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Protein in the envelope enclosing the cell nucleus - a new piece of the puzzle in research on cancer and stem cells?
17 June 2009
A research team led by Professor Einar Hallberg at the Department of Life Sciences at Södertörn University in Sweden has discovered a new protein in the inner membrane of the cell nucleus. This protein may play an important role in cell division and now provides a new piece of the puzzle to study in cancer research.
All living organisms are made up of cells. The cell consists of different "compartments" that have different functions. In one of the compartments, the cell nucleus, there is genetic information about how the organism's proteins should look like, and when they should be produced. The cell nucleus is enclosed by a double lipid membrane that is called the nuclear envelope. All transports in and out of the nucleus take place through pores in the nuclear envelope. It is estimated that there are some 100 different proteins in the nuclear envelope, but today scientists do not yet know precisely how they function.
The protein that the Södertörn researchers have now discovered, called Samp1, normally exists in the membrane envelope that surrounds the cell nucleus (Fig. 1). During cell division it turned out that it was part of the process that distributes the chromosomes evenly between the daughter cells, the so-called "mitotic spindle" (Fig. 2). The protein was therefore named Samp1 (Spindle associated membrane protein 1).
"This discovery was unexpected, since it was previously not believed that integral proteins that are embedded in membranes could be in the mitotic spindle. Nor was it previously understood what functions such proteins would have there," says Professor Hallberg.
The distribution of chromosomes during cell division is extremely rigidly regulated, and the slightest error can lead to the development of tumors. Samp1 will now be a key piece of the puzzle to study in cancer research.
The study, which was recently published in Journal of Cell Science "An integral protein of the inner nuclear membrane localizes to the mitotic spindle in mammalian cells", Journal of Cell Science 122, 2100-2107), was part of a doctoral thesis at the Karolinska Institutet that was defended at Södertörn University by Dr. Charlotta Buch on February 20 this year.
Einar Hallberg's research team discovered in their study that the Samp1 protein has connections to the cell skeleton outside the cell nucleus. This takes place between cell divisions, when the protein is in the inner membrane of the cell nucleus. It is possible that Samp1 may play an important role when mechanical signals from the outside of the cell are transmitted to the genes in the cell nucleus. Professor Hallberg's research group is now focusing on investigating what role Samp1 might have in the transmission of mechanical signals from the outside of the cell to the genes.
Recently mechanical signaling has been shown to be extremely important in how the body's cells are organized to form various organs. For instance, cultured stem cells develop into nerve cells, muscle cells, or bone cells depending on the stiffness of the material they grow on. Increased knowledge about mechanical signaling is of great importance to stem cell research and future regenerative medicine.
Fig.1. The cell nucleus is enclosed by an envelope consisting of two parallel membranes, the outer membrane and the inner membrane. There are proteins in the inner membrane that interact with "nuclear lamina" and organize the genes. This is where the newly discovered Samp1 protein is found. It is thought to have connections to the centrosome, which in turn has contact with the surface of the cell via the cell skeleton.
Fig 2: During cell division it is important that the chromosomes (red) become evenly divided between the daughter cells. This is ensured by the so-called mitotic spindle, where Samp1 (green) is located.