Ratcheting in post-translational protein translocation: a mathematical model

Area of Science:

• Molecular Biology
• Biophysics
• Computational Biology

Background:

• Protein translocation across the endoplasmic reticulum (ER) membrane is crucial for cellular function.
• The role of molecular chaperones like BiP (Kar2p) in this process is complex and not fully understood.
• Existing models often simplify the dynamic nature of polypeptide movement.

Purpose of the Study:

• To develop a non-steady-state mathematical model for post-translational protein translocation.
• To investigate the mechanism of polypeptide movement biased by BiP (Kar2p) binding (ratcheting model).
• To analyze the efficiency and energetics of translocation against opposing forces.

Main Methods:

• Development of a non-steady-state mathematical model.
• Simulation of polypeptide chain movement as a stochastic process (Brownian ratchet).
• Fitting model parameters to experimental data to determine translocation dynamics.

Main Results:

• The model accurately describes experimental data, supporting the BiP (Kar2p) ratcheting mechanism.
• Translocation occurs against a significant free energy gradient (approx. 25 kJ/mol).
• The BiP ratchet is optimized for fast translocation, minimal ATP consumption, and efficient BiP dissociation.

Conclusions:

• The BiP (Kar2p) ratcheting model provides a robust explanation for protein translocation across the ER membrane.
• Polypeptide unfolding in the cytosol may provide the force driving translocation.
• The system is highly efficient, balancing speed, energy, and chaperone dynamics.