Functional analysis of the C-terminal extension of telomerase reverse transcriptase. A putative "thumb" domain.

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.