The researchers propose a two-step regulatory mechanism for non-Rabl configurations of centromeres in the nucleus.
Centromeres are chromosomal domains that join pairs of sister chromatids during cell division. When cells divide, the centromeres are pulled to opposite ends of the cell. Once divided, the centromeres are distributed within the nucleus in a Rabl or non-Rabl configuration, named after the 19th century cytologist Carl Rabl. In the Rabl configuration, the distribution of centromeres remains unchanged as they cluster on one side of the nucleus, whereas in the non-Rabl configuration, centromeres are scattered throughout the nucleus.
The biological function and molecular mechanism of Rabl and non-Rabl configurations have remained a mystery since the 1800s. Now, a collaboration led by researchers at the University of Tokyo (Japan), along with other institutions in Japan and Switzerland, has discovered the molecular mechanism behind the non-Rabl configurations.
Using cytogenic and molecular analyses, the researchers studied a plant known to have a non-Rabl centromere configuration called Arabidopsis thaliana, also known as thale cress, as well as a mutant of this plant with a Rabl configuration. They found that two protein complexes work together to determine the distribution of the centromere during cell division: condensin II (CII) and linker of the nucleoskeleton and cytoskeleton (LINC).
“Centromere distribution for the non-Rabl configuration is independently regulated by the CII-LINC complex and a nuclear lamina protein known as CROWDED NUCLEI (CRWN),” explained Sachihiro Matsunaga (University of Tokyo), the main author of the article.
The researchers proposed a two-step mechanism for the distribution of non-Rabl centromeres. First, the CII-LINC protein complex mediates centromere dispersal from late anaphase to telophase (two phases towards the end of the cell division machinery). Then follow the second step; CRWN that stabilizes centromeres dispersed in the nuclear lamina, which is a protein mesh attached to the inner nuclear membrane, within the nucleus.
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The researchers then investigated the biological significance of centromere distribution and analyzed gene expression in Arabidopsis thaliana and the Rabl structure mutant. The researchers hypothesized that the spatial arrangement of centromeres also changes the spatial arrangement of genes, so they expected to observe changes in gene expression between Rabl and non-Rabl plants. They did not observe this. However, they found that when DNA damage stress was applied, the Rabl mutant plant grew organs at a slower rate than the unaltered plant.
“This suggests that precise control of centromere spatial arrangement is required for organ growth in response to DNA damage stress, and there is no difference in tolerance to DNA damage stress in ‘DNA between the non-Rabl and Rabl organisms,” Matsunaga said. “This suggests that the proper spatial arrangement of DNA in the nucleus regardless of Rabl configuration is important for the stress response.”
Matsunaga revealed that the next steps will be to identify the source of energy that changes the spatial arrangement of specific regions of DNA and the mechanisms that underlie this process.
“These discoveries will lead to the development of technology to artificially organize DNA in the nucleus into an appropriate spatial arrangement,” Matsunaga explained. “This technology is expected to make it possible to create stress-resistant organisms, as well as affect new properties and functions by altering the spatial arrangement of DNA rather than editing its nucleotide sequence.”