Telomerases protect the ends of linear chromosomes from shortening. They are composed of an RNA (TLC1 in S. cerevisiae) and several proteins. TLC1 undergoes several maturation steps before it is exported into the cytoplasm to recruit the Est proteins for complete assembly. The mature telomerase is subsequently reimported into the nucleus, where it fulfills its function on telomeres. Here, we show that TLC1 export into the cytoplasm requires not only the Ran GTPase-dependent karyopherin Crm1/Xpo1 but also the mRNA export machinery. mRNA export factor mutants accumulate mature and export-competent TLC1 RNAs in their nuclei. Moreover, TLC1 physically interacts with the mRNA transport factors Mex67 and Dbp5/Rat8. Most importantly, we show that the nuclear export of TLC1 is an essential step for the formation of the functional RNA containing enzyme, because blocking TLC1 export in the mex67-5 xpo1-1 double mutant prevents its cytoplasmic maturation and leads to telomere shortening.
Model for the life cycle of TLC1. TLC1 is generated in the nucleus by RNA polymerase II (RNAP II). Upon association of the Sm ring, the RNA is processed and the poly(A) tail removed by the nuclear exosome. The CAP is hypermethylated and a trimethylguanine (TMG) CAP is generated. The Ran-dependent nuclear export signal (NES) export receptor Crm1/Xpo1 associates via a currently unknown NES containing protein (X). Proper nuclear export additionally requires the mRNA export machinery; the export receptor heterodimer Mex67-Mtr2 associates and supports transport through the nuclear pore complex (NPC). Upon transit Mex67-Mtr2 is displaced by the DEAD box RNA helicase Dbp5 that is located at the cytoplasmic filaments of the NPC, where it interacts with Rat7/Nup159. The subsequent cytoplasmic maturation of the telomerase requires the association of several proteins including Est1, Est2, and Est3. The mature telomerase is reimported into the nucleus via the importins Mtr10 and Kap122, where it finally associates with the telomere ends.