Scientists at the Stanford University School of Medicine say that they have discovered a protein that ferries the critically important clump of proteins around to repair the ends of chromosomes that are lost during normal replication.
The researchers say that the importance of their work lies in the fact that without such ongoing maintenance, stem cells would soon cease dividing and embryos would fail to develop.
"This is the first new protein component of telomerase that has been identified in 10 years. And it's likely to be a valuable target for anti-cancer therapies," said Dr. Steven Artandi, associate professor of haematology.
In a research article published in the journal Science, the researchers has revealed that he and graduate student Andrew Venteicher carried out this research in collaboration with scientists at the National Cancer Institute-Frederick and the University of Georgia.
Telomerase is normally expressed in adult stem cells and immune cells, as well as in cells of the developing embryo. In these cells, the enzyme caps off the ends of newly replicated chromosomes, allowing unfettered cell division.
The researchers point out that in the absence of telomerase, cells stop dividing or die within a limited number of generations.
Unfortunately, they say, the enzyme is also active in many cancer cells.
Artandi said that he and his colleagues have found that blocking the inappropriate expression of the protein, called TCAB1, in human cancer cells keeps telomerase from reaching its DNA targets, called telomeres, and may limit the cell's life span.
"There are currently no effective telomerase inhibitors. We've never really understood before how the enzyme gets to the telomeres; it's been a complete black box. Now we're starting to piece together how it happens, and that gives us more opportunities to interfere with its function," he said.
Telomerase has been subject of intense research for years, but scientists have been stymied by the enzyme's large size and extreme rarity.
Few cells in the adult body make the huge protein complex, and even they make only tiny amounts, which is why only some members have been identified.
"It's been incredibly challenging to figure out all the protein components of telomerase," said Artandi, who refers to the unknown members of the complex as 'dark matter'.
"We know how big the enzyme is, and it's clear that the known components don't add up to the total. Now we've identified one more member," the researcher added.
In their study, the researchers involved a highly sensitive protein identification technique called mass spectrometry to ferret out TCAB1's presence in telomerase, based on its ability to bind to another known component of the enzyme.
While Artandi's lab had used the same technique last year to identify for the first time two other proteins pontin and reptin that are important for assembling the massive complex, this time they identified TCAB1, a protein of previously unknown function.
Unlike pontin and reptin, TCAB1 is a true component of telomerase.
It is not required for the enzyme's activity, but rather recruits the telomerase complex to processing and holding areas in the nucleus of the cell called Cajal (pronounced "cuh-hall") bodies, which apply the finishing touches to a variety of proteins that use small molecules of RNA to conduct their activities.
When appropriate, TCAB1 then chauffeurs the telomerase complex to the waiting end of a newly replicated chromosome.
"TCAB1 is absolutely necessary for the telomerase to make this jump from Cajal bodies to telomeres," said Artandi.
"When we inhibited its activity in human cancer cells, the telomeres grew shorter," implying the cancer cells would die more quickly, the researcher added.
Before this study, scientists did not know any function of TCAB1.
"Andy (Venteicher) found that TCAB1 binds not only telomerase, but also a specific class of small, non-coding RNA molecules that also end up in the Cajal bodies," said Artandi.
The researcher added that the protein may be a common biological shuttle responsible for delivering a variety of molecules to their destinations.
He and his collaborators plan to continue their study of TCAB1, and also to identify other telomerase components.
"This is a story that's been unfolding over decades. Telomerase is such a high-priority target for many diseases, but it's hard to attack when you know very little about it. But that's changing now," said Artandi.