Figure 13–71 Two ways of sorting plasma membrane proteins in a polarized epithelial cell. (a) In the direct pathway, proteins destined for different plasma membrane domains are sorted and packaged into different transport vesicles. The lipid-raft-dependent delivery system to the apical domain described in the text is an example of the direct pathway.
(b) In the indirect pathway, a protein is retrieved from the inappropriate plasma membrane domain by endocytosis and then transported to the correct domain via early endosomes—that is, by transcytosis. The indirect pathway, for example, is used in liver hepatocytes to deliver proteins to the apical domain that lines bile ducts.
Having selected a unique set of cargo molecules, the rafts then bud from the TGN whaT we don’T know into transport vesicles destined for the apical plasma membrane. This process is similar to the selective partitioning of some membrane proteins into the special-• how are targeting and fusion ized lipid domains in caveolae at the plasma membrane discussed earlier. proteins such as snares regulated,
Membrane proteins destined for delivery to the basolateral membrane con-so that they can be returned to their tain sorting signals in their cytosolic tail. When present in an appropriate struc-respective donor compartments in an inactive state?
tural context, these signals are recognized by coat proteins that package them into appropriate transport vesicles in the TGN. The same basolateral signals that are recognized in the TGN also function in early endosomes to redirect the proteins and endocytic events to keep its back to the basolateral plasma membrane after they have been endocytosed.
plasma membrane a constant size?
summary • can newly formed daughter cells generate a golgi apparatus de novo, or do they have to inherit it?
ion. Whereas the regulated pathways operate only in specialized secretory cells, a constitutive secretory pathway operates in all eukaryotic cells, characterized by continual vesicle transport from the TGN to the plasma membrane. In the regulated their own membranes?
pathways, the molecules are stored either in secretory vesicles or in synaptic vesicles, which do not fuse with the plasma membrane to release their contents until they • how does a cell maintain the receive an appropriate signal. Secretory vesicles containing proteins for secretion right amount of every component bud from the TGN. The secretory proteins become concentrated during the forma-(organelles, molecules), and how does tion and maturation of the secretory vesicles. Synaptic vesicles, which are confined it change these amounts as needed to nerve cells and some endocrine cells, form from both endocytic vesicles and from (for example, to greatly expand the endoplasmic reticulum when the cell endosomes, and they mediate the regulated secretion of small-molecule neurotrans needs to produce large amounts of mitters at the axon terminals of nerve cells.
secreted proteins)?
Proteins are delivered from the TGN to the plasma membrane by the constitutive pathway unless they are diverted into other pathways or retained in the Golgi apparatus. In polarized cells, the transport pathways from the TGN to the plasma membrane operate selectively to ensure that different sets of membrane proteins, secreted proteins, and lipids are delivered to the different domains of the plasma membrane.
Which statements are true? Explain why or why not.
13–1 In all events involving fusion of a vesicle to a target membrane, the cytosolic leaflets of the vesicle and target bilayers always fuse together, as do the leaflets that are not in contact with the cytosol.
13–2 There is one strict requirement for the exit of a protein from the ER: it must be correctly folded.
13–3 All the glycoproteins and glycolipids in intracellular membranes have oligosaccharide chains facing the lumenal side, and all those in the plasma membrane have oligosaccharide chains facing the outside of the cell.
Discuss the following problems.