How is cell division regulated
Insights into the regulation of cell division
Research Report 2010 - Friedrich Miescher Laboratory for Biological Working Groups in the Max Planck Society
Cell division - one becomes two
All living things are made up of cells that have divided from other cells. The division of cells into two daughter cells was first observed in the 19th century, when microscopes were developed with which one could observe the cells, often only a hundredth of a millimeter in size. Tracking cell division under the microscope is as fascinating yesterday as it is today, as the cells undergo a series of visible changes (Fig. 1). The DNA is converted into a compact form so that the individual chromosomes become visible; long protein polymers, called microtubules, form and pull the two copies of a chromosome to opposite ends of the cell; finally, the two newly formed daughter cells cord off.
During this complex process, daughter cells receive all the components they need to survive. All of this happens with impressive reliability. The human body, for example, is created from a single fertilized egg cell through billions of error-free cell divisions. The researchers at the Friedrich Miescher Laboratory are interested in how the processes involved in cell division are regulated and coordinated with one another. They focus their studies on yeast cells Schizosaccharomyces pombe. This may seem exotic, but the work of the last few decades has shown that it is possible to research fundamental processes in these cells which, compared to yeast, take place in the far more complex cells of mammals.
Kinases cause important changes in cell division
Cell division is not only a dramatic one, but also a rapid process in relation to the time that passes between cell divisions. How can it be that both the shape of the cell and its structure change so dramatically within a very short time? The chemical modification of proteins has a significant influence on this. Chemical groups are added or removed, which changes the properties of the protein. The addition of phosphate groups to certain amino acids in a polypeptide chain, called phosphorylation, is a common form of chemical change that is also essential during cell division. The phosphorylation reaction is catalyzed by other proteins called kinases. The phosphorylation of a protein can have different effects: The folding of the polypeptide chain or the binding to other molecules can change or the protein can be stabilized or degraded. Kinases typically effect the phosphorylation of not just one but several proteins. In this way they can bring about a number of changes at the same time, as is necessary during cell division.
The kinases that play a role in cell division are already largely known. For example, a cyclin-dependent kinase is necessary to initiate cell division. Aurora kinase, in turn, is activated when cells start dividing and is necessary for the chromosomes to be compressed and properly attached to the microtubules (Fig. 2 [1, 2]). In order to understand how these kinases influence cellular processes, one has to know their substrates, i.e. the proteins that are phosphorylated with the help of the kinase. However, identifying these has proven to be very difficult .
Identifying Kinase Substrates - Finding Needles in a Haystack
In order to uncover all substrates of the Aurora kinase, researchers at the Friedrich Miescher Laboratory together with colleagues at the University of Tübingen have now used a new method that was originally developed at the Max Planck Institute for Biochemistry . With the help of a method based on mass spectrometry, almost all proteins in the cell can now be examined for phosphorylation. Thanks to an extension developed at the Max Planck Institute for Biochemistry, the so-called SILAC method, it is even possible to directly compare the phosphorylations in two different cell samples. This makes it possible to study how the inhibition of a kinase affects the intracellular phosphorylation (Fig. 3). As with the proverbial search for the needle in the haystack, one must be able to distinguish the few phosphorylations that are lost from the thousands of phosphorylations that persist even when the kinase is inhibited. The researchers at the Friedrich Miescher Laboratory have now succeeded in doing this. As soon as the Aurora kinase was inhibited, the phosphorylation of a few proteins decreased significantly. The fact that these included proteins that were already known to be phosphorylated by Aurora kinase shows that the method works reliably. In addition to these known substrates, an even larger number of previously unknown substrates were found, and the type of substrates found indicates that the Aurora Kinase fulfills previously unexpected functions during cell division. The researchers are now working to elucidate this in detail.
Aurora kinases are essential for cell division, and understanding how they work is of fundamental scientific importance. There is also evidence that inhibiting aurora kinases can slow tumor growth. Specific inhibitors for aurora kinases are therefore currently being tested in humans for their effectiveness in treating tumors . It is still unclear whether the Aurora substrates identified by the researchers in yeast are also modified by Aurora in human cells. However, based on various observations, this seems likely. If so, treatment with Aurora Kinase Inhibitors could also have effects that were not previously expected. It is still unclear whether this is helpful in stopping tumor growth or possibly leading to undesirable side effects.
In addition to their scientific and clinical significance, the results obtained also suggest that the technique used here can be used to identify the substrates of other kinases. Kinases are among the most common types of protein in the cell and - in addition to their role in cell division - are involved in almost all signal transmission pathways in the cell. Clarifying which proteins are phosphorylated by a particular kinase will take the researchers an important step forward in their endeavors to understand complex cellular processes.
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