Saccharomyces cerevisiae (Baker's yeast)
Endocytosis is the process by which cells ingest fluid, molecules and particles. It involves the invagination of the plasma membrane and its subsequent pinching off to form endocytic vesicles (endosome). Some of the endocytosed components are recycled back to the plasma membrane while the rest are transported to the lysosomes where they are degraded. In the multivesicular-body (MVB) pathway, transmembrane proteins, such as receptors, and lipids are ingested in vesicles derived from the plasma membrane. They are then delivered to the early endosomes which mature into late endosomes through the formation of multivesicular bodies. These bodies move into the cell along microtubules and eventually become late endosomes by fusing with each other or preexisting late endosomes. The late endosome is then converted into the lysosome by proteins received from the trans Golgi network. The endocytosed proteins and lipids are degraded by hydrolytic enzymes in lysosomes. Entry of proteins into the MVB pathway and their final degradation in the lysozomes requires ubiquitination (binding of ubiquitin to the protein) and three protein complexes, "endosomal sorting complex required for transport" (ESCRT)-I, -II and -III.
The structure of the core of the Y-shaped ESCRT-II complex of yeast is shown here. One molecule of the vesicular protein sorting (Vps)22 and the carboxy-terminal domain of Vps36 bind to form one arm of Y. The coiled-coil of Vps22 which putatively binds other ESCRT complexes, along with the flexible linker with the ubiquitin-binding NZF domain of Vps36 to which the cargo (protein or lipid to be transported) binds, protrude from the arm of the Y. The other arm and the stalk of the Y are formed by two molecules of Vps25. The authors suggest a mode for the movement of the cargo across a scaffold of ESCRT complexes through the MVB pathway, based on this structure.
Protein Data Bank (PDB)
Hierro, A. Sun, J. Rusnak, A.S. Kim, J. Prag, G. Emr, S.D. Hurley, J.H.; "Structure of ESCRT-II endosomal trafficking complex"; Nature; (2004) 431:221-225 PubMed:15329733.
author: Ashwini Patil