In order to divide, cells must first replicate their chromosomes. Cells use an array of proteins to accomplish the job, including a large enzyme complex that synthesizes new strands of DNA.
In a paper published in the journal Molecular Cell, University of Minnesota researchers report that a particular protein, called minichromosome maintenance protein 10 (Mcm10), protects the enzyme from destruction and, like a molecular tugboat, escorts it to its "port"--the location on a chromosome where DNA replication will begin. Mcm10's versatility implies that it is indispensable for cell division. Therefore, drugs that target Mcm10 could be effective in stopping the uncontrolled cell division seen in cancerous tumors.
The work grew out of a desire to learn the identity of the "tugboat," said Anja Bielinsky, an assistant professor of biochemistry, molecular biology and biophysics, in whose laboratory the work was performed. The first author on the paper is Robin Ricke, a graduate student. The two scientists worked with baker's yeast, an organism often used to study basic biological mechanisms.
First, a little background on how DNA replicates itself: Before DNA replication can begin, the two strands in the DNA double helix must be unwound. Next, multiple molecules of a certain protein attach to the strands to keep them from spontaneously sticking together again. Only then can the star of the show--DNA polymerase alpha, the enzyme complex that synthesizes new DNA strands--be escorted to the specific sites on the DNA strands where it can attach and go to work.
"The big question was, How is polymerase alpha recruited to the first site?" said Bielinsky. "We found that Mcm10 brings the DNA polymerase alpha complex to the chromosomal sites where replication originates. It does this by attaching to the protein that keeps the two DNA strands from reconnecting. But what came as a complete surprise was that Mcm10 also stabilizes the polymerase alpha complex. In cells lacking Mcm10, the catalytic subunit of the complex--the part that attaches to DNA--was degraded so it could not attach." If the complex cannot attach to DNA, cell division is stopped cold.
The proteins that bring about DNA replication have been highly conserved during evolution; that is, the proteins are virtually identical whether the organism is yeast, an invertebrate or a human. Therefore, whatever is learned from yeast--which are much easier to work with than are human cells--is likely applicable to humans, Bielinsky said. Among the next steps for her lab is to find exactly how Mcm10 interacts with the polymerase alpha complex, because disrupting this interaction in a cancer cell might prevent it from multiplying. Also, the researchers want to know what it is in the cell that causes the degradation of the polymerase alpha complex when no Mcm10 is around to protect it. Bielinsky and her group are now beginning to work with chicken blood cells to confirm that Mcm10 works the same way in vertebrates as in yeast.