MutS and MutL activate DNA helicase II in a mismatch-dependent manner

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• DNA mismatch repair (MMR) is essential for maintaining genomic stability in bacteria.
• Key proteins like MutS, MutL, and DNA helicase II orchestrate the excision of mismatched DNA segments.
• Understanding the synergistic roles of these proteins is vital for elucidating MMR mechanisms.

Purpose of the Study:

• To investigate the functional interaction between MutL and DNA helicase II during Escherichia coli mismatch repair.
• To quantify the effect of MutL on the helicase activity of DNA helicase II.
• To determine the requirements for MutS, MutL, and DNA helicase II in DNA strand displacement.

Main Methods:

• In vitro assays using purified proteins (MutS, MutL, DNA helicase II) and defined DNA substrates.
• Enzyme kinetics to measure helicase unwinding activity on single-stranded and gapped circular DNA.
• Analysis of protein-DNA interactions and ATP-dependent DNA strand displacement.

Main Results:

• MutL enhances DNA helicase II unwinding activity by over 10-fold on a conventional substrate.
• MutS and MutL cooperatively activate DNA helicase II on a model substrate with a mismatched region.
• ATP and the presence of a mismatch are essential for oligonucleotide displacement mediated by MutS, MutL, and DNA helicase II.

Conclusions:

• MutL plays a critical regulatory role in activating DNA helicase II for mismatch repair.
• The combined action of MutS, MutL, and DNA helicase II is necessary for efficient processing of mismatched DNA.
• DNA helicase II is specifically implicated in mismatch-provoked excision, unlike the homologous Rep helicase.