Nascent SecM chain interacts with outer ribosomal surface to stabilize translation arrest

Area of Science:

• Molecular Biology
• Bacterial Protein Synthesis
• Gene Regulation

Background:

• SecM is a bacterial secretion monitor protein that regulates gene expression by arresting translation elongation.
• The C-terminal arrest sequence of SecM is known to stop translation within the ribosomal exit tunnel.
• Previous studies suggested the nascent SecM chain outside the ribosome stabilizes translation arrest, but the mechanism was unclear.

Purpose of the Study:

• To elucidate the molecular mechanism by which the nascent SecM chain stabilizes translation arrest.
• To identify specific residues in the nascent SecM chain responsible for stabilizing translation arrest.

Main Methods:

• Alanine/serine-scanning mutagenesis of residues 57-98 in the nascent SecM chain.
• Prediction of secondary structure, including alpha-helix formation.
• Photocross-linking experiments to determine proximity to ribosomal components.

Main Results:

• Residues 57-98 of the nascent SecM chain were identified as crucial for stabilizing translation arrest.
• Specific key residues (D79, Y80, W81, H84, R87, I90, R91, F95) were pinpointed for their role in stabilization.
• These residues are located on or near an alpha-helix with an arginine patch that likely interacts with the negatively charged ribosomal surface.
• Photocross-linking revealed that Y80 is positioned near ribosomal protein L23, adjacent to the exit tunnel.

Conclusions:

• The folded nascent SecM chain emerging from the ribosomal exit tunnel interacts with the ribosome's outer surface.
• This interaction, mediated by specific residues and an alpha-helical structure, stabilizes translation arrest.
• The study reveals a novel mechanism of translational control involving nascent polypeptide interactions with the ribosome.