DNA mismatch repair proteins MLH1 and PMS2 can be imported to the nucleus by a classical nuclear import pathway

Area of Science:

• Molecular biology
• Cancer research
• Protein structure and function

Background:

• The MutLα heterodimer, composed of MLH1 and PMS2 proteins, is essential for DNA mismatch repair (MMR) in humans.
• Defects in MMR proteins are linked to various cancers, particularly colon cancer.
• Nuclear localization of MutLα is critical for its function, mediated by Nuclear Localization Sequences (NLSs) in its linker region.

Purpose of the Study:

• To investigate the binding interactions of putative MLH1 and PMS2 NLSs with importin-α.
• To clarify the controversial NLS sequences within the MLH1 and PMS2 proteins.
• To understand the role of MLH1 and PMS2 in the nuclear transport of the MutLα heterodimer.

Main Methods:

• Crystal structure determination of importin-α bound to MLH1 and PMS2 NLS peptides.
• Isothermal titration calorimetry to quantify binding affinities between NLS peptides and importin-α.

Main Results:

• Both MLH1 and PMS2 NLS peptides bind to importin-α as monopartite NLSs.
• MLH1-NLS demonstrates a significantly higher binding affinity to importin-α compared to PMS2-NLS.
• These findings support previous studies suggesting distinct roles in nuclear import.

Conclusions:

• MLH1 plays a more dominant role in the nuclear import of the MutLα heterodimer compared to PMS2.
• The differential binding affinities provide insights into the regulation of MutLα nuclear transport.
• Understanding these mechanisms is crucial for cancer research and therapeutic strategies targeting MMR deficiency.