Interaction mapping between Saccharomyces cerevisiae Smc5 and SUMO E3 ligase Mms21

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• The Smc5-Smc6 holocomplex (Smc5/6) is essential for chromosome stability, DNA replication, homologous recombination, and DNA damage repair.
• While Smc5 and Smc6 form the core, the functions of associated non-SMC elements, including the SUMO E3 ligase Mms21 (Nse2), are not fully understood.
• Understanding the interaction between Smc5 and Mms21 is critical for elucidating Mms21's roles within the Smc5/6 complex.

Purpose of the Study:

• To precisely define the interaction region between the Smc5 and Mms21 subunits of the Saccharomyces cerevisiae Smc5/6 holocomplex.
• To characterize the binding stoichiometry and affinity of the Smc5-Mms21 interaction.
• To establish a robust methodology for analyzing large multiprotein complex structures.

Main Methods:

• Limited proteolysis coupled with mass spectrometry and N-terminal sequencing to map protein interaction interfaces.
• Yeast two-hybrid assays to identify initial interaction sites.
• X-ray crystallography to determine the overall molecular arrangement.
• Isothermal titration calorimetry (ITC) to quantify binding affinity and stoichiometry.

Main Results:

• The study precisely mapped the interaction region between Smc5 and Mms21.
• Isothermal titration calorimetry revealed that Mms21 binds to Smc5 with a 1:1 stoichiometry and a K(d) of 0.68 μM.
• The combination of biochemical and biophysical techniques provides a powerful approach for structural analysis of large protein complexes.

Conclusions:

• The precise interaction interface between Smc5 and Mms21 has been defined.
• The Smc5-Mms21 interaction is characterized by a high affinity and a 1:1 binding ratio.
• The employed methodological combination offers a versatile strategy for investigating the structure and function of large multiprotein assemblies like Smc5/6.