Delineation of the high-affinity single-stranded telomeric DNA-binding domain of Saccharomyces cerevisiae Cdc13.

Cdc13 is an essential protein from Saccharomyces cerevisiae that caps telomeres by protecting the C-rich telomeric DNA strand from degradation and facilitates telomeric DNA replication by telomerase. In vitro, Cdc13 binds TG-rich single-stranded telomeric DNA with high affinity and specificity. A previously identified domain of Cdc13 encompassing amino acids 451-694 (the 451-694 DBD) retains the single-stranded DNA-binding properties of the full-length protein; however, this domain contains a large unfolded region identified in heteronuclear NMR experiments. Trypsin digestion and MALDI mass spectrometry were used to identify the minimal DNA-binding domain (the 497-694 DBD) necessary and sufficient for full DNA-binding activity. This domain was completely folded, and the N-terminal unfolded region removed was shown to be dispensable for function. Using affinity photocrosslinking to site-specifically modified telomeric single-stranded DNA, the 497-694 DBD was shown to contact the entire 11mer required for high-affinity binding. Intriguingly, both domains bound single-stranded telomeric DNA with much greater affinity than the full-length protein. The full-length protein exhibited the same rate of dissociation as both domains, however, indicating that the full-length protein contains a region that inhibits association with single-stranded telomeric DNA.