The Product of the Survival of Motor Neuron(SMN) Gene is a Human Telomerase-associated Protein

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Abstract
Telomerase is a ribonucleoprotein (RNP) complex that is minimally composed of a protein catalytic subunit, the telomerase reverse transcriptase (TERT), and an RNA component, the telomerase RNA. Thesurvival of motor neuron (SMN) gene codes for a protein involved in the biogenesis of certain RNPs. Here, we report that SMN is a telomerase-associated protein. Using in vitro binding assays and immunoprecipitation experiments, we demonstrate an association between SMN and the telomerase RNP in vitro and in human cells. The specific immunopurification of SMN from human 293 cells copurified telomerase activity, suggesting that SMN associates with a subset of the functional telomerase holoenzyme. Our results also indicate that the human telomerase RNA and the human (h) TERT are not associated with Sm proteins, in contrast to Saccharomyces cerevisiae telomerase. Immunofluorescence analysis showed that hTERT does not specifically colocalize with wild-type SMN in gems or Cajal bodies. However, a dominant-negative mutant of SMN (SMNΔN27) previously characterized to elicit the cellular reorganization of small nuclear RNPs caused the accumulation of hTERT in specific SMNΔN27-induced cellular bodies. Furthermore, coexpression of SMNΔN27 and hTERT in rabbit reticulocyte lysates decreased the efficiency of human telomerase reconstitution in vitro. Our results establish SMN as a novel telomerase-associated protein that is likely to function in human telomerase biogenesis.