Regulation of the APC/C by mitotic phosphorylation

Eukaryotic cells pass their genomes from one cell generation to the next by first replicating their DNA, resulting in chromosomes containing two sister chromatids. During DNA replication, these sister chromatids become physically connected. Later in mitosis, chromosomes are attached to both poles of the mitotic spindle. The physical linkage between sister chromatids is then destroyed, and the spindle segregates sister chromatids towards opposite poles of the dividing cell. This enables the generation of two daughter cells, which contain identical sets of chromosomes. Errors in this chromosome segregation process can lead to the formation of cells with wrong chromosome numbers, a condition that can contribute to the formation of diseases such as cancer. The separation of chromosomes into the two sister chromatids is initiated by a large protein complex called the anaphase-promoting complex/cyclosome (APC/C). To ensure proper chromosome segregation, the APC/C has to be activated at the right time once all chromosomes have become attached to the mitotic spindle. Previous work has shown that the mitotic activation of the APC/C depends on the attachment of phosphate groups (phosphorylation) to subunits of the APC/C. However, it is unknown how these modifications result in APC/C activation. The aim of this study is to use a combination of mass spectrometric, biochemical and electron microscopic techniques to understand how the APC/C is activated in mitosis by phosphorylation. The results of this study will provide mechanistic insight into the regulation of chromosome segregation and may also help to understand the etiology of diseases that are caused by defects in this process.

Watching Molecular Machines at Work When one cell divides into two - that is how all forms of life are propagated - the newly born daughter cells have to be equipped with everything they will need in their tiny lives. Most important of all is that they inherit a complete copy of the genetic information from their mother cell. If this is not the case because a wrong number of chromosomes - on which the genetic information is stored - gets passed on during cell division, the daughter cells will often not survive, or worse, contribute to the development of diseases such as cancer or conditions such as Down Syndrome. Segregating chromosomes correctly is therefore of great importance and cells use complex molecules to carry out this process. How one of these "molecular machines" works has now been elucidated by Jan-Michael Peters from the Research Institute of Molecular Pathology (IMP) in Vienna, Holger Stark from the Max Planck Institute for Biophysical Chemistry in Göttingen and Brenda Schulman from St. Jude Children's Research Hospital in Memphis. They describe their findings in a series of four papers that have been published this year in PNAS, Cell and Molecular Cell. Molecular complex switches itself on The teams of Brenda Schulman, Holger Stark and Jan-Michael Peters have applied these approaches to visualise a molecular machine called the APC/C. "APC/C initiates chromosome segregation and it does this only after the mother cell has completed all other steps that are necessary for cell division. We knew all of this, because otherwise daughter cells with the wrong chromosome numbers would be born - with catastrophic consequences", explains Jan-Michael Peters, "But we did not know how the APC/C is switched on at the right time". The work of Brenda Schulman, Holger Stark and Jan-Michael Peters has now directly visualised the APC/C machine before and after it is switched on. "Interestingly, this revealed that the APC/C can switch itself on, like a smart hybrid car knows when to switch from the electric to the gas engine or vice versa", says Brenda Schulman. "Without directly being able to see the APC/C in detail by electron microscopy, we would have never been able to find out", adds Holger Stark. "In the future, the new technology will allow us to visualise and understand molecular processes at a level we could so far only dream of." In the long run, the scientists hope that their work will help to understand how errors in chromosome segregation and the diseases and syndromes caused by them can be prevented.