Transient B12-dependent methyltransferase complexes revealed by small-angle X-ray scattering

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• The Wood-Ljungdahl pathway is crucial for carbon fixation in many microorganisms.
• Protein-protein interactions between methyltransferase (MeTr) and corrinoid iron-sulfur protein (CFeSP) are essential for methyl group transfer.
• Previous studies provided crystal structures of MeTr and CFeSP, but their solution structures and transient interactions remain uncharacterized.

Purpose of the Study:

• To investigate the transient interactions between MeTr and CFeSP in solution.
• To determine the solution structures and stoichiometry of CFeSP/MeTr complexes.
• To understand how solution conditions influence these protein-protein interactions.

Main Methods:

• Anaerobic small-angle X-ray scattering (SAXS) was employed to study protein interactions in solution.
• A global analysis approach was developed for deconvolution of heterogeneous protein mixtures.
• Anaerobic isothermal titration calorimetry (ITC) was used to complement SAXS data.

Main Results:

• Two distinct active CFeSP/MeTr complexes with different oligomeric compositions were identified in solution.
• One complex exhibited a 2:1 CFeSP/MeTr stoichiometry, similar to crystallographic findings.
• The other complex best fit a 1:1 CFeSP/MeTr arrangement, indicating solution-dependent binding cooperativity.

Conclusions:

• Solution conditions significantly affect the binding cooperativity between CFeSP and MeTr.
• SAXS is a valuable technique for elucidating the dynamic behavior of transient protein interactions in solution.
• The findings reveal a more complex solution behavior of CFeSP/MeTr interactions than previously suggested by crystallography.