Functional interaction between telomere protein TPP1 and telomerase

Human chromosome end-capping and Telomerase regulation require POT1 (Protection of Telomeres 1) and TPP1 proteins, which bind to the 3' ssDNA extension of human telomeres. POT1-TPP1 binding to telomeric DNA activates Telomerase repeat addition processivity. We now provide evidence that this POT1-TPP1 activation requires specific interactions with Telomerase, rather than it being a DNA substrate-specific effect. First, Telomerase from the fish medaka, which extends the same telomeric DNA primer as human Telomerase, was not activated by human POT1-TPP1. Second, mutation of a conserved glycine, Gly100 in the TEN (Telomerase essential N-terminal) domain of TERT, abolished the enhancement of Telomerase processivity by POT1-TPP1, in contrast to other single amino acid mutations. Chimeric human-fish telomerases that contained the human TEN domain were active but not stimulated by POT1-TPP1, showing that additional determinants of processivity lie outside the TEN domain. Finally, primers bound to mouse POT1A and human TPP1 were activated for extension by human Telomerase, whereas mPOT1A-mTPP1 was most active with mouse Telomerase, indicating that these mammalian telomerases have specificity for their respective TPP1 proteins. We suggest that a sequence-specific interaction between TPP1 in the TPP1-POT1-telomeric DNA complex and the G100 region of the TEN domain of TERT is necessary for high-processivity Telomerase action.