How SecB maintains clients in a translocation competent state

Area of Science:

• Molecular Biology
• Protein Folding
• Bacterial Secretion

Background:

• Bacterial secretory proteins require specific chaperones to maintain solubility and prevent premature folding before reaching the SecYEG translocase.
• The SecB chaperone's precise mechanism in regulating client protein folding and its interaction dynamics remain incompletely understood.

Purpose of the Study:

• To elucidate the mechanism by which the SecB chaperone maintains a model client, maltose binding protein (MBP), in a non-folded, translocation-competent state.
• To investigate the interplay between SecB, the client protein's signal peptide, and the SecA translocase.

Main Methods:

• Single-molecule Förster Resonance Energy Transfer (smFRET) to monitor protein folding dynamics.
• Hydrogen-deuterium exchange mass spectrometry (HDX-MS) to assess protein structure and interactions.
• Analysis of client protein mutations affecting folding intermediates.

Main Results:

• SecB functions as an unfoldase, reverting partial folding, and subsequently as a holdase, inhibiting further folding of MBP.
• The presence of a signal peptide (SP) delays MBP folding and enhances its interaction stability with SecB.
• Mutations disrupting specific folding units (foldons) prolong the SecB-bound state of MBP.
• SecA forms a quaternary complex with MBP:SecB, further stabilizing the client's unfolded state before translocation.

Conclusions:

• SecB employs a dual unfoldase and holdase strategy to maintain secretory proteins in a translocation-ready state.
• The SecA translocase interacts with the SecB-client complex to ensure client stability until translocation.
• This study reveals a detailed molecular mechanism for chaperone-mediated protein translocation in bacteria.