Telomerase synthesizes DNA sequences that protect the integrity of chromosome ends. A model for how the components of this enzyme complex co-assemble offers insight into its structure and function. See Article p.187
Early research on the mechanism of DNA replication uncovered a startling fact: each round of cell division slightly shortens the ends of chromosomal DNA, which if left unchecked would eventually lead to the loss of essential genetic material1. This 'end replication problem' is solved by telomeres — structures at the ends of chromosomes that contain a series of non-coding DNA repeats, and which become shorter themselves but protect the coding regions from damage2. To synthesize and maintain telomeres in rapidly dividing cells, the enzyme telomerase comes into play3. The discovery that telomerase has a key role in the immortalization of cancer cells by preventing telomere loss during cancer-cell division4 has sparked efforts to understand the chemical structure of this enzyme complex. But despite more than two decades of research, many details of telomerase structure have remained unclear, until Jiang and colleagues' report5 on page 187 of this issue*.
Telomerase has been particularly refractory to structural studies because of its naturally low abundance and its complexity. This holoenzyme — a multi-subunit complex consisting of several proteins and an evolutionarily conserved telomerase RNA — has proven challenging to reconstitute in large quantities in vitro. Instead, structural biologists have focused on fragments of the complex that were more easily tamed, providing glimpses into parts of it6,7.
Much of the pioneering structural work was performed on telomerase from the model organism Tetrahymena thermophila. More recently, biochemical studies of naturally assembled telomerase complexes from this well-characterized protozoan established the entire complement of molecules that comprise the functional holoenzyme7. With the molecular players identified, and partial high-resolution structures accumulating, a void emerged in understanding how the pieces of the telomerase complex assemble. Jiang et al. present a structure of the T. thermophila telomerase obtained by electron microscopy (EM), providing the first view of how the telomerase puzzle fits together.
The initial structure that the authors solved using EM reconstruction techniques provided a three-dimensional 'envelope' of the holoenzyme's overall structure at 25 ångströms resolution. To identify the position of each subunit within this structure, the researchers turned to an elegant combination of genetic manipulation and biochemical purification techniques. This approach allowed site-specific attachment of bulky antibodies to the complex, which could in turn be visualized as additional density in the EM reconstructions when compared with the unlabelled enzyme. Similarly, they engineered a unique RNA sequence into the complex to localize the telomerase RNA. This sequence binds to a viral coat protein that, like the antibody labelling, introduced unambiguous EM density. These experiments were sufficiently specific to identify the locations of all but one of the seven proteins in the complex, as well as a crucial region of the telomerase RNA.
Previous biochemical work suggested that, functionally, telomerase can be divided into two parts: a catalytic core consisting of components essential for the enzyme's DNA-extension activity in vivo, and a set of accessory factors that promote multiple rounds of telomere-DNA synthesis8. Jiang and co-workers' structure reveals that the holoenzyme is spatially organized around this division, with the catalytic core (consisting of the essential telomerase RNA TER and the proteins p65 and TERT) on one side and the accessory factors (p75, p19, p45 and Teb1) on the other (Fig. 1). The authors propose a complete model of the catalytic core that is consistent with existing biochemical data of known inter-subunit interactions, thus providing a validation of their approach.
The structure also reveals a surprise: the previously uncharacterized holoenzyme protein p50 serves as a crucial bridge between the catalytic core and the accessory factors, and, on the basis of in vitro studies, has a central role in promoting high levels of telomerase activity. The unexpected importance of the p50 protein in supporting telomerase holoenzyme assembly and function suggests that it should be the subject of future study. In particular, it would be of great interest to identify p50-related proteins in other biological systems, such as vertebrates and yeast, and to determine whether these proteins perform similar roles in mediating the assembly of multi-subunit biological complexes.
The structure reported in the present paper provides a much-anticipated model for telomerase structure and establishes a crucial framework for future structure–function analyses of this holoenzyme. For instance, it proposes sites of RNA–protein and protein–protein interaction that should be characterized. Also, it remains unclear whether the structural organization between DNA-handling and core telomerase factors is preserved in the mammalian telomerase enzyme. A recently solved9 EM structure of human telomerase suggests that, unlike T. thermophila telomerase, the human enzyme can form a functional dimer. However, the structure gives no information about the interaction of human telomerase with known telomerase-associated DNA-binding proteins.
Ultimately, a complete mechanistic understanding of telomerase assembly and function will require high-resolution structures that provide atomic-level insight into the functional contributions of proteins, RNA and the DNA substrates within the telomerase complex. Given the close link between telomerase activity and cancer, mechanistic information derived from telomerase structural studies could be useful for the design and optimization of anticancer drugs targeting this holoenzyme. Historically, EM reconstructions like that reported by Jiang et al. have been milestones on the path towards solving the X-ray crystal structures of other essential cellular machines, including the ribosome and RNA polymerase. So an even clearer picture of the telomerase holoenzyme might be in sight.
*This article and the paper5 under discussion were published online on 3 April 2013.