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A sizable fraction of proteins destined for ER are first synthesized in the cell cytosol and then transported across the ER membrane–a process called post-translational translocation. Similar to cotranslationally translocated proteins, these proteins also use the Sec translocon complex to enter the ER lumen.
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Bacterial protein secretion involves translocation systems to ensure proteins reach their designated locations, including the plasma membrane, periplasm, outer membrane, or the external environment. These translocation systems are vital for bacterial physiology, supporting processes like membrane assembly, enzymatic activity in the periplasm, and interactions with the external environment. The division of labor between Sec and Tat pathways ensures efficiency in handling proteins with diverse...
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Translocation of proteins across membranes is an ancient process that occurs even in bacteria and archaebacteria. In fact, the components of the translocation machinery are still conserved between prokaryotes and eukaryotes.
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Secretory vesicles, also known as dense core vesicles (DCVs), are membrane-bound vesicles that transport secretory proteins, such as hormones or neurotransmitters. Regulated secretory vesicles transport proteins from the trans-Golgi network to the exterior of the cell. Proteins present in regulated secretory vesicles are required to be rapidly exocytosed in large amounts upon a specific stimulus.
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Integral membrane proteins are proteins adhered to the lipid bilayer of a cell organelle or membrane. They can be of two types: transmembrane integral proteins that span the lipid bilayer and monotopic proteins that are attached to either side of the membrane but do not pass through it.
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How SecB maintains clients in a translocation competent state.

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SecB chaperone prevents bacterial secretory protein folding, acting as both an unfoldase and holdase. This ensures proteins remain translocation-competent until delivery to the SecYEG translocase.

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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.