[Endosomes and toxin translocation]

Area of Science:

• Cell Biology
• Molecular Toxicology
• Biochemistry

Background:

• Toxins such as ricin, diphtheria toxin (DT), and Pseudomonas exotoxin A (PE) pose significant threats to animal cells.
• The translocation of these toxins across the endosome membrane is a critical step determining their cellular toxicity.
• Understanding these mechanisms is key to developing targeted interventions.

Purpose of the Study:

• To investigate the distinct molecular strategies employed by ricin, DT, and PE for translocation into the cell cytosol.
• To elucidate the role of endosomal pH, ATP hydrolysis, and protein unfolding in the toxin entry process.
• To engineer a more potent Pseudomonas exotoxin A (PE) mutant through targeted genetic modification.

Main Methods:

• Purification of endosomes from lymphocytes for studying toxin translocation.
• Analysis of toxin entry mechanisms under varying conditions, including pH and ATP availability.
• Utilizing dihydrofolate reductase (DHFR) conjugates to assess protein unfolding during translocation.
• Employing a deletion approach to create and evaluate modified PE mutants.

Main Results:

• Ricin translocation depends solely on cytosolic ATP hydrolysis.
• Pseudomonas exotoxin A (PE) translocation requires initial exposure to low endosomal pH followed by ATP hydrolysis.
• Diphtheria toxin (DT) translocation is driven by the pH gradient between the endosome and cytosol.
• Evidence suggests ricin and PE require unfolding for membrane translocation.
• A modified PE mutant with enhanced translocation and increased cytotoxicity was generated.

Conclusions:

• Ricin, DT, and PE exhibit distinct mechanisms for crossing the endosome membrane, highlighting diverse cellular entry strategies.
• The findings provide insights into the biophysical requirements for toxin translocation, including pH, ATP, and protein conformation.
• Engineering of PE demonstrated the potential for developing more effective toxin-based agents or countermeasures.