Rap1 and Cdc13 have complementary roles in preventing exonucleolytic degradation of telomere 5' ends

Area of Science:

• Molecular biology
• Genetics
• Biochemistry

Background:

• Telomeres possess a single-stranded 3' overhang, and excessive length can trigger DNA damage responses.
• Telomere attrition is linked to aging and cancer, necessitating understanding of telomere maintenance mechanisms.
• While genetic studies identified key players in preventing 5' end hyper-resection, their precise molecular mechanisms remain unclear.

Purpose of the Study:

• To investigate the in vitro mechanism by which N. castellii Cdc13 and Rap1 protect telomeric 5' ends from degradation by λ-exonuclease.
• To elucidate the roles of Cdc13 and Rap1 in maintaining the proper length of the 3' overhang at telomeres.

Main Methods:

• Utilized an in vitro DNA 5' end protection assay with homogeneous telomeric repeat sequences from N. castellii.
• Assessed the protective capabilities of Cdc13 and Rap1 against λ-exonuclease degradation at defined binding sites relative to the 5' end.

Main Results:

• Both Cdc13 and Rap1 demonstrated efficient protection when bound close to the 5' end.
• Rap1 provided protection even when bound to double-stranded DNA away from the 5' end.
• The DNA-binding domain and wrapping loop region of Rap1 were found to be essential for 5' end protection.
• Rap1 also facilitated protection when its binding site spanned the double-stranded/single-stranded DNA junction.

Conclusions:

• Cdc13 and Rap1 play complementary roles in maintaining the appropriate length of the 3' overhang.
• Rap1 has a significant role in 5' end protection, contributing to telomere stability.