Crystal structures of N-terminally truncated telomerase reverse transcriptase from fungi

Abstract

Telomerase plays critical roles in cellular aging, in the emergence and/or development of cancer, and in the capacity for stem-cell renewal, consists of a catalytic telomerase reverse transcriptase (TERT) and a template-encoding RNA (TER). TERs from diverse organisms contain two conserved structural elements: the template-pseudoknot (T-PK) and a helical three-way junction (TWJ). Species-specific features of the structure and function of telomerase make obtaining a more in-depth understanding of the molecular mechanism of telomerase particularly important. Here, we report the first structural studies of N-terminally truncated TERTs from Candida albicans and Candida tropicalis in apo form and complexed with their respective TWJs in several conformations. We found that Candida TERT proteins perform only one round of telomere addition in the presence or absence of PK/TWJ and display standard reverse transcriptase activity. The C-terminal domain adopts at least two extreme conformations and undergoes conformational interconversion, which regulates the catalytic activity. Most importantly, we identified a conserved tertiary structural motif, called the U-motif, which interacts with the reverse transcriptase domain and is crucial for catalytic activity. Together these results shed new light on the structure and mechanics of fungal TERTs, which show common TERT characteristics, but also display species-specific features.

Zipper head mechanism of telomere synthesis by human telomerase

“…S11b ). 28 , 29 , 49 Similar to human TERT Leu980 , these non-glycine zipper head residues very likely also limit the length of the primer–template duplex in the active site and facilitate RAP during repetitive telomere extension. Therefore, we propose an evolutionally conserved zipper head mechanism for all eukaryotic telomerases in which a non-glycine residue, not its amino acid identity at the zipper head position of TERT, is essential for the repetitive telomere extension.…”