Coincident localization of secretory and plasma membrane proteins in organelles of the yeast secretory pathway

Immunoelectron microscopy of Saccharomyces cerevisiae cells embedded in Lowicryl K4M has been used to localize invertase and plasma membrane (PM) ATPase in secretory organelles. sec mutant cells incubated at 37 degrees C were prepared for electron microscopy, and thin sections were incubated with polyclonal antibodies, followed by decoration with protein A-gold. Specific labeling of invertase was seen in the lumen of the endoplasmic reticulum, Golgi apparatus, and secretory vesicles in mutant cells that exaggerate these organelles. PM ATPase accumulated within the same organelles. Double-immune labeling revealed that invertase and PM ATPase colocalized in secretory vesicles. These results strengthen the view that secretion and plasma membrane assembly are biosynthetically coupled in yeast.