Saccharomyces cerevisiae mutants that have a post-Golgi block in the e
xocytic 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 centrif
ugation. The major population of vesicles contains Bgl2p, an endogluca
nase 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 p
opulations also contain exoglucanase activity; the major exoglucanase
normally secreted from the cell, encoded by EXG1, is carried in the po
pulation containing periplasmic enzymes. Electron microscopy shows tha
t both vesicle groups have an average diameter of 100 nm. The late sec
retory mutants sec1, sec4, and seed accumulate both vesicle population
s, 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 do
uble 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.