Dynamic conformational equilibria in the physiological function of the Bombyx mori pheromone-binding protein

Area of Science:

• Biochemistry
• Structural Biology
• Insect Physiology

Background:

• Bombyx mori pheromone-binding protein (BmorPBP) exhibits pH-dependent conformational changes crucial for pheromone binding.
• At basic pH, BmorPBP has an open cavity (BmorPBP(B)), while at acidic pH (4.5), helix α7 occupies this cavity (BmorPBP(A)).
• Helix α7 originates from the C-terminal dodecapeptide (131-142), which is disordered in the ligand-bound state.

Purpose of the Study:

• To investigate the mechanism of pheromone ligand ejection at low pH.
• To elucidate the role of the C-terminal dodecapeptide in BmorPBP's conformational dynamics and ligand binding.
• To understand the pH-dependent regulation of pheromone transport in Bombyx mori.

Main Methods:

• Solution Nuclear Magnetic Resonance (NMR) studies.
• Investigation of a truncated BmorPBP variant (BmorPBP(1-128)) lacking the C-terminal helix-forming peptide.
• Analysis of protein structure and dynamics across a pH range (6.5 to 4.5).

Main Results:

• The truncated BmorPBP(1-128) at pH 6.5 resembles the B-type conformation.
• At pH 4.5, BmorPBP(1-128) retains a B-type structure in slow equilibrium with an unobservable low-pH conformation.
• Complete NMR spectral recovery occurred upon returning to pH 6.5, indicating reversible conformational changes.

Conclusions:

• The C-terminal tetradecapeptide plays a dual role in reversible pheromone binding and transport.
• This peptide governs dynamic equilibria between protein conformations at acidic pH.
• It competes with the pheromone ligand for the internal binding cavity, facilitating ligand ejection.