Human MSH2 (hMSH2) protein controls ATP processing by hMSH2-hMSH6

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• The human MutS homolog 2-human MutS homolog 6 (hMSH2-hMSH6) complex plays a crucial role in DNA mismatch repair (MMR).
• Understanding the ATP binding and hydrolysis mechanisms of hMSH2-hMSH6 is vital for elucidating MMR pathways and associated diseases like Lynch syndrome.
• Coordination of ATP processing between hMSH2 and hMSH6 subunits is critical for complex function.

Purpose of the Study:

• To investigate the detailed mechanics of ATP binding and hydrolysis by the hMSH2-hMSH6 complex.
• To elucidate the role of magnesium ions in regulating ADP/ATP exchange and complex formation.
• To understand how these mechanisms contribute to the formation of the MMR sliding clamp.

Main Methods:

• Biochemical assays to study ATP binding and hydrolysis kinetics.
• Analysis of hMSH2-hMSH6 complex formation under varying magnesium and nucleotide conditions.
• DNA binding assays to characterize the sliding clamp formation and stability.

Main Results:

• hMSH2-hMSH6 function is strictly controlled by hMSH2 and magnesium in a complex with ADP.
• Magnesium destabilization triggers ADP release, facilitating high-affinity ATP binding by hMSH6, which in turn enhances ATP binding by hMSH2.
• Both subunits must be ATP-bound to form a stable, hydrolysis-independent hMSH2-hMSH6 sliding clamp essential for MMR, remaining on DNA for approximately 8 minutes in the presence of magnesium.

Conclusions:

• A precise, stepwise kinetic mechanism governs hMSH2-hMSH6 function, mimicking G protein switches.
• These findings refine models of DNA mismatch repair.
• The elucidated mechanism may offer insights into the MSH2 allele frequency observed in Lynch syndrome and hereditary nonpolyposis colorectal cancer.