Structure of Active, Dimeric Human Telomerase

Telomerase contains a large RNA subunit TER and a protein catalytic subunit TERT. Whether telomerase functions as monomer or dimer has been a matter of debate. Here we report biochemical and labeling data that show that in vivo assembled human telomerase contains two TERT subunits and binds two telomeric DNA substrates. Importantly, catalytic activity requires both TERT active sites to be functional, demonstrating that human telomerase functions as a dimer. We also present the three-dimensional structure of active, full-length human telomerase dimer, determined by single-particle electron microscopy in negative stain. Telomerase has a bilobal architecture, with the two monomers linked by a flexible interface. The monomer reconstruction at 23Å resolution, and fitting of the atomic structure of the beetle TERT subunit reveals the spatial relationship between RNA and protein subunits, providing insights into the telomerase architecture.