Regulating telomerase activity impacts DNA double-strand break (DSB) repair, as well as the length of telomeres at the ends of our chromosomes. The misregulation of telomere maintenance is also a hallmark of all cancers. However, despite its critical importance, the mechanism by which cells maintain a homeostatic telomere length and properly regulate telomerase activity remains ill defined. Using yeast genetics, we found that the Saccharomyces cerevisiae homolog of the human RECQL4 helicase, called Hrq1, acts as a telomerase inhibitor at both DSBs and telomeres. By investigating the mechanism biochemically, we discovered that Hrq1 acts synergistically with the Pif1 helicase to “tune” telomerase activity up and down. These data led to a model wherein the concerted activity of two helicases can stimulate or inhibit telomerase, as needed, at the level of individual telomeres during each cell cycle. Currently, we are further examining this interaction between Hrq1 and Pif1, as well as determining the roles of other helicases and telomere-binding proteins in the modulation of homeostatic telomere length.
Hrq1 and Pif1 synergistically modulate in vitro telomerase activity. (Left) Alone, Pif1 both stimulates and inhibits telomerase activity in a concentration-dependent manner; high concentrations of Hrq1 slightly stimulate telomerase. However, the combined effect of both helicases dampens the telomerase stimulation displayed by Pif1 and enhances the inhibition. (Middle) Combining inactive Hrq1 with wild-type Pif1 yields a super-telomerase inhibitor. (Right) Combining inactive Pif1 with wild-type Hrq1 greatly stimulates telomerase activity. Thus, the activity of these helicases, likely controlled by PTMs in vivo (see Project 3), tunes telomerase activity to generate a homeostatic telomere length.
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