Direct demonstration that homotetrameric chaperone SecB undergoes a dynamic dimer-tetramer equilibrium

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Structure

Background:

• SecB is a cytosolic bacterial chaperone essential for protein translocation.
• Chaperones play critical roles in maintaining cellular proteostasis.
• Understanding SecB's quaternary structure is key to its function.

Purpose of the Study:

• To elucidate the native quaternary structure and assembly dynamics of the bacterial chaperone SecB.
• To investigate the role of cysteine residues in SecB oligomerization.

Main Methods:

• Size exclusion chromatography to analyze protein size and interactions.
• Native polyacrylamide gel electrophoresis (native PAGE) to assess protein assembly.
• Chemical modification using 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) to probe disulfide bond involvement.

Main Results:

• SecB exists in a dynamic equilibrium between dimer and tetramer forms in solution.
• Mixing distinct tetrameric SecB species resulted in the formation of a hybrid tetramer, confirming dimer exchange.
• Chemical modification of cysteine residues with DTNB led to irreversible dissociation to dimers, indicating their role in stabilizing the tetramer.

Conclusions:

• SecB forms a structural dimer of dimers, with differential stability at the dimer-dimer interfaces.
• One dimer interface is less stable, allowing for dynamic dimer-tetramer equilibrium.
• Cysteine residues are critical for the stability of the tetrameric structure of SecB.