Mms22-Rtt107 axis attenuates the DNA damage checkpoint and the stability of the Rad9 checkpoint mediator

Area of Science:

• Cellular biology
• Molecular genetics
• DNA damage response

Background:

• The DNA damage checkpoint is crucial for genome stability, halting cell cycle progression upon genotoxic stress.
• While essential, prolonged checkpoint activation can hinder cell survival, necessitating downregulation mechanisms.
• Strategies for dampening the DNA damage checkpoint are not fully elucidated.

Purpose of the Study:

• To investigate the mechanisms by which cells downregulate the DNA damage checkpoint.
• To identify novel pathways involved in checkpoint control.
• To understand how Rtt107 contributes to checkpoint regulation.

Main Methods:

• Utilized budding yeast as a model organism.
• Investigated protein interactions between Rtt107, Mms22, and ubiquitin ligase complexes.
• Assessed the role of these interactions in the degradation of the checkpoint protein Rad9.
• Examined the localization of the ubiquitin ligase complex on chromatin.
• Compared the Rtt107-Mms22 pathway with the Rtt107-Slx4 pathway.

Main Results:

• Identified a novel pathway involving scaffold protein Rtt107, its partner Mms22, and an associated ubiquitin ligase complex.
• Demonstrated that this pathway promotes the proteasomal degradation of the key checkpoint factor Rad9.
• Showed that Rtt107 binding to Mms22 enriches the ubiquitin ligase on chromatin, targeting chromatin-bound Rad9.
• Revealed that the Rtt107-Mms22 axis functions in parallel to the Rtt107-Slx4 axis, which displaces Rad9 from chromatin.

Conclusions:

• Budding yeast employs a bifurcated "anti-Rad9" strategy, mediated by Rtt107, to optimally downregulate the DNA damage checkpoint.
• The Rtt107-Mms22 pathway facilitates Rad9 degradation via proteasomal machinery.
• The Rtt107-Mms22 and Rtt107-Slx4 pathways provide distinct but complementary mechanisms for checkpoint inactivation.