Structure and stability of monomeric lambda repressor: NMR evidence for two-state folding

Area of Science:

• Biochemistry
• Structural Biology
• Biophysics

Background:

• Protein folding reactions are complex, often involving intermediate states that complicate analysis.
• Proving a protein folds via a two-state mechanism (without intermediates) requires rigorous evidence.
• The N-terminal domain of bacteriophage lambda repressor (lambda 6-85) is a suitable model for studying folding.

Purpose of the Study:

• To rigorously test for the absence of equilibrium intermediates in the folding of lambda 6-85.
• To validate the use of proton chemical shifts as sensitive local probes for detecting partially folded species.
• To compare folding behavior using local probes (chemical shifts) and a global probe (circular dichroism).

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy to assign resonances and analyze structure.
• Circular Dichroism (CD) spectroscopy to monitor global structural changes.
• Thermal and urea denaturation experiments using chemical shifts and CD signals.

Main Results:

• Proton chemical shifts and CD signals showed identical denaturation curves across various conditions.
• Analysis of chemical shifts, hydrogen exchange, coupling constants, and NOEs confirmed structural similarity to known forms.
• No evidence of populated intermediates was found during the denaturation of lambda 6-85.

Conclusions:

• The bacteriophage lambda repressor N-terminal domain (lambda 6-85) exhibits strict two-state folding.
• Proton chemical shifts provide a sensitive and reliable method for detecting folding intermediates.
• This study provides strong evidence for the absence of populated intermediates in lambda 6-85 folding.