FIGURE 2.

Cdc13 at the crossroads. Cdc13 forms separate complexes with different functions at telomeres. Cell cycle regulated post-translational modifications control the balance between these complexes. Siz1- and/or Siz2-dependent SUMOylation on Lys909 located in the C-terminus of Cdc13 promotes its interaction with Stn1 and formation of the CST complex. This modification peaks in early to mid S-phase (Hang et al., 2011). Cyclin-dependent kinase Cdk1 (Cdc28) phosphorylates Cdk1 on Thr308 located in the telomerase recruitment domain of Cdc13 Li et al., 2009). This modification occurs in late S to G2-phases, and it favors the interaction of Cdc13 with Est1 at the expense of its interaction with Stn1, thus promoting telomere uncapping and telomerase recruitment (Li et al., 2009). When telomerase action is accomplished, Cdc13 switches back to interaction with Stn1, and CST complex recruits polα-primase for the C-strand synthesis. Other modifications are likely involved in fine-tuning the balance between the complexes formed by Cdc13 (Wu et al., 2013).