麻豆传媒

鈥淒NA replication is a major source of errors that can lead to cancer,鈥� explained Li, a Professor in the Department of Biochemistry & Cell Biology at Stony Brook University, a scientist at Brookhaven Lab, and lead author of the paper. 鈥淭he entire genome鈥攁ll 46 chromosomes鈥攇ets replicated every few hours in dividing human cells,鈥� Li said, 鈥渟o studying the details of how this process works may help us understand how errors occur.鈥�

The investigative team includes scientists from Stony Brook University, the U.S. Department of Energy鈥檚 Brookhaven National Laboratory, Rockefeller University, and the University of Texas. Their research builds on previous collaborative work led by Dr. Li. In 2015, they produced the of the complete DNA-copying protein complex, called the replisome. That study revealed a surprise about the location of the DNA-copying enzymes鈥擠NA polymerases.

In the new paper, titled 鈥淪tructure of the eukaryotic replicative CMG helicase suggests a pumpjack motion for translocation,鈥� the research team focused on the atomic-level details of the 鈥渉elicase鈥� portion of the protein complex鈥攖he part that encircles and splits the DNA double helix so the polymerases can synthesize two daughter strands by copying from the two separated parental strands of the 鈥渢wisted ladder.鈥�

The scientists produced high-resolution images of the helicase using a technique known as cryo-electron microscopy (cryo-EM). One advantage of this method is that the proteins can be studied in solution, which is how they exist in the cells.

鈥淵ou don鈥檛 have to produce crystals that would lock the proteins in one position,鈥� Li said, adding that this is essential because the helicase is a molecular 鈥渕achine鈥� made of 11 connected proteins that must be flexible to work. 鈥淵ou have to be able to see how the molecule moves to understand its function.鈥�

鈥淭he whole mechanism operates similar to an old style pumpjack oil rig, with one part of the protein complex forming a stable platform, and another part rocking back and forth,鈥� Li explained. 鈥淓ach rocking motion could nudge the DNA strands apart and move the helicase along the double helix in a linear fashion,鈥� he suggested.

Using computer software to sort out the images revealed that the helicase has two distinct conformations鈥攐ne with components stacked in a compact way, and one where part of the structure is tilted relative to a more 鈥渇ixed鈥� base.

The atomic-level view allowed the scientists to map out the locations of the individual amino acids that make up the helicase complex in each conformation. Then, combining those maps with existing biochemical knowledge, they came up with a mechanism for how the helicase works.

鈥淥ne part binds and releases energy from a molecule called ATP. It converts the chemical energy into a mechanical force that changes the shape of the helicase,鈥� Li said. After kicking out the spent ATP, the helicase complex goes back to its original shape so a new ATP molecule can come in and start the process again.

鈥淚t looks and operates similar to an old style pumpjack oil rig, with one part of the protein complex forming a stable platform, and another part rocking back and forth,鈥� Li said. Each rocking motion could nudge the DNA strands apart and move the helicase along the double helix in a linear fashion, he suggested.

This linear translocation mechanism appears to be quite different from the way helicases are thought to operate in more primitive organisms such as bacteria, where the entire complex is believed to rotate around the DNA, Li said. But there is some biochemical evidence to support the idea of linear motion, including the fact that the helicase can still function even when the ATP hydrolysis activity of some, but not all, of the components is knocked out by mutation.

鈥淲e acknowledge that this proposal may be controversial and it is not really proven at this point, but the structure gives an indication of how this protein complex works and we are trying to make sense of it,鈥� he said.

The study was funded by the U.S. National Institutes of Health and the Howard Hughes Medical Institute (HHMI), with additional support from the Brookhaven Lab Biology Department. High-resolution cryo-EM data were collected at HHMI and the University of Texas Health Science Center.

About Stony Brook University Part of the State University of New York system, Stony Brook University encompasses 200 buildings on 1,450 acres. Since welcoming its first incoming class in 1957, the University has grown tremendously, now with more than 25,000 students and 2,500 faculty. Its membership in the prestigious Association of American Universities (AAU) places Stony Brook among the top 62 research institutions in North America. U.S. 麻豆传媒 & World Report ranks Stony Brook among the top 100 universities in the nation and top 40 public universities, and Kiplinger names it one of the 35 best values in public colleges. One of four University Center campuses in the SUNY system, Stony Brook co-manages Brookhaven National Laboratory, putting it in an elite group of universities that run federal research and development laboratories. A global ranking by U.S. 麻豆传媒 & World Report places Stony Brook in the top 1 percent of institutions worldwide. It is one of only 10 universities nationwide recognized by the National Science Foundation for combining research with undergraduate education. As the largest single-site employer on Long Island, Stony Brook is a driving force of the regional economy, with an annual economic impact of $4.65 billion, generating nearly 60,000 jobs, and accounts for nearly 4 percent of all economic activity in Nassau and Suffolk counties, and roughly 7.5 percent of total jobs in Suffolk County.

Greg FilianoMedia Relations ManagerSchool of MedicineStony Brook UniversityOffice of Communications and Marketing631-444-9343[email protected]