair any errors that creep in
during DNA duplication. However, the same protein machinery also
performs another function: in germ cells that divide to from gametes –
egg cells and sperm – it is responsible for mixing the genetic
information of the original maternal and paternal side during cell
division. The same mechanism therefore has to resolve two conflicting
problems: in normal cell division, called mitosis, it ensures genetic
preservation, while in the cell division to produce gametes, or meiosis,
it ensures genetic diversity.
Both tasks are vital. If DNA repair does not work in mitosis, this
can lead to cancer and other diseases. If, on the other hand, the
exchange of DNA in meiosis does not function correctly, the fertility
and health of the offspring may be damaged. “Although these processes
are crucial for our health, very little was known till now about how the
whole system functions and is regulated,” says Joao Matos, Professor
for Biochemistry at ETH Zurich. His team has now studied the responsible
proteins and discovered how they differentiate between the two tasks.
A complex task
The scientists began by cultivating a large number of yeast cells in
the laboratory, as these cells only contain a minute quantity of the
proteins involved. The production of the yeast cells was therefore
extremely complex: the researchers cultivated cells in 120 6-litre
containers in such a way that the division occurred simultaneously in
all yeast cells. Mitosis and meiosis are highly complex processes that
take place in precisely orchestrated phases. Only synchronized cell
cultures can thus differentiate which proteins are important in which
phase, and how they work together.
Scientists already knew that yeast, along with plants, animals and
humans, have a group of seven enzymes involved in the reproduction of
DNA: the recombination intermediates processing enzymes (RIPEs). For the
first time, ETH scientists were able to isolate these RIPEs from the
cell cultures and identify them in the mass spectrometer – from a
specific phase of cell division in each instance. At the same time, they
used this method to identify a series of other proteins that help
regulate cell division.
The same components, but rewired
Joao Matos and his team were eventually able to identify which RIPEs
are important for which phase of cell division and which helper proteins
interact with the RIPEs in each case. The first unexpected result: the
quantity of RIPEs remains almost constant in all phases of mitosis and
meiosis. “Unlike many other processes, the cells do not regulate cell
division and DNA repair through the production of the proteins
involved,” Matos explains. Instead, the helper proteins interact
systematically with the RIPE enzymes in order to enable or disable them
in a specific phase. “All the components are always there, but are
rewired depending on the task”, says the ETH professor.
For example, the researchers discovered that three of the RIPEs lose
almost all their interaction partners precisely in the so-called
metaphase of meiosis, in other words when the maternal and paternal DNA
is mixed. In return, another protein complex is formed at this point.
“This must be responsible for mixing up the maternal and paternal DNA,”
Matos concludes. In addition, ETH researchers have identified a number
of new helper proteins whose role was previously unknown.
Key to understanding disease
The results from the yeast cells can be transferred to humans, as for
every helper protein involved there is an equivalent in humans that
functions in the same or very similar fashion. So the Matos research
group and fellow scientists can build on this knowledge. They can now
study specific proteins to discover whether, and how, they are involved
in the development of diseases and ultimately find a remedy to combat
them.
Reference
Wild P, et al. Network Rewiring of Homologous
Recombination Enzymes during Mitotic Proliferation and Meiosis.
Molecular Cell, available online 24 July 2019. doi: 10.1016/j.molcel.2019.06.022
