Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Visualizing the end of the human genome

24.11.2004


Scientists have glimpsed the three-dimensional structure of a protein that protects the ends of human chromosomes, a function that is essential for normal cell division and survival. By visualizing the protein as it surrounds the end of a chromosome, the scientists have learned how the protein homes in on a specific DNA sequence and acts like a protective cap to prevent erosion of chromosome ends.



The researchers, led by Howard Hughes Medical Institute President Thomas R. Cech, whose laboratory is at the University of Colorado at Boulder, published their findings in an advance online publication in Nature Structural and Molecular Biology on November 21, 2004. Ming Lei and Elaine R. Podell in Cech’s lab were co-authors. According to Cech, the findings raise new questions about essential cellular functions taking place at the end of the chromosome.

During normal DNA replication, the very ends of a DNA molecule are lost. In order to prevent erosion, chromosomes are capped with a specialized region of DNA known as a telomere – a short, repetitious DNA sequence that does not code for any protein. In humans, an entire telomere is thousands of base pairs long, and is made up of a repeating sequence of six nucleotides. The final 100 to 300 nucleotides at the very end extend beyond the double helix as a single-stranded DNA "tail." The telomeres of normal cells gradually become shorter and shorter with each cell division, a characteristic sign of cellular aging. But cells also possess a unique enzyme known as telomerase that can lengthen telomeres by adding DNA to the ends of the chromosome using its own RNA template. In most cells, telomerase activity is very low after embryonic development, and regulation of telomerase is critical, because too much telomerase activity can promote tumor development.


In 2001, Dr. Peter Baumann in Cech’s laboratory discovered POT1 (for "protection of telomeres"), the only protein known to bind to human telomeric DNA tails. Pot1 plays an important role in capping the ends of chromosomes and in regulating telomere length. "Before that discovery," he said, "people weren’t even in agreement that there was a protein at the very ends of human chromosomes." At the same time, Cech’s team found a version of the POT1 protein in fission yeast. Other versions of POT1 have since been found in plants and mice – each recognizing a telomeric sequence that is unique to that organism. POT1 is critical to normal cell division and survival; experiments in fission yeast have shown that without it, most cells die immediately. Cells that do manage to survive quickly lose their telomeres, which interferes with normal cell division and eventually leads to massive DNA errors and abnormal, circular chromosomes. In human cells grown in the laboratory, too much POT1 can be disruptive, causing abnormal lengthening or shortening of telomeres.

Prior to determining the structure of human POT1, the researchers’ prediction of what it might look like was based on their understanding of the yeast version of the protein. In yeast, POT1 wraps around the end of a chromosome via a region known as an oligonucleotide/oligosaccharide-binding fold (OB-fold) – a shape found in many proteins that recognize and bind to DNA or RNA. The repeating six-nucleotide telomeric unit fits precisely within this fold, with many POT1 molecules binding to each chromosome end. Cech and his colleagues expected human POT1 to have a similar design, but the results of their biochemical analyses of the protein did not fit easily with this model. For example, when the scientists added the protein to short pieces of DNA containing the six nucleotides that make up a human telomeric repeat, the human POT1 protein bound poorly.

To their surprise, they found that POT1 required a stretch of telomeric DNA containing at least ten nucleotides for efficient recognition and binding of DNA. "We were confused about how ten nucleotides was even a binding site, because it wasn’t a multiple of six." Cech said. "If you need to coat something that has a repeating motif of six, you need to bind some multiple of six." To understand how human POT1 recognized and bound to the telomere, the researchers crystallized a form of POT1 bound to the critical ten-nucleotide segment of DNA. They then used x-ray diffraction to reveal the structure of the complex. Unexpectedly, they found that unlike the yeast version of the protein, human POT1 contained two distinct OB-folds. The grooves of the two folds align with one another, forming a continuous channel where the telomeric DNA can fit. They also learned that while the protein would bind to a ten-nucleotide sequence, the structure could also accommodate twelve nucleotides. "So it turns out it doesn’t bind one six, it binds two times six," Cech said. On a single chromosome end, he said, there might be eight to 24 POT1 molecules coating the DNA tail.

The structure of the complex suggests that the end of the chromosome is tightly protected by POT1, and the researchers were able to verify this with additional biochemical experiments. When the POT1-DNA complex was treated with a solution that would normally modify the DNA at specific sites, no such changes occurred – indicating that those sites were completely enclosed by the POT1 protein.

According to Cech, the findings raise important questions about the regulation of telomerase. When telomeric DNA is buried within POT1, telomerase cannot access the DNA to elongate the telomere. "This is something that could keep the cell from making telomeres all day long," he said. "We think this is one level at which telomerase is regulated." Therefore, he said, an important next step will be to determine the cellular mechanism that switches the telomere to the on state so that elongation can occur. "This is the end of the human genome. If you march out to the ends of human chromosome, what’s there? Now we know what is there – at least part of the time," Cech said. "There may be other states of the telomere, as well, but we think this is right where the action is."

Jennifer Michalowski | EurekAlert!
Further information:
http://www.hhmi.org

More articles from Life Sciences:

nachricht Navigational view of the brain thanks to powerful X-rays
18.10.2017 | Georgia Institute of Technology

nachricht Separating methane and CO2 will become more efficient
18.10.2017 | KU Leuven

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Neutron star merger directly observed for the first time

University of Maryland researchers contribute to historic detection of gravitational waves and light created by event

On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...

Im Focus: Breaking: the first light from two neutron stars merging

Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.

Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....

Im Focus: Smart sensors for efficient processes

Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).

When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...

Im Focus: Cold molecules on collision course

Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.

How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...

Im Focus: Shrinking the proton again!

Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.

It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ASEAN Member States discuss the future role of renewable energy

17.10.2017 | Event News

World Health Summit 2017: International experts set the course for the future of Global Health

10.10.2017 | Event News

Climate Engineering Conference 2017 Opens in Berlin

10.10.2017 | Event News

 
Latest News

Osaka university researchers make the slipperiest surfaces adhesive

18.10.2017 | Materials Sciences

Space radiation won't stop NASA's human exploration

18.10.2017 | Physics and Astronomy

Los Alamos researchers and supercomputers help interpret the latest LIGO findings

18.10.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>