Spin label method reveals barnase-barstar interaction: a temperature and viscosity dependence approach

Area of Science:

• Biochemistry
• Structural Biology
• Biophysics

Background:

• Barnase (Bn) is an enzyme inhibited by barstar (Bs).
• Understanding protein-protein interactions is crucial in molecular biology.
• Spin labeling is a technique used to probe molecular dynamics.

Purpose of the Study:

• To investigate the protein-protein interaction between barnase (Bn) and barstar (Bs) using spin labeling.
• To characterize the structural dynamics of barstar (Bs) upon binding to barnase (Bn).

Main Methods:

• Site-directed mutagenesis of barstar (Bs) to create a single cysteine mutant (C40A).
• Selective modification of the C40A barstar mutant with two different spin labels.
• Electron Paramagnetic Resonance (EPR) spectroscopy to analyze spin label mobility.
• Temperature-viscosity dependence approach to determine order parameter (S) and rotational correlation time (tau).

Main Results:

• Binding of barnase (Bn) to barstar (Bs) significantly restricted spin label mobility at residue C82.
• The order parameter (S) increased upon complex formation, indicating reduced local motion.
• Rotational correlation times (tau) correlated with molecular mass, suggesting rigid protein structures for barstar (Bs), its complex with barnase (Bn), and barstar dimers.
• Formation of barstar-barstar complexes was also detected via restricted spin label mobility.

Conclusions:

• Spin labeling effectively monitors barnase-barstar complex formation and protein rigidity.
• The study confirms that barstar, its complex with barnase, and barstar dimers behave as rigid entities.
• The employed EPR-based methodology is applicable to studying various spin-labeled macromolecular complexes.