Parallel secretory pathways to the cell surface in yeast

Saccharomyces cerevisiae mutants that have a post-Golgi block in the exocytic pathway accumulate 100-nm vesicles carrying secretory enzymes as well as plasma membrane and cell-wall components. We have separated the vesicle markers into two groups by equilibrium isodensity centrifugation. The major population of vesicles contains Bg12p, an endoglucanase destined to be a cell-wall component, as well as Pma1p, the major plasma membrane ATPase. In addition, Snc1p, a synaptobrevin homologue, copurifies with these vesicles. Another vesicle population contains the periplasmic enzymes invertase and acid phosphatase. Both vesicle populations also contain exoglucanase activity; the major exoglucanase normally secreted from the cell, encoded by EXG1, is carried in the population containing periplasmic enzymes. Electron microscopy shows that both vesicle groups have an average diameter of 100 nm. The late secretory mutants sec1, sec4, and sec6 accumulate both vesicle populations, while neither is detected in wild-type cells, early sec mutants, or a sec13 sec6 double mutant. Moreover, a block in endocytosis does not prevent the accumulation of either vesicle species in an end4 sec6 double mutant, further indicating that both populations are of exocytic origin. The accumulation of two populations of late secretory vesicles indicates the existence of two parallel routes from the Golgi to the plasma membrane.