Characterization of Yeast Extracellular Vesicles: Evidence for the Participation of Different Pathways of Cellular Traffic in Vesicle Biogenesis

TLDR

Yeast extracellular vesicles mediate transport of proteins, lipids, polysaccharides, and pigments across the cell wall, yet the cellular pathways that generate them remain largely unknown. The study aimed to characterize extracellular vesicle production in wild‑type and mutant *Saccharomyces cerevisiae* strains. Using transmission electron microscopy, light‑scattering analysis, lipid extraction, and proteomics, the authors examined WT cells and mutants defective in Sec4p or Snf7p, which impair Golgi‑derived exocytosis and multivesicular body formation, respectively. Bilayered vesicles 100–300 nm in diameter were isolated, and proteomic and lipid analyses revealed that mutations in both Golgi‑derived and MVB secretion pathways altered vesicle composition and release kinetics but did not abolish production, demonstrating that both conventional and unconventional pathways are required for yeast extracellular vesicle biogenesis.

Abstract

Background Extracellular vesicles in yeast cells are involved in the molecular traffic across the cell wall. In yeast pathogens, these vesicles have been implicated in the transport of proteins, lipids, polysaccharide and pigments to the extracellular space. Cellular pathways required for the biogenesis of yeast extracellular vesicles are largely unknown. Methodology/Principal Findings We characterized extracellular vesicle production in wild type (WT) and mutant strains of the model yeast Saccharomyces cerevisiae using transmission electron microscopy in combination with light scattering analysis, lipid extraction and proteomics. WT cells and mutants with defective expression of Sec4p, a secretory vesicle-associated Rab GTPase essential for Golgi-derived exocytosis, or Snf7p, which is involved in multivesicular body (MVB) formation, were analyzed in parallel. Bilayered vesicles with diameters at the 100–300 nm range were found in extracellular fractions from yeast cultures. Proteomic analysis of vesicular fractions from the cells aforementioned and additional mutants with defects in conventional secretion pathways (sec1-1, fusion of Golgi-derived exocytic vesicles with the plasma membrane; bos1-1, vesicle targeting to the Golgi complex) or MVB functionality (vps23, late endosomal trafficking) revealed a complex and interrelated protein collection. Semi-quantitative analysis of protein abundance revealed that mutations in both MVB- and Golgi-derived pathways affected the composition of yeast extracellular vesicles, but none abrogated vesicle production. Lipid analysis revealed that mutants with defects in Golgi-related components of the secretory pathway had slower vesicle release kinetics, as inferred from intracellular accumulation of sterols and reduced detection of these lipids in vesicle fractions in comparison with WT cells. Conclusions/Significance Our results suggest that both conventional and unconventional pathways of secretion are required for biogenesis of extracellular vesicles, which demonstrate the complexity of this process in the biology of yeast cells.