Zipper head mechanism of telomere synthesis by human telomerase

Futang Wan,Guohui Li,Jian Wu,Yongbo Ding,Xiangyu Yan,Pengfei Lan,Yuebin Zhang,Shaobai Li,Zhenfang Wu,Ming Lei,Lin Yang

doi:10.1038/s41422-021-00586-7

Abstract

Telomerase, a multi-subunit ribonucleoprotein complex, is a unique reverse transcriptase that catalyzes the processive addition of a repeat sequence to extend the telomere end using a short fragment of its own RNA component as the template. Despite recent structural characterizations of human and Tetrahymena telomerase, it is still a mystery how telomerase repeatedly uses its RNA template to synthesize telomeric DNA. Here, we report the cryo-EM structure of human telomerase holoenzyme bound with telomeric DNA at resolutions of 3.5 Å and 3.9 Å for the catalytic core and biogenesis module, respectively. The structure reveals that a leucine residue Leu980 in telomerase reverse transcriptase (TERT) catalytic subunit functions as a zipper head to limit the length of the short primer–template duplex in the active center. Moreover, our structural and computational analyses suggest that TERT and telomerase RNA (hTR) are organized to harbor a preformed active site that can accommodate short primer–template duplex substrates for catalysis. Furthermore, our findings unveil a double-fingers architecture in TERT that ensures nucleotide addition processivity of human telomerase. We propose that the zipper head Leu980 is a structural determinant for the sequence-based pausing signal of DNA synthesis that coincides with the RNA element-based physical template boundary. Functional analyses unveil that the non-glycine zipper head plays an essential role in both telomerase repeat addition processivity and telomere length homeostasis. In addition, we also demonstrate that this zipper head mechanism is conserved in all eukaryotic telomerases. Together, our study provides an integrated model for telomerase-mediated telomere synthesis.

Highlights

Eukaryotic linear chromosomes face two major challenges, known as the end-replication and end-protection problems.[1,2] Telomeres, highly ordered DNA–protein complexes located at chromosomal ends, are evolved to solve these two problems and ensure complete genome replication and genome stability.[3]
A model of telomere synthesis by human telomerase Based on our structural and computational data reported here combined with previous studies, we propose an integrated model for human telomerase-mediated telomere synthesis (Fig. 7a)
At the beginning of each cycle, the 3′ terminus of telomeric overhang pairs with the 3′ alignment region of the RNA template to form a duplex with three W-C and one wobble base pairs at positions 2–5, which fits into the active site defined by the zipper head residue Leu[980] (Fig. 7a, Stage 1)

Summary

Introduction

Eukaryotic linear chromosomes face two major challenges, known as the end-replication and end-protection problems.[1,2] Telomeres, highly ordered DNA–protein complexes located at chromosomal ends, are evolved to solve these two problems and ensure complete genome replication and genome stability.[3]. Telomerase deficiency is linked to several human telomere syndromes including dyskeratosis congenita and Hoyeraal-Hreidarsson syndrome.[12,13]

Methods

Results

Conclusion