Activation of Saccharomyces cerevisiae Mlh1-Pms1 Endonuclease in a Reconstituted Mismatch Repair System

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Mismatch repair (MMR) is crucial for genomic stability.
• Previous studies reconstituted an Mlh1-Pms1-independent MMR reaction.
• The precise mechanisms of Mlh1-Pms1 endonuclease activation and its role in different MMR pathways remain incompletely understood.

Purpose of the Study:

• To reconstitute and characterize Mlh1-Pms1 endonuclease activation and MMR reactions in vitro.
• To elucidate the roles of key MMR proteins, including Msh2-Msh6, PCNA, RFC, and Exo1, in these reconstituted systems.
• To investigate the relationship between in vitro reconstituted reactions and in vivo MMR pathways, particularly Exo1-independent MMR.

Main Methods:

• Reconstitution of a mispair-dependent Mlh1-Pms1 endonuclease activation reaction.
• Reconstitution of a Mlh1-Pms1-dependent 3' nick-directed MMR reaction.
• Utilized various Saccharomyces cerevisiae proteins, including Msh2-Msh6, PCNA, RFC, Exo1, RPA, and DNA polymerase δ.
• Analysis of mutant proteins to determine functional requirements.

Main Results:

• Successfully reconstituted both Mlh1-Pms1 endonuclease activation and a 3' nick-directed MMR reaction.
• Both reactions require Mg(2+) and Mn(2+) for optimal activity.
• Msh2-Msh6-mediated recognition and recruitment of Mlh1-Pms1 are essential, but sliding clamp formation is not required.
• Mutant proteins defective in Exo1-independent MMR in vivo showed partial defects in the reconstituted reactions, linking in vitro findings to in vivo processes.

Conclusions:

• The reconstituted reactions accurately reflect key aspects of Mlh1-Pms1 activation in Exo1-independent MMR.
• This system provides a powerful tool for dissecting the molecular mechanisms of MMR.
• Facilitates future studies on both Exo1-independent and Exo1-dependent MMR pathways.