21st century genetics: Mass spectrometry of yeast telomerase

Telomerase is a specialized reverse transcriptase that maintains the ends of chromosomes in almost all eukaryotes. The core of telomerase consists of telomerase RNA and the reverse transcriptase that uses a short segment without the RNAto template the addition of telomeric repeats. In addition, one or more accessory proteins are required for telomerase action in vivo. The beststudied accessory protein is Est1, which is conserved from yeasts to humans. In budding yeast, Est1 has two critical in vivo functions: By interaction with Cdc13, a telomere-binding protein, it recruits telomerase to telomeres, and it also increases telomerase activity. Although budding yeast telomerase is highly regulated by the cell cycle, Est1 is the only telomerase subunit whose abundance is cell cycle-regulated. Close to 400 yeast genes are reported to affect telomere length, although the specific function of most of them is unknown. With the goal of identifying novel telomerase regulators by mass spectrometry, we developed methods for purifying yeast telomerase and its associated proteins. We summarize the methods we used and describe the experiments that show that four telomerase-associated proteins identified by mass spectrometry, none of which had been linked previously to telomeres, affect telomere length and cell cycle regulation of telomerase by controlling Est1 abundance.