Unraveling the stepwise maturation of the yeast telomerase

Telomerases elongate the ends of chromosomes required for cell immortality through their reverse transcriptase activity. By using the model organism Saccharomyces cerevisiae we defined the order in which the holoenzyme matures. First, a longer precursor of the telomerase RNA, TLC1 is transcribed and exported into the cytoplasm, where it associates with the protecting Sm-ring, the Est- and the Pop-proteins. This partly matured telomerase is re-import into the nucleus via Mtr10 and a novel TLC1-import factor, the karyopherin Cse1. Remarkably, while mutations in all known transport factors result in short telomere ends, mutation in CSE1 bypasses this defect and become Type I like survivors. Interestingly, both import receptors contact the Sm-ring for nuclear import, which therefore resembles a quality control step in the maturation process of the telomerase. The re-imported immature TLC1 is finally trimmed into the ~1150 nucleotide long mature form. TMG-capping of TLC1 finalizes maturation, leading to mature telomerase.