About 10 years ago, Lundblad discovered that Cdc13, a protein that binds single-stranded telomeric DNA, plays a central role at the telomeres of baker's yeast. But the function of two Cdc13-associated proteins, Stn1 and Ten1, remained unclear.
When members of the Lundblad laboratory searched the Protein Data Bank for relatives of Stn1, they dug up Rpa2, the middle subunit of the RPA complex. In particular, Stn1 and Rpa2 share similarities in a region known as oligonucleotide/oligosaccharide-binding fold or OB-fold, a protein fold that is commonly used to recognize and bind to either DNA or RNA.
Based on these findings, Lundblad and her colleagues developed a model, which predicts that Cdc13, Stn1 and Ten1 come together at the very end of chromosomes to form a telomere-dedicated RPA-like complex they dubbed the t-RPA complex.
Graduate student and first author Hua Gao tested this model, using detailed biochemical experiments which revealed that Stn1 and Ten1 behave just like their counterparts in the conventional RPA complex. However, Stn1 and Ten1 have the same predilection for single-stranded telomeric DNA as Cdc13, thereby helping to ensure that this complex only targets chromosome tips.
According to Lundblad, this finding raises important questions about the potential biological parallels between the conventional RPA complex and the newly discovered t-RPA complex.
"Whereas the conventional RPA complex acts elsewhere in the genome to raise a red flag to attract the attention of the repair machinery," says Lundblad, "curiously, the t-RPA complex performs the exact opposite function at chromosome ends, ensuring that these tips do not become targets for DNA repair."