The more complex a machine is - the more likely it is to break, simply because there are more potential breaking points. Yet, cells are highly complex entities and are extremely robust. We want to understand the underlying basis: what makes biological systems reliable? Which design principles are used? Do they resemble the principles that humans use in engineering?
To study these questions, we look at cell division, a process that is essential for life. When cells divide, a multitude of changes need to happen in a very short timeframe, and any error can be fatal. Hence, reliability is crucial. We use fission yeast in our experiments to identify general principles that have made cells successful in populating the earth for the past 3.5 billion years.
The mechanism of spindle assembly checkpoint signaling
When cells divide, they need to pass on copies of the genetic information to both daughter cells. This step is controlled by a signaling pathway called the spindle assembly checkpoint. The checkpoint detects chromosomes that are not properly attached to the mitotic spindle and therefore are at risk to be missegregated in anaphase. If wrongly attached chromosomes are detected, the checkpoint delays anaphase until the error has been corrected. Although it is clear, which proteins play a role, it is far from clear how the signaling network is constructed.
Can spindle assembly checkpoint signaling be simplified?
The picture that emerges for checkpoint signaling is a highly complex one. Yet the task is simple: detect badly attached chromosomes, create a signal and prevent anaphase. We want to construct a synthetic pathway that does precisely this and uses less components than the natural checkpoint. By comparing the cellular and the synthetic pathway, we want to answer whether the complexity is necessary for functionality or is a byproduct of the evolutionary process that created the checkpoint.
How are protein levels and noise kept within an acceptable range?
Cells are confronted with the inherent problem that biochemical reactions are stochastic events. A reaction occurs when two molecules run into each other. If many copies are present, this happens frequently and is not a problem. However, in reactions that involve genomic DNA (1 or 2 copies per cell) or mRNA (often only a few copies per cell), this causes considerable variability and results in fluctuations of mRNA and protein abundance (called 'noise'). We know that the spindle assembly checkpoint needs both precise relative levels between its proteins and low protein variability. Yet, the mRNAs coding for checkpoint proteins are present at only 4 - 9 copies per cell. We want to know how cells nevertheless keep noise in check and make sure that precise protein levels are reached.
How are multiple cellular processes coordinated with each other?
The separation of the chromosomes in anaphase is one of the most dramatic events of the cell cycle. The separation is irreversible and therefore needs to be highly accurate and tightly coordinated with mitotic exit. We combine perturbation experiments, live cell imaging and computational modeling to understand how this high degree of coordination is achieved. We want to find general principles that allow cells to achieve extreme accuracy of highly dynamic processes.
Stefan Legewie, Alex Anyaegbunam / IMB, Mainz, Germany / computational modeling / regulation of anaphase
Andrea Ciliberto / IFOM, Milan, Italy / computational modeling / spindle assembly checkpoint
Lenwood S. Heath, Yanshen Yang / Department of Computer Science, Virginia Tech / bioinformatics / spindle assembly checkpoint
Jan Hasenauer / Helmholtz Zentrum Muenchen, Germany / stochastic modeling and statistics / spindle assembly checkpoint
Boris Macek, Karsten Krug, Alejandro Carpy / Proteome Center, University of Tuebingen, Germany / mass spectrometry / proteome and phosphoproteome across the cell cycle
Michael Knop, Susanne Trautmann / EMBL and ZMBH Heidelberg / fluorescence correlation spectroscopy
Gunnar Raetsch, Chris Widmer, Philipp Drewe / Memorial Sloan-Kettering Cancer Center / image recognition
Daniel Rauh / Chemical Genomics Centre of the Max Planck Society and Technical University Dortmund / chemistry / analog-sensitive kinases