18 December

Thesis defense Banafsheh Etemad and Eelco Tromer: how is the DNA split during cell division?

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During their PhDs, Banafsheh Etemad and Eelco Tromer, both researchers from the Geert Kops group, studied the way in which the DNA is split in a dividing cell. Etemad and Tromer successfully defended their theses on December 12, 2017.

A dividing cell. DNA is in blue, kinetochores are in red, and microtubules are in green.

When a cell is divided, it transfers half of its genetic material to the one daughter cell, and the other half to the other cell. For that to happen in the final stage of mitosis (cell division), the DNA must first be divided into two equal parts of chromosomes. Errors during this process could lead to abnormal DNA contents in the daughter cell, a phenomenon often observed in cancer cells and developmental defects like Down syndrome. Therefore, understanding the molecular pathways that regulate chromosome segregation is essential for unraveling the processes that underlie disease.

Signal proteins
For a successful chromosome segregation, the tubular parts of the cellular skeleton, the microtubules (in green), need to pull the chromosomes (in blue) apart. A mechanism, the SAC (Spindle Assembly Checkpoint), slows down this process until success. Chromosome and microtubules unite at the place where the two equal parts, the sister chromatids, are attached. This is the place of the kinetochores (in red), protein structures that enable the attachment of microtubules. When a chromosome and a microtubule are attached incorrectly, thanks to SAC, cell division is halted. Only when the microtubules are attached to the chromosomes correctly, the SAC signal disappears and mitosis continues.

‘By researching how proteins influence each other, we can see what the cell does and what it should be doing.’

In her thesis, Banafsheh Etemad describes the interaction between kinetochores and microtubule that stops the SAC signal. For this, she researched the way these two bind to each other. Etemad proved that the traction forces of the microtubules is not enough to stop SAC signaling, but the confirmation of microtubule attachment is. She also collected data on the amount of SAC proteins and the number of microtubules attached to kinetochores necessary to stop the signaling. Furthermore, Etemad searched for homologues – organisms that have a comparable SAC mechanism. For this, she used the smut fungus Ustilago maydis. Etemad believes that researching this mechanism in other organisms could provide more understanding of the SAC signaling system and mitosis in general. ‘We can better understand SAC when other model organisms are available. By researching how proteins influence each other, we can see what the cell does and what it should be doing.’

Ancestral eukaryote
Eelco Tromer describes his research on the evolution of the kinetochore. This protein complex consists of 75 proteins and is specific to eukaryotes – the group of organisms that have cells with at least one nucleus. In contrast, the other two groups we use to distribute all life forms are characterized by the absence of nuclei and organelles. To research the evolution of kinetochores systematically, Tromer studied a hundred different eukaryotes. By investigating the DNA sequence of their kinetochores, he was able to reconstruct what the kinetochore structure must have looked like in the last eukaryotic common ancestor, who lived approximately two billion years ago.

Using a self-developed method, Tromer demonstrated how the kinetochores of this ancestral eukaryote have been evolved to those of the eukaryotes in present day. He concludes: ‘The protein structure in the last eukaryotic common ancestor was relatively complex, and various parts of the original kinetochore have been lost in time. Especially the connective function between chromosome and microtubule has been conserved.’ Also, while various protein complexes co-evolved, the kinetochore as a whole evolved relatively fast. It is food for thought as to why a cellular system as essential as this, developed this quickly.