Abstract
The telomere repeat units of Candida species are substantially longer and more complex than those in other organisms, raising interesting questions concerning the recognition mechanisms of telomere-binding proteins. Herein we characterized the properties of Candida parapsilosis Cdc13A and Cdc13B, two paralogs that are responsible for binding and protecting the telomere G-strand tails. We found that Cdc13A and Cdc13B can each form complexes with itself and a heterodimeric complex with each other. However, only the heterodimer exhibits high-affinity and sequence-specific binding to the telomere G-tail. EMSA and crosslinking analysis revealed a combinatorial mechanism of DNA recognition, which entails the A and B subunit making contacts to the 3′ and 5′ region of the repeat unit. While both the DBD and OB4 domain of Cdc13A can bind to the equivalent domain in Cdc13B, only the OB4 complex behaves as a stable heterodimer. The unstable Cdc13ABDBD complex binds G-strand with greatly reduced affinity but the same sequence specificity. Thus the OB4 domains evidently contribute to binding by promoting dimerization of the DBDs. Our investigation reveals a rare example of combinatorial recognition of single-stranded DNA and offers insights into the co-evolution of telomere DNA and cognate binding proteins.
Highlights
The ends of linear eukaryotic chromosomes, or telomeres, play critical roles in maintaining genome stability [1,2,3]
To test the generality of this conclusion, we examined the abilities of C. parapsilosis Cdc13A and Cdc13B (CPAR2 602150) to form homo-oligomers and/or hetero-oligomers in co-expression/pull down experiments
These results for C. parapsilosis Cdc13A and Cdc13B are similar to what has been described for the C. albicans homologs [20] and suggest that the AB dimer is generally more stable
Summary
The ends of linear eukaryotic chromosomes, or telomeres, play critical roles in maintaining genome stability [1,2,3]. Because the G-strand is typically longer than the complementary C-strand, most chromosomes terminate in 3 -overhangs commonly referred to as G-tails Both the duplex region of telomeres and the G-tails are bound by protective proteins, and these proteins collectively block DNA repair factors from engaging in aberrant ‘repair’ of the natural chromosome ends as if they are double strand breaks. Cdc homologues display considerable plasticity, with the Saccharomyces and Kluyveromyces family members carrying four OB fold domains ( referred to as large Cdc13s), and the Candida family members carrying just two OB folds (referred to as the small Cdc13s) (Figure 1A) [16] These domains mediate distinct functions in a modular fashion.
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