Sequential phosphorylation of CST subunits by different cyclin-Cdk1 complexes orchestrate telomere replication

ABSTRACT

Telomeres are nucleoprotein structures that cap the ends of linear chromosomes. Telomere homeostasis is central to maintaining genomic integrity. In budding yeast, Cdk1 phosphorylates the telomere-specific binding protein, Cdc13, promoting the recruitment of telomerase to telomere and thereby telomere elongation. Cdc13 is also an integral part of the CST (Cdc13-Stn1-Ten1) complex that is essential for telomere capping and counteracting telomerase-dependent telomere elongation. Therefore, telomere length homeostasis is a balance between telomerase-extendable and CST-unextendable states. In our earlier work, we showed that Cdk1 also phosphorylates Stn1 which occurs sequentially following Cdc13 phosphorylation during cell cycle progression. This stabilizes the CST complex at the telomere and results in telomerase inhibition. Hence Cdk1-dependent phosphorylations of Stn1 acts like a molecular switch that drives Cdc13 to complex with Stn1-Ten1 rather than with telomerase. However, the underlying mechanism of how a single cyclin-dependent kinase phosphorylates Cdc13 and Stn1 in temporally distinct windows is largely unclear. Here, we show that S phase cyclins are necessary for telomere maintenance. The S phase and mitotic cyclins facilitate Cdc13 and Stn1 phosphorylation respectively, to exert opposing outcomes at the telomere. Thus, our results highlight a previously unappreciated role for cyclins in telomere replication.

KEYWORDS:

• Budding yeast
• cell cycle
• Cdk1
• CST complex
• cyclins
• phosphorylation
• telomere

Disclosure of potential conflicts of interest

The authors have no potential conflict of interest to be reported.

Acknowledgments

We thank Dr. Uttam Surana and Dr. Hong Hwa Lim for technical support, invaluable discussions and critical reading of this manuscript. We thank Dr. Mardo Koivomagi for his helpful suggestions on purification of Cyclin-Cdk1 complexes and in vitro kinase assays.

Funding

The study is funded by MOE Tier 2 grant to S.L.

Author contributions

V. G. and S. L. conceived and coordinates the study and wrote the paper. V.G. and C.R.T. performed and analyzed the experiments. All authors reviewed the data and approved the final version of the manuscript.