Signal recognition particle prevents N-terminal processing of bacterial membrane proteins

Area of Science:

• Molecular Biology
• Immunology
• Bacterial Pathogenesis

Background:

• Bacterial proteins initiate synthesis with N-formylated methionine, a formyl group often removed co-translationally by peptide deformylase.
• N-formylated peptides are recognized by the innate immune system, suggesting a role in bacterial infection responses.
• The precise source of these immunologically active N-formylated peptides remains unclear.

Purpose of the Study:

• To investigate the mechanism regulating the retention of N-terminal formyl groups on bacterial proteins.
• To identify factors that prevent the co-translational deformylation of bacterial proteins.
• To elucidate the origin of N-formylated peptides involved in innate immunity.

Main Methods:

• Development of a novel assay to measure peptide deformylase activity on translating ribosomes.
• Investigating the effect of signal recognition particle (SRP) binding to nascent polypeptide chains on deformylation.
• Assessing the influence of trigger factor chaperone on deformylation and SRP inhibition.

Main Results:

• The binding of SRP to signal sequences on nascent ribosomal proteins inhibits peptide deformylase activity.
• Deformylation of nascent proteins lacking signal sequences is not affected by SRP.
• The trigger factor chaperone does not influence either deformylation or SRP-mediated inhibition of deformylation.

Conclusions:

• SRP binding to signal sequences prevents the removal of the N-terminal formyl group during protein synthesis.
• Bacterial inner-membrane proteins, particularly those with N-out topology, can retain their N-terminal formyl group.
• These retained formyl groups on specific bacterial proteins may serve as the source of immunologically active peptides during infection.