Mispair-bound human MutS-MutL complex triggers DNA incisions and activates mismatch repair

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• DNA mismatch repair (MMR) is crucial for maintaining genomic stability.
• The roles of MutS and MutL ATPases in MMR initiation and execution remain incompletely understood.
• The mechanism of MutS-MutL complex coordination, particularly the necessity of ATP-dependent sliding, is debated.

Purpose of the Study:

• To elucidate the precise role of human MutSα sliding in DNA mismatch repair.
• To differentiate between sliding and protein-protein interactions in MMR initiation and execution.
• To resolve ambiguities in the MMR pathway concerning mismatch recognition and removal.

Main Methods:

• Functional DNA mismatch repair (MMR) assays were employed.
• Experimental systems were designed to inhibit protein sliding on DNA.
• The recruitment and activity of key MMR proteins, including MutSα, MutLα, and Exonuclease 1 (Exo1), were analyzed.

Main Results:

• MutSα sliding is not essential for MMR initiation but is critical for the final removal of mispaired bases.
• MutSα recruits MutLα to form a complex that nicks the daughter strand 5' to the mismatch.
• Exonuclease 1 (Exo1) is recruited to the nick for 5'→3' excision, and its activity is dependent on MutSα dissociation from the mismatch.

Conclusions:

• ATP-dependent sliding of MutSα is required for efficient mismatch excision, not initiation.
• The study clarifies the sequential events in MMR, involving nicking, recruitment of Exo1, and ATP-dependent dissociation for complete repair.
• These findings provide novel mechanistic insights into DNA mismatch repair and its regulation.