Research
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Hauf Lab - Research
Background
Multicellular organisms can consist of of trillions of cells, all of which have been generated through divisions from a single cell. Even in an adult human body, millions of cell divisions still happen every minute, which is necessary to continuously regenerate body tissues. When cells divide, both daughter cells have to receive the complete genetic information in order to be able to function properly. The genetic information is packaged in chromosomes. Chromosomes are duplicated before cell division and the two copies are distributed to the emerging daughter cells during cell division by the microtubules of the mitotic spindle. Errors in the distribution of the chromosomes to the daughter cells can result in cell death, contribute to tumorigenesis or, when happening in germ cells, lead to abortion or birth defects. Cell division is not only crucial for multicellular organisms but is a hallmark of all life on earth. Because of this central importance, the molecules contributing to the proper segregation of chromosomes are highly similar even in only distantly related organisms. In our lab, we study chromosome segregation using fission yeast (Schizosaccharomyces pombe). Fission yeast is very easy to manipulate genetically, which is essential for many of the experiments we conduct. We want to understand which molecules contribute to proper chromosome segregation and how all the proteins that are involved act together. By later combining what we have learned from fission yeast with results from other eukaryotes, we also want to elucidate how the network of interacting molecules has changed during evolution and how the changes serve the needs of a particular organism.
Coordination of chromosome segregation with the cell cycle
For cell division to be executed correctly, the mechanical events of chromosome segregation need to be coordinated with cell cycle progression. In particular, anaphase and exit from mitosis should only be initiated once all chromosomes have been attached correctly to microtubules of the mitotic spindle, which is controlled by a signaling network that has been called ‘spindle assembly checkpoint’. Some proteins, including Bub1 and Aurora, have a dual role both in ensuring proper chromosome attachment and in checkpoint signaling. We want to elucidate the precise molecular function of these proteins in the different processes they influence, and ultimately want to understand why these different functions have been combined in one protein. Both Aurora and Bub1 are protein kinases. We work on the identification of the substrates of Aurora, and we dissect the different functions of Bub1 using separation-of-function mutants. To identify Aurora substrates, we use both a candidate approach as well as an unbiased approach, where we make use of an analog-sensitive version of Ark1.
Quantitative analysis of the spindle assembly checkpoint
The spindle assembly checkpoint (SAC) is the signaling mechanism that controls the onset of anaphase depending on the attachment state of the chromosomes to the mitotic spindle. Malfunction of this checkpoint leads to chromosome segregation errors and has been implicated in tumorigenesis. Most if not all of the SAC proteins are known but how they act together to build a highly sensitive signaling network remains unclear. We plant to combine mathematical modeling and experiments to generate and test models for the SAC in fission yeast. To be able to base our models on realistic parameters, we have put fluorescent tags on SAC- and SAC-related proteins and we determine the abundance of SAC proteins and protein complexes using biochemical and advanced microscopy techniques. We furthermore rigorously test and refine our models by predicting as well as experimentally determining how SAC activity changes when we modify the abundance or activity of SAC proteins. In the course of this analysis, we made the unexpected finding that Bub3, which has been considered a core component of the SAC, is not essential for SAC activity in fission yeast. In other eukaryotes in contrast, Bub3 seems to be essential for SAC function. Interestingly, the physical interaction of Bub3 with other SAC proteins as well as the requirement for Bub3 in recruiting other SAC proteins to kinetochores is conserved between fission yeast and other eukaryotes. Thus, fission yeast Bub3 clearly remains a component of the signaling mechanism, but is not essential for SAC activity. We will try to understand why this is the case, and we expect that this analysis will help us to elucidate how the signaling network is constructed and how it has been changed in evolution to accommodate the requirements of specific organisms.
