RNA component of the telomerase complex: a refined three-dimensional model
A new 3-D model of the key domain of the telomerase enzyme RNALifeSciencesToday based on UCLA materials:
New 3-D model of RNA 'core domain' of enzyme telomerase may offer clues to cancer, agingTelomerase is an enzyme that "guards" our DNA at the ends of chromosomes, in the so–called telomeres.
In the absence of telomerase activity, each time a cell divides, the telomeres become shorter. This is an integral part of the natural aging process, since telomerase is inactive in most cells of the human body. Eventually, the telomeres that function as protective caps at the ends of chromosomes become so short that the cells die.
But in some cells, such as cancer cells, telomerase, consisting of RNA and proteins, is very active and constantly adds DNA to telomeres, preventing their shortening and prolonging the life of the cell.
Biochemists from the University of California at Los Angeles (University of California - Los Angeles, UCLA) have obtained a three–dimensional structural model of the main domain of the RNA enzyme telomerase. Since this enzyme plays a crucial role in the aging process and the development of cancer, understanding its structure may lead to the development of new approaches to the treatment of diseases.
Model of the "key domain" of RNA telomerase,
received by Julie Feigon, Qi Zhang and their colleagues
"We still don't know exactly how RNA and proteins work this magic – prolong the ends of our telomeres – but now we are one step closer to understanding this process," says Julie Feigon, professor of the UCLA Department of Chemistry and Biochemistry, the main author of the study published in the Proceedings of the National Academy of Sciences (Qi Zhang et al., Structurally conserved five nucleotide bulge determines the overall topology of the core domain of human telomerase RNA).
The key domain of the RNA component of telomerase is necessary for the enzyme to add repeats to the ends of chromosomes, structures containing our genes. It contains a template used to encode these repeats.
"Telomerase is an amazing complex," says Feigon, who began studying the DNA structure of telomeres in the early 90s, which sparked her interest in telomerase. "There is an opinion that by activating telomerase, we can increase life expectancy. However, we don't need our cells to retain the ability to divide indefinitely. As they age more and more, they accumulate all possible types of DNA damage and defects. Therefore, in most cells we do not need a high level of telomerase activity."
Since cancer cells divide rapidly, their telomeres shorten faster than in normal cells. But while telomerase has a low level of activity in most types of healthy cells in our body, a high level of its activity in cancer cells helps to restore their telomeres. Cancer cells, according to Feigon, "become immortal" thanks to telomerase, which promotes tumor progression.
"Understanding how telomerase works has a huge potential for the treatment of diseases," the scientist believes.
She and her laboratory staff study the structure of telomerase at a very deep level, which allows us to get an idea of its function. However, Feigon emphasizes that the laboratory conducts basic scientific research and does not develop cancer treatment methods.
The study is funded by the National Institutes of Health (National Institutes of Health), the National Science Foundation (National Science Foundation) USA and other organizations.
Telomerase is an enzyme that lengthens telomeres
(photo from the website allscienceconsidered.wordpress.com )
The key domain of RNA telomerase consists of three parts: the "pseudo–node" necessary for the activity of the enzyme, in the center of which three strands of RNA converge, forming a triple helix; the "inner convex loop", the value of which was often underestimated and which turned out to be very important; the "spiral expansion" - all of which Feigon and her colleagues modeled using a new method developed by them. The structures were determined using the most modern nuclear magnetic resonance (NMR) spectroscopy.
By joining together three parts of the RNA component of telomerase - a pseudonode, an internal convex loop and a spiral extension, and her colleagues created a three–dimensional model.
The structure of the pseudonode of the RNA component of human telomerase
(photo of the Feigon Laboratory from the UCLA website)
"To get a three-dimensional model of the main domain, we put three parts of it together, doing it with high resolution for the first time. From the point of view of studying the function of telomerase, it was amazing, because for the first time we got a constructive model of the shape of this important part of RNA," says Feigon, who was elected to the National Academy of Sciences of the USA in 2009.
The new study, in her opinion, may lead to the identification of targets for drugs.
"If we want to find drug targets in telomerase, we need to know how it functions at every stage of the cell cycle," says Feigon. "If the three-dimensional structure of each protein or nucleic acid involved in the vital activity of the cell is known, the probability of targeting them with small molecules or other pharmaceuticals to block or activate increases immeasurably."
There are diseases in which a mutation in telomerase RNA or in a telomerase protein leads to inactivation of the enzyme.
"We are trying to get a general picture from the point of view of structural biology, including the functions of the enzyme and how it can be inactivated," says Qi Zhang, a postdoctoral fellow in the Feigon laboratory and lead author of the paper. "What we are reporting already contains a lot of information."
Scientists who described how telomeres protect chromosomes received the 2009 Nobel Prize in Physiology or Medicine. Still, very little is known about the structural biology of the enzyme; its full three-dimensional structure has not yet been established. At the same time, almost all information about the three-dimensional structure of vertebrate telomerase RNA was obtained in the Feigon laboratory.
"If we know a lot about the biochemistry of the enzyme, then almost nothing is known about how the ribonucleic and protein components interact with each other in a three–dimensional structure," comments Feigon. "We decided to study the structure of the inner loop and its dynamics. Having determined the structure, we found a completely unexpected assembly, leading to a large bend in the RNA. Then we conducted a biochemical study that showed that this bend and its plasticity are important for telomerase activity. It turned out that the internal convex loop is very important for determining the topology of the domain."
The structure and dynamics of the inner convex loop are important for the catalytic activity of the enzyme.
"We studied the database of all the RNA structures that had been identified by that time, and it turned out that there is another structure that has the same type of loop of five nucleotides. The second structure belonged to the RNA domain of the hepatitis C virus. This turned out to be a big surprise for us. What surprised us even more was that the nucleotide sequence of the viral loop was completely different, and the structure was almost identical. It is also very important for the function of the virus: if it is destroyed, the hepatitis C virus becomes less pathogenic," explains Feigon.
To activate telomerase, telomerase RNA and a protein called telomerase reverse transcriptase (telomerase reverse transcriptase – TERT) are needed. Chromosomes consist of sequences of nucleotides represented by the letters A, C, G, and T. C always binds to G, while A – to T. When connected, the nucleotides make up a three-letter code in which amino acids are encoded. The corresponding amino acids are combined in the synthesis of proteins.
"Telomerase contains an RNA template, which is the code of telomere DNA repeats," explains Feigon. "Instead of the letter A, such a template contains T, and instead of G – C. Copying DNA from an RNA template instead of copying RNA from DNA is called reverse transcription. The main domain of telomerase includes a template that allows such transcription. The HIV virus also has a reverse transcriptase that copies the RNA template into DNA. Reverse transcriptases usually copy RNA into DNA, but do not contain RNA; in telomerase, an RNA component is required for the protein to function."
Telomerase is unique because its RNA template is part of the enzyme itself and is used to copy first one telomeric repeat, then another, and so on. All synthesized repeats are connected to each other. Thus, telomerase restores telomeres. Telomerase has its own internal RNA used to copy into DNA, but this template is only about 10 of the 451 nucleotides that make up the core of the RNA component.
Telomerase is extremely difficult to characterize in terms of structure due to its size and complexity, as well as due to the low level of content in normal cells.
The Feigon laboratory studies the 3-D structure of DNA and RNA and the problem of mutual recognition of DNA, RNA and proteins regulating the activity of genes. Feigon was the first to use nuclear magnetic resonance spectroscopy to determine the structure of DNA and RNA. In her research, she uses a wide range of molecular biological, biochemical and biophysical methods.
Intuition often plays an important role in science. When Feigon started her research on telomeres and telomerase in the early 90s, she didn't even think about cancer. She was interested in the structure of DNA.
Portal "Eternal youth" http://vechnayamolodost.ru09.11.2010