![]() Cargo receptors span the membrane to bridge cargo and coat Understanding how local cooperative effects of interactions among cargo, lipid, adaptor, and scaffold components mutually influence vesicle formation remains a key question in many vesicle transport events. Finally, adaptors bind directly to the structural scaffolding elements of the various coats, thus bridging the membrane, cargo, and membrane deformation machinery. However, cargo adaptors also often have affinity for lipids, which can contribute both to the specificity of recruitment and to the global affinity of the adaptor for a donor organelle. During the lifetime of the vesicle, coat proteins are shed from the vesicle surface to expose fusion machinery therefore, interactions between coat and vesicle components must be reversible. Most binary cargo–coat interactions measured in vitro are relatively low affinity, which may be important in the context of coat dynamics during traffic. Interaction between coat and signal is responsible for capture of cargo into the forming vesicles. Central to the appropriate sorting of cargo, specific coat subunits (known as cargo adaptors) contain binding surfaces that recognize sorting signals present in the cytoplasmic domains of cargo proteins. Subsequent biochemical, structural, and genetic dissection of clathrin and other vesicle systems has defined how these different coat assemblies couple cargo sorting with the general formation of vesicles. Studies on the internalization of cell surface receptors via clathrin-mediated endocytosis first established the principle that specific protein-based signals mediate capture of cargo into vesicles. By coupling cargo selection to vesicle formation, cells can achieve efficient protein sorting as an in-built outcome of the transport pathway itself. Vesicle coats perform two central functions: deforming the membrane into a spherical vesicle and populating the vesicle with specific cargo. COPII-coated vesicles transport cargo proteins from the ER to the Golgi COPI-coated vesicles transport cargo in the retrograde direction (from the cis-Golgi back to the ER) and between Golgi cisternae and clathrin-coated vesicles form from the plasma membrane and the TGN to fuse with endosomes or lysosomes ( Fig. The three main vesicular frameworks found across eukaryotic life (clathrin, COPI, and COPII) come from evolutionarily related coat proteins. Conserved sets of cytoplasmic proteins generate distinct classes of transport vesicles, which are largely classified by the protein coats that drive their formation. This fission and fusion transport strategy allows secretory proteins to cross membrane barriers without perturbing the functional segregation conferred by organelles. Transport of proteins between organelles within the secretory pathway occurs via spherical membrane-bounded vesicles that bud from a donor organelle and fuse with an acceptor in another part of the cell. Principles of selective capture into transport vesicles Here, we consider how cells satisfy the sorting needs of the diverse set of proteins that navigate the ER–Golgi interface, an impressive feat considering the extent of cargo protein heterogeneity. Finally, retrieval from the Golgi to the ER ensures that immature cargoes or escaped ER resident proteins are efficiently transported back to the ER. However, traffic can also occur in a nonselective manner called bulk flow. Many secretory proteins are actively sorted during ER export. At this point, cells seem to distinguish between native and nonnative proteins, ensuring that only appropriately folded and assembled cargo protein undergo forward transport. At the ER, proteins destined for the extracellular space or to organelles along the route are packaged into vesicles that transport them to the Golgi apparatus. ER-to-Golgi transport is the first step in the secretory pathway. Between 20% and 30% of the cell’s proteome is destined for either the extracellular environment or the internal endomembrane system. Yet, within the connected compartments of the secretory pathway, this material continuously exchanges as membranes and cargo proteins undergo dynamic traffic. Eukaryotic cells are highly compartmentalized, with separate organelles each characterized by specific protein and lipid compositions. ![]()
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