Figure 13–3 A “road-map” of the secretory and endocytic pathways. (a) In this schematic roadmap, which was introduced in chapter 12, the endocytic and secretory pathways are illustrated with green and red arrows, respectively. In addition, bluearrows denote retrieval pathways for the backflow of selected components. (b) The compartments of the eukaryotic cell involved in vesicle transport. The lumen of each membrane-enclosed compartment is topologically equivalent to the outside of the cell. all compartments shown communicate with one another and the outside of the cell by means of transport vesicles. In the secretory pathway (red arrows), protein molecules are transported from the er to the plasma membrane or (via endosomes) to lysosomes. In the endocytic pathway (green arrows), molecules are ingested in endocytic vesicles derived from the plasma membrane and delivered to early endosomes and then (via late endosomes) to lysosomes. Many endocytosed molecules are retrieved from early endosomes and returned (some via recycling endosomes) to the cell surface for reuse; similarly, some molecules are retrieved from the early and late endosomes and returned to the golgi apparatus, and some are retrieved from the golgi apparatus and returned to the er. all of these retrieval pathways are shown with blue arrows,as in part (a).
functional differences between its compartments. Finally, we consider the function of the Golgi apparatus, lysosomes, secretory vesicles, and endosomes, as we trace the pathways that connect these organelles.
The Molecular MechanIsMs of MeMbrane TransporT and The MaInTenance of coMparTMenTal dIversITy
Vesicle transport mediates a continuous exchange of components between the ten or more chemically distinct, membrane-enclosed compartments that collectively comprise the secretory and endocytic pathways. With this massive exchange, how can each compartment maintain its special identity? To answer this question, we must first consider what defines the character of a compartment. Above all, it is the composition of the enclosing membrane: molecular markers displayed on the cytosolic surface of the membrane serve as guidance cues for incoming traffic to ensure that transport vesicles fuse only with the correct compartment. Many of these membrane markers, however, are found on more than one compartment, and it is the specific combination of marker molecules that gives each compartment its molecular address.
How are these membrane markers kept at high concentration on one compartment and at low concentration on another? To answer this question, we need to consider how patches of membrane, enriched or depleted in specific membrane components, bud off from one compartment and transfer to another.
We begin by discussing how cells segregate proteins into separate membrane domains by assembling a special protein coat on the membrane’s cytosolic face. We consider how coats form, what they are made of, and how they are used to extract specific cargo components from a membrane and compartment lumen for delivery to another compartment. Finally, we discuss how transport vesicles dock at the appropriate target membrane and then fuse with it to deliver their cargo.
There are various Types of coated vesicles
Most transport vesicles form from specialized, coated regions of membranes. They bud off as coated vesicles, which have a distinctive cage of proteins covering their cytosolic surface. Before the vesicles fuse with a target membrane, they discard their coat, as is required for the two cytosolic membrane surfaces to interact directly and fuse.
The coat performs two main functions that are reflected in a common two-layered structure. First, an inner coat layer concentrates specific membrane proteins in a specialized patch, which then gives rise to the vesicle membrane. In this way, the inner layer selects the appropriate membrane molecules for transport. Second, an outer coat layer assembles into a curved, basketlike lattice that deforms the membrane patch and thereby shapes the vesicle.