Parallel secretory pathways to the cell surface in yeast

Area of Science:

• Cell Biology
• Molecular Biology
• Yeast Genetics

Background:

• The exocytic pathway transports proteins and lipids to the cell surface.
• Understanding vesicle trafficking is crucial for cellular function.
• Saccharomyces cerevisiae serves as a model organism for studying eukaryotic cell biology.

Purpose of the Study:

• To investigate the nature and origin of vesicles accumulating in yeast mutants with a post-Golgi block.
• To determine if distinct vesicle populations exist within the late secretory pathway.
• To elucidate the mechanisms of protein and cell wall component transport in yeast.

Main Methods:

• Equilibrium isodensity centrifugation to separate vesicle populations.
• Analysis of vesicle markers (Bg12p, Pma1p, Snc1p, invertase, acid phosphatase, EXG1).
• Electron microscopy to determine vesicle size.
• Genetic analysis using various yeast secretory (sec) and endocytosis mutants.

Main Results:

• Two distinct 100-nm vesicle populations accumulate in late secretory mutants (sec1, sec4, sec6).
• One population contains cell wall (Bg12p) and plasma membrane (Pma1p) components, plus Snc1p.
• The second population carries periplasmic enzymes (invertase, acid phosphatase) and exoglucanase (EXG1).
• Both vesicle types originate from the exocytic pathway, not endocytosis.

Conclusions:

• The accumulation of two distinct vesicle populations indicates two parallel exocytic routes from the Golgi to the plasma membrane.
• These findings challenge a single, unified late secretory pathway.
• This provides new insights into the complex organization of vesicle trafficking in yeast.