The yeast endocytic membrane transport system.

Progress has been made recently in visualizing the structures and organelles responsible for endocytic membrane traffic from the cell surface to the lysosome-like vacuole in Saccharomyces cerevisiae. This, together with the recent discovery of several new membrane trafficking pathways connecting these organelles, has led to a quantum leap in our understanding of the S. cerevisiae endocytic pathway. We now know that although the cortical actin cytoskeleton is required for the internalization step of endocytosis, the internalization event occurs at furrow-like invaginations of the plasma membrane, which are distinct from cortical actin patches. Internalized material is taken into the cell in the form of small (30-50 nm diameter) vesicles and delivered to tubulo-vesicular early endosomes at the cell periphery. Subsequently, the internalized material arrives in multivesicular late endosomes adjacent to the vacuole. Recent microscopy evidence suggests that transfer from late endosomes to the vacuole may involve direct fusion of late endosomes with the vacuole. The visualization of the S. cerevisiae endocytic pathway has revealed similarities to endocytic pathways visualized in higher eukaryotes.