Interaction between human mismatch repair recognition proteins and checkpoint sensor Rad9-Rad1-Hus1

Area of Science:

• Molecular Biology
• Cell Cycle Regulation
• DNA Repair Mechanisms

Background:

• Eukaryotic cells utilize the 9-1-1 complex (Rad9, Rad1, Hus1) and mismatch repair (MMR) factors like hMutS alpha for DNA integrity.
• The 9-1-1 complex is structurally analogous to PCNA, a known interactor with hMutS alpha.
• MNNG-induced G2/M cell cycle arrest involves hMutS alpha and hMutL alpha, highlighting their role in DNA damage response.

Purpose of the Study:

• To investigate the physical and functional interactions between the 9-1-1 complex subunits and MMR proteins (hMSH2, hMSH3, hMSH6).
• To determine the role of the 9-1-1 complex in hMutS alpha-mediated DNA binding and MNNG-induced DNA damage response.

Main Methods:

• Co-immunoprecipitation assays to assess physical interactions between 9-1-1 subunits and MMR proteins.
• DNA binding assays to evaluate the effect of the 9-1-1 complex and its subunits on hMutS alpha binding to G/T-containing DNA.
• Immunofluorescence microscopy in HeLa cells (wild-type, MSH6-deficient, and Rad9-knockdown) to observe protein localization after MNNG treatment.

Main Results:

• Each subunit of the human 9-1-1 complex physically interacts with hMSH2, hMSH3, and hMSH6.
• The 9-1-1 complex and its individual subunits (Rad9, Rad1, Hus1) significantly stimulate hMutS alpha binding to G/T-containing DNA.
• Human Rad9 and hMSH6 colocalize to nuclear foci upon MNNG exposure, dependent on MSH6 presence. Rad9 knockdown affects MSH6 localization.

Conclusions:

• The 9-1-1 complex physically interacts with key mismatch repair proteins, including hMutS alpha.
• The 9-1-1 complex enhances the DNA binding activity of hMutS alpha, suggesting a role in DNA damage recognition.
• These findings indicate that the 9-1-1 complex is an integral component of the mismatch repair pathway involved in responding to MNNG-induced DNA damage.