Precursor in cotranslational secretion of diphtheria toxin

Area of Science:

• Molecular Biology
• Microbiology
• Cell Biology

Background:

• Diphtheria toxin is a critical virulence factor produced by Corynebacterium diphtheriae.
• Understanding the synthesis and secretion pathway of diphtheria toxin is crucial for developing targeted interventions.

Purpose of the Study:

• To elucidate the site of diphtheria toxin synthesis and secretion.
• To identify and characterize any precursor forms of diphtheria toxin.

Main Methods:

• Extracellular labeling of peptides from intact Corynebacterium diphtheriae.
• Cell fractionation and completion of nascent peptide chains by isolated polysomes in vitro.
• Immune precipitation and enzymatic assays to characterize toxin and precursor molecules.

Main Results:

• Diphtheria toxin is cotranslationally formed and secreted by membrane-bound polysomes, not free polysomes.
• A 68,000-dalton precursor molecule is identified, which is converted to 62,000-dalton diphtheria toxin.
• The precursor molecule possesses N-terminal characteristics distinct from the mature toxin, indicating an N-terminal extension.

Conclusions:

• Diphtheria toxin is synthesized and secreted via a cotranslational mechanism involving membrane-bound polysomes.
• A membrane-associated enzymatic process is responsible for processing the diphtheria toxin precursor into its mature, secreted form.