LARP3, LARP7, and MePCE are Involved in the Early Stage of Human Telomerase RNA Biogenesis

Human telomerase assembly is a highly dynamic process. Using biochemical approaches, we found that LARP3 and LARP7/MePCE are involved in the early stage and that their binding to hTR is destabilized when the mature hTR is produced. LARP3 plays a negative role in preventing the processing of the 3'-extended long (exL) form and binding of LARP7 and MePCE. Interestingly, the tertiary structure of the exL form prevents LARP3 binding and facilitates hTR biogenesis. Supporting this process, LARP3 at low levels promotes hTR maturation, increases telomerase activity, and elongates telomeres. LARP7 and MePCE knockdown inhibits the conversion of the 3'-extended short (exS) form into mature hTR and the cytoplasmic accumulation of hTR, resulting in telomere shortening. Our data suggest that LARP3 and LARP7/MePCE mediate the processing of hTR precursors and thus control the production of functional telomerase. Overall design: To investigate the function of PARN, LARP7, and MePCE in hTR 3'-end processing. We isolated the RNA from PARN, LARP7, and MePCE knockdown cell lines in HeLa cells, followed by 3' RACE sequencing.