Function of Rad17/Mec3/Ddc1 and its partial complexes in the DNA damage checkpoint

Area of Science:

• Molecular Biology
• Cellular Biology
• Genetics

Background:

• The Saccharomyces cerevisiae Rad17/Mec3/Ddc1 (Rad17/3/1) complex, analogous to the human 9-1-1 complex, is crucial for DNA damage response.
• It functions upstream in the signal transduction pathway, activating checkpoint systems and leading to cell cycle arrest.
• Structural similarities exist between the Rad17/3/1 complex and the replication clamp PCNA, with Rad17/3/1 loading onto DNA via Rad24-RFC.

Purpose of the Study:

• To investigate the interactions between the checkpoint clamp subunits (Rad17, Mec3, Ddc1).
• To determine the functional activity of partial checkpoint clamp complexes.
• To assess if individual subunits or pairs can form a functional DNA-loading clamp.

Main Methods:

• Studied interactions between Rad17, Mec3, and Ddc1 subunits.
• Assessed the DNA-loading capability of partial clamp complexes using Rad24-RFC.
• Utilized genetic analysis by examining the effects of overexpressing specific subunits in deletion strains (MEC3Δ, DDC1Δ).

Main Results:

• None of the possible partial complexes (e.g., dimers) could form a clamp that is loaded onto DNA by the Rad24-RFC clamp loader.
• Overexpression of DDC1 or RAD17 in a MEC3Δ strain did not rescue DNA damage sensitivity.
• Overexpression of MEC3 or RAD17 in a DDC1Δ strain also failed to rescue DNA damage sensitivity.

Conclusions:

• The complete heterotrimeric Rad17/Mec3/Ddc1 complex is essential for its function as a DNA damage checkpoint clamp.
• Partial complexes or individual subunits lack the necessary structure or interactions to be loaded onto DNA by Rad24-RFC.
• The integrity of the entire Rad17/3/1 complex is indispensable for proper DNA damage response and cell cycle control in Saccharomyces cerevisiae.