Proteins that reside withinthe reticuloendothelial system or that are secreted from the plasma membrane enter the ER by cotranslational transfer directly from the ribosome. They are transported through the Golgi in the anterograde (forward) direction. Specific signals may cause them to be retained in the ER or a Golgi stack, or directed to other organelles such as endosomes. The default pathway is to be transported to the plasma membrane. Retrograde transport is less well characterized, but proteins that reside in the ER are retrieved from the Golgi by virtue of specific signals; an example is the C-terminal KDEL.
Proteins are transported between membranous surfaces as cargoes in membrane-bound coated vesicles. The vesicles form by budding from donor membranes; they unload their cargos by fusing with target membranes. The protein coats are added when the vesicles are formed and must be removed before they can fuse with target membranes. Anterograde transport does not result in any net flow of membrane from the ER to the Golgi and/or plasma membrane, so membrane moving with anterograde transport must be returned to the ER by a retrograde mechanism.
Modification of proteins by addition of a preformed oligosaccharide starts in the endoplasmic reticulum. High mannose oligosaccharides are trimmed. Complex oligosaccharides are generated by further modifications that are made during transport through the Golgi, determined by the order in which the protein encounters the enzymes localized in the various Golgi stacks. Proteins are sorted for different destinations in the trans Golgi. The signal for sorting to lysosomes is the presence of mannose-6-phosphate.
Different types of vesicles are responsible for transport to and from different membrane systems. The vesicles are distinguished by the nature of their protein coats.
COP-I-coated vesicles are responsible for retrograde transport from the Golgi to the ER. COP-I vesicles are coated with coatomer. One of the proteins of coatomer, β-COP, is related to the β-adaptin of clathrin-coated vesicles, suggesting the possibility of a common type of structure between COP-I-coated and clathrin-coated vesicles.
COP-II vesicles undertake forward movement from the ER to Golgi. Vesicles that transport proteins along the Golgi stacks have not yet been identified. Vesicles responsible for constitutive (bulk) movement from the Golgi to the plasma membrane also have not been identified. An alternative model for anterograde transport proposes that cis-Golgi cisternae actually become trans-Golgi cisternae, so that there is a continuous process of cisternal maturation from the cis to the trans face.
In the pathway for regulated secretion of proteins, proteins are sorted into clathrin-coated vesicles at the Golgi trans face. Some vesicles may fuse into (larger) secretory granules. Vesicles also move to endosomes, which control trafficking to the cell surface. Secretory vesicles are stimulated to unload their cargos at the plasma membrane by extracellular signals. Similar vesicles are used for endocytosis, the pathway by which proteins are internalized from the cell surface. The predominant protein in the outer coat of these vesicles is clathrin. The inner coat contains an adaptor complex, consisting of adaptin subunits, which bind to clathrin and to cargo proteins. There are (at least) three types of adaptor complex, with different specificities.
Budding and fusion of all types of vesicles is controlled by a small GTP-binding protein. This is ARF for clathrin and COP-I-coated vesicles, and Sar1P for COP-II-coated vesicles. When activated by GTP, ARF/Sar1p inserts into the membrane and causes coat proteins to assemble. This leads ultimately to budding. Further proteins, such as dynamin, may be required to "pinch off" the budding vesicle from the donor membrane. When ARF/Sar1p is inactivated because GTP is hydrolyzed to GDP, it withdraws from the membrane and the coat proteins either disassemble spontaneously (COP-coated vesicles) or are caused to do so by other proteins (clathrin-coated vesicles).
Vesicles initially recognize appropriate target membranes by a tethering reaction in which a tethering complex recognizes a Rab protein on the vesicle and brings the vesicle close to the membrane. Rabs are prenylated monomeric GTP-binding proteins. The fusion reaction is triggered when a v-SNARE on the vesicle pairs specifically with a t-SNARE on the target membrane. Pairing occurs by a coiled-coil interaction in which the SNARE complex lies parallel to the membrane surface. This causes the inner leaflets of the membranes to fuse to form a hemifusion complex; this is followed by fusion of the outer leaflets . The 20S fusion complex includes the soluble ATPase NSF and SNAP, and uses hydrolysis of ATP to release the SNAREs after pairing, which allows them to recycle.
Receptors may be internalized either continuously or as the result of binding to an extracellular ligand. Receptor-mediated endocytosis initiates when the receptor moves laterally into a coated pit. The cytoplasmic domain of the receptor has a signal that is recognized by proteins that are presumed to be associated with the coated pit. An exposed tyrosine located near the transmembrane domain is a common signal; it may be part of the sequence NPXY. When a receptor has entered a pit, the clathrin coat pinches off a vesicle, which then migrates to the early endosome.
The acid environment of the endosome causes some receptors to release their ligands; the ligand are carried to lysosomes, where they are degraded, and the receptors are recycled back to the plasma membrane by means of coated vesicles. A ligand that does not dissociate may recycle with its receptor. In some cases, the receptor-ligand complex is carried to the lysosome and degraded.