Nucleolar localization of hTERT protein is associated with telomerase function

Telomerase is a ribonucleoprotein (RNP) complex that prevents telomeric erosion in eukaryotic cells. Although there are also other associated proteins in this complex, the catalytic activity of this complex is composed of two components. One is a reverse transcriptase subunit, TERT (telomerase reverse transcriptase); another is an RNA template subunit, TR (telomerase RNA). However, where these two parts are assembled in mammalian cells is unclear. In the present study, we investigated the intracellular distribution of human TERT (hTERT) protein and observed that hTERT protein in individual cells could concentrate in or be excluded from the nucleolus. Further we have identified a nucleolar targeting signal in the hTERT protein. Point mutations that disrupted this signal region interrupted telomerase RNP complex formation, decreased telomerase activity, and caused telomere shortening in cells transfected with mutated hTERT. Our results indicate that the amino acid sequence of the extreme N-terminus (1-15) of hTERT, which targets nucleolar localization of the protein, is required for full telomerase function.