FIGURE 2: Schematic model of the mechanism(s) that keep telomere length homeostasis.

Long telomeres carry many copies of the Rap1 protein, and its associated Rif1 and Rif2 proteins, which prevent recruitment of telomerase. Telomeres in this state are said to be in a “non-extendable” configuration [69]. Telomeres shorten as a consequence of the “end replication problem” or by the action of exonucleases (Pacman). It is still unclear what circumstances allow recruitment of nucleases. Short telomeres carry a low number of Rap1/Rif units, and the CST is able to recruit telomerase, thus elongating the telomere. It is assumed that the presence of an active telomerase prevents recruitment of shortening nucleases.

69. Teixeira MT, Arneric M, Sperisen P, Lingner J (2004). Telomere length homeostasis is achieved via a switch between telomerase- extendible and -nonextendible states. Cell 117(3): 323-35. http://dx.doi.org/10.1016/s0092-8674(04)00334-4