Tying up the Ends: Recognition of ssDNA at Telomeres

Abstract

Telomere dysfunction has been implicated in several diseases including cancer and aging. The two main roles of telomeres are end-protection and length regulation and the shelterin complex lies at the heart of both of these functions. Pot1 is the single-stranded DNA-binding component of shelterin and is important for genome stability, telomere length regulation, and C-strand resection, all functions that rely on its ssDNA-binding ability. We have structurally and biochemically characterized the dual OB-fold DNA-binding domain of Pot1 from S. pombe, providing insights into potential mechanisms of telomere length regulation and elucidates novel and broadly applicable features of protein/nucleic acid recognition. X-ray crystal structures of the second OB-fold of Pot1, Pot1pC, bound to various ssDNA sequences reveal a unique plasticity at the DNA-binding interface. Global rearrangements of the entire interface allow the accommodation of complementary base substitutions despite the abundance of apparently base-specific hydrogen bonds. This structural plasticity likely explains the ability of S. pombe Pot1 to accommodate the natural heterogeneity in S. pombe telomeric sequence. Furthermore, these mechanisms of accommodation allow for a high-affinity/low-specificity binding mode that is likely utilized by other sequence nonspecific ssDNA and ssRNA-binding proteins. We also use NMR and biochemical techniques to study how the behavior of the individual OB-folds is modulated in the context of the complete DNA-binding domain. Multiple DNA-binding domains often exist within a single protein or complex in biology, but the functional importance of these tandem arrangements is rarely known. This work reveals a malleable interface between domains that allows for multiple DNA-binding modes that can be utilized to provide access for telomerase.