Abstract
Telomerase is an RNA-protein complex responsible for the extension of one strand of telomere terminal repeats. The catalytic protein subunit of telomerase, known generically as telomerase reverse transcriptase (TERT), exhibits significant homology to reverse transcriptases (RTs) encoded by retroviruses and retroelements. The mechanisms of telomerase may therefore be similar to those of the conventional reverse transcriptases. In this report, we explore potential similarity between these two classes of proteins in a region with no evident sequence similarity. Previous analysis has implicated a C-terminal domain of retroviral RTs (known as the "thumb" domain) in template-primer binding and in processivity control. The equivalent region of TERTs, although similar to one another, does not exhibit significant sequence homology to retroviral RTs. However, we found that removal of this region of yeast TERT similarly resulted in a decrease in the stability of telomerase-DNA complex and in the processivity of telomerase-mediated nucleotide addition. Moreover, the C-terminal domain of TERT exhibits a nucleic acid binding activity when recombinantly expressed and purified. Finally, amino acid substitutions of conserved residues in this region of TERT were found to impair telomerase activity and processivity. We suggest that mechanistic similarity between telomerase and retroviral RTs may extend beyond the regions with apparent sequence similarity.
Highlights
Telomerase is an RNA-protein complex responsible for the extension of one strand of telomere terminal repeats
We suggest that mechanistic similarity between telomerase and retroviral reverse transcriptases (RTs) may extend beyond the regions with apparent sequence similarity
Removal of the C-terminal Extension of telomerase reverse transcriptase (TERT) Causes a Reduction in the Stability of Telomerase-DNA Interaction—To probe the function of the CTE, we generated a deletion mutant of Est2p that removed most of the amino acids beyond the last conserved RT motif (Fig. 1A, motif E)
Summary
Telomerase is an RNA-protein complex responsible for the extension of one strand of telomere terminal repeats. The catalytic reverse transcriptase protein subunit (TERT), first purified from Euplotes aediculatus as p123, was found to be homologous to Est2p, a yeast protein required for telomere maintenance [7,8,9] Both proteins possess RT-like motifs, alterations therein can cause inactivation of telomerase activity and reduction in telomere lengths. Some other crucial RT residues (e.g. a Gln in motif BЈ) appear to be less important or even dispensable for telomerase function [9] Together, these results suggest that despite the high degree of sequence divergence (Ͻ20% sequence identity), TERT and conventional RTs may possess very similar polymerization mechanisms
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