Visualising the ends of chromosomes 21/11/04. By the Howard Hughes Medical Institute How the POT1 protein prevents erosion of chromosome ends.

Researchers have visualised the three-dimensional structure of the POT1 protein as it surrounds the ends of a chromosome. In doing so, they 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 Cech at the University of Colorado at Boulder, published their findings on 21 November 2004 in Nature Structural and Molecular Biology.

During normal DNA replication, the very ends of a DNA molecule are lost. In order to prevent erosion, chromosomes are capped with a specialised 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 ageing.

Background: Telomeres

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, and so 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.

Having crystallised a form of POT1 bound to a critical ten-nucleotide segment of DNA, the researchers used X-ray diffraction to reveal the structure of the complex and found that human POT1 has two distinct regions called oligonucleotide/oligosaccharide-binding folds (OB-folds) – shapes found in many proteins that recognise and bind to DNA or RNA. The grooves of the two folds align with one another, forming a continuous channel where the telomeric DNA can fit.

The POT1 protein binds to the end of a human chromosome by way of two oligonucleotide/oligosaccharide-binding folds, shown here in green and blue. Single-stranded telomeric DNA is represented in red.

Technology centre: X-ray crystallography

The structure of the complex suggests that the end of the chromosome is tightly protected by POT1, findings that 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," said Cech. "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.

Adapted from a press release by the Howard Hughes Medical Institute.

Further reading

Lei M, Podell ER, Cech TR (2004) Structure of human POT1 bound to telomeric single-stranded DNA provides a model for chromosome end-protection. Nat Struct Mol Biol Nov 21; [Epub ahead of print]. Abstract