Endocytotic uptake and retrograde transport of a virally encoded killer toxin in yeast

Area of Science:

• Cell Biology
• Molecular Biology
• Mycology

Background:

• The K28 killer toxin is a viral protein that kills sensitive yeast strains.
• It functions by blocking DNA synthesis in the nucleus.
• The toxin is a heterodimer requiring specific processing and transport mechanisms.

Purpose of the Study:

• To elucidate the mechanism of K28 killer toxin entry and intracellular transport in yeast.
• To identify the key molecular signals and pathways involved in toxin activity.
• To understand the role of the secretory pathway in toxin function.

Main Methods:

• Utilized yeast end3/4 mutants, Deltaerd2 (lacking HDEL receptor), and erd1 mutants.
• Employed site-directed mutagenesis to investigate the beta-HDEL motif.
• Assessed toxin resistance and biological activity.
• Investigated ER-to-cytosol export using Sec61p, Kar2p, and Cne1p.

Main Results:

• K28 toxin enters target cells via endocytosis.
• The HDEL motif at the beta-C-terminus is crucial for retrograde transport within the secretory pathway.
• Mutations affecting endocytosis, HDEL receptor, or Golgi-to-ER recycling confer toxin resistance.
• Kex1p cleavage unmasks the HDEL signal in the Golgi.
• ER-to-cytosol export involves the Sec61p translocon and ER chaperones Kar2p and Cne1p.

Conclusions:

• The K28 killer toxin utilizes a novel retrograde transport mechanism within the yeast secretory pathway.
• Endocytosis, HDEL-mediated retrieval, and ER-associated degradation pathways are essential for toxin activity.
• Understanding this pathway provides insights into protein trafficking and cellular defense mechanisms in yeast.