Cryo-EM structure confirms the existence of dimeric formation of telomerase with two behaviourally independent catalytic cores which are crucial to telomere maintenance

Telomeres, large nucleoprotein complexes which cap the ends of eukaryotic nuclear chromosomes, are integral to genome stability. Telomeres are gradually shortened during cell division and rely on the enzyme telomerase to maintain them by adding repetitive sequences to the ends of chromosomes. The core of telomerase consists of telomerase reverse transcriptase (TERT), a protein which makes telomeric DNA, and telomerase RNA (hTR in humans), which holds the template sequence used by TERT to copy the telomeric repeats. The combined molecular mass of TERT and hTR is roughly 289 kilodaltons (kDa) – under half of the weight of the entire human telomerase structure which is estimated at 600 kDa. This disparity has sparked a debate over telomerase’s composition, with some favouring a dimeric organisation with two copies of TERT and hTR, and others championing a monomeric structure with additional accessory proteins making up the extra weight.
Kelly Nguyen’s group, in the LMB’s Structural Studies Division, previously published the first atomic structure of human telomerase, revealing a monomeric assembly and the identity of additional factors. Though this structure supports the monomeric model of telomerase, biochemical data points to the existence of a dimeric form which Kelly’s group set out to confirm. Their newly published structure is the first direct visualisation of a human telomerase dimer.
Sebastian Balch initiated the study by collecting and analysing a large electron cryomicroscopy (cryo-EM) dataset to explore diverse, architectural formations of human telomerase. This highlighted that although human telomerase is predominantly monomeric, it can present as the dimeric complex. Zala Sekne then performed cryo-EM analyses to determine the structure of the telomerase dimer bound to telomeric DNA. The structure revealed two intertwined telomerase complexes, arranged in an X-shaped configuration, each containing a catalytic core and a Hinge and ACA ribonucleoprotein (H/ACA RNP). This dimeric assembly has a molecular weight of around 1.2 megadaltons (MDa) and contains double the number of subunits as the monomeric structure.
Interestingly, the group found no interactions between the two catalytic cores within the dimer. This usurps previously held assumptions about the nature of the telomerase dimer. Group member Elsa Franco-Echevarría worked with Sebastian and Patryk Ludzia (also Kelly’s group), Yiliang Ding’s group at the John Innes Centre and Rhiju Das’s group at Stanford University to conduct biochemical and cellular analyses of the complex’s behaviour. They found clear evidence to suggest that the dimer is indeed catalytically active, but that the two catalytic cores engage with telomeric DNA independently and demonstrate no clear cooperation.
To better understand the functional significance of this dimer, the group introduced targeted mutations to disrupt the interface between hTR of one telomerase molecule in the dimer with the H/ACA RNP of the other. Disrupted dimerization interface resulted in compromised telomerase assembly and shortened telomeres in cells. This confirms that dimerization is mostly mediated by H/ACA RNP and that the telomerase dimer, though lower in abundance than the monomeric structure, is crucial to telomere maintenance.
Notably, this interface was also found to contain a cluster of several mutations associated with premature ageing disorders. It has long been known that several ageing disorders present with shortened telomeres, and this now suggests that the pathogenic mechanisms behind these diseases rely on defects in dimer formation. This finding highlights a new avenue for future therapeutic studies to explore.
This study confirms the existence of a dimeric state of human telomerase and provides detailed insight into its structure. The findings broaden our understanding of telomere maintenance and provide an explanation as to why this is impacted in the case of some ageing diseases. Additionally, it has been previously established that telomerase activity is markedly increased in cancer cells, in which it enables limitless replication (leading to its nickname ‘the immortality enzyme’). Insights into the mechanisms behind telomerase thus present a potential therapeutic target which could be harnessed in future pharmaceutical interventions.
This work was funded by UKRI MRC, the Wellcome Trust, EMBO, Stanford Bio-X, China Scholarship Council, UKRI BBSRC, Jane Coffin Childs Fund, the National Institute of Health and the Howard Hughes Medical Institute.
Further references
Cryo-EM structure of human telomerase dimer reveals H/ACA RNP-mediated dimerization. Balch, S., Sekne, Z., Franco-Echevarría, E., Ludzia, P., Kretsch, R.C., Sun, W., Yu, H., Ghanim, G.E., Thorkelsson, S., Ding, Y., Das, R., Ngyuyen, T.H.D. Science
Kelly’s group page
Yiliang Ding – John Innes Centre
Rhiju Das – Stanford University
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