Oligomerization of the telomerase reverse transcriptase from Euplotes crassus

The telomerase ribonucleoprotein reverse transcriptase uses its RNA subunit as a template to synthesize telomeric repeats and maintain telomere tracts on chromosome ends. In the ciliate Euplotes crassus, the core telomerase ribonucleoprotein particle undergoes a developmentally programmed assembly into three higher order complexes after mating. Here, we provide evidence using oligonucleotide-directed affinity purification that all of the E.crassus telomerase complexes contain at least two enzyme active sites. Furthermore, we show using co-immunoprecipitation experiments that EcTERT, the telomerase catalytic subunit, undergoes multimerization in vitro. Two independent interaction domains were identified in EcTERT, one at the N-terminus that spans amino acids 186-354 and one at the C-terminus that spans amino acids 755-857. Unexpectedly, we found that TERT can form head-to-head, tail-to-tail and head-to-tail oligomers in vitro, implying that E.crassus telomerase has the potential to assume different conformations in vivo. Together, these data indicate that oligomerization is a conserved feature of telomerase and that the minimal functional unit of the enzyme is a dimer.