University of North Carolina School of Medicine scientists havediscovered that a protein known as Cdt1, which is required for DNAreplication, also plays an important role in a later step of thecell cycle, mitosis. The finding presents a possible explanationfor why so many cancers possess not just genomic instability, butalso more or less than the usual 46 DNA-containing chromosomes. The new research, which was published online ahead of print by thejournal Nature Cell Biology , is the first to definitively show such a dual role for a DNAreplication protein. “It was such a surprise, because we thought we knew what thisprotein’s job was — to load proteins onto the DNA in preparationfor replication,” said Jean Cook, PhD, associate professor ofbiochemistry and biophysics and pharmacology at the UNC School ofMedicine and senior study author. Smart Car Turbocharger
“We had no idea it also hada night job, in a completely separate part of the cell cycle.” The cell cycle is the series of events that take place in a cellleading to its growth, replication and division into two daughtercells. It consists of four distinct phases: G1 (Gap 1), S (DNAsynthesis), M (mitosis) and G2 (Gap 2). Cook’s research focuses onG1, when Cdt1 places proteins onto the genetic material to get itready to be copied. In this study, Cook ran a molecular screen to identify otherproteins that Cdt1 might be interacting with inside the cell. Sheexpected to just find more entities that controlled replication,and was surprised to discover one that was involved in mitosis.That protein, called Hec1 for “highly expressed incancer,” helps to ensure that the duplicated chromosomes areequally divided into daughter cells during mitosis, or celldivision. Turbo Wastegate Actuator
Cook hypothesized that either Hec1 had a job in DNAreplication that nobody knew about, or that Cdt1 was the one withthe side business. Cook partnered with Hec1 expert Edward (Ted) D. Salmon, PhD,professor of biology and co-senior author in this study, to explorethese two possibilities. After letting Cdt1 do its replication job,the researchers interfered with the protein’s function to see if itadversely affected mitosis. Other Series
Using a high-powered microscope thatrecords images of live cells, they showed that cells where Cdt1function had been blocked did not undergo mitosis properly. Once the researchers knew that Cdt1 was involved in mitosis, theywanted to pinpoint its role in that critical process. They furthercombined their genetic, microscopy and computational methods todemonstrate that without Cdt1, Hec1 fails to adopt the conformationinside the cells necessary to connect the chromosomes with thestructure that pulls them apart into their separate daughter cells. Cook says cells that make aberrant amounts of Cdt1, like that seenin cancer, can therefore experience problems in both replicationand mitosis. One current clinical trial is actually trying to rampup the amount of Cdt1 in cancer cells, in the hopes of pushing themfrom an already precarious position into a fatal one.
The research was funded by the National Institutes of Health. Studyco-authors from UNC were Dileep Varma; Srikripa Chandrasekaran;Karen T. Reidy; and Xiaohu Wan.