Vesicular Traffic Secretion And Endocytosis

Electron micrograph of clathrin cages, like those that surround clathrin-coated transport vesicles, formed by the in vitro polymerization of clathrin heavy and light chains. [John Heuser, Washington University School of Medicine.]

In the previous chapter we explored how proteins are targeted to and translocated across the membranes of different intracellular organelles. In this chapter we turn our attention to the mechanisms that allow soluble and membrane proteins synthesized on the rough endoplasmic reticulum (ER) to move to their final destinations via the secretory pathway. A single unifying principle governs all protein trafficking in the secretory pathway: transport of membrane and soluble proteins from one membrane-bounded compartment to another is mediated by transport vesicles that collect " cargo" proteins in buds arising from the membrane of one compartment and then deliver these cargo proteins to the next compartment by fusing with the membrane of that compartment. Importantly, as transport vesicles bud from one membrane and fuse with the next, the same face of the membrane remains oriented toward the cytosol. Therefore once a protein has been inserted into the membrane or the lumen of the ER, the protein can be carried along the secretory pathway, moving from one organelle to the next without being translocated across another membrane or altering its orientation within the membrane.

Figure 17-1 outlines the major routes for protein trafficking in the secretory pathway. Once newly synthesized proteins are incorporated into the ER lumen or membrane as discussed in Chapter 16, they can be packaged into anterograde (forward-moving) transport vesicles. These vesicles fuse with each other to form a flattened membrane-bounded compartment known as the cis-Golgi cisterna. Certain proteins, mainly ER-localized proteins, are retrieved from the cis-Golgi to the ER via a different set of retrograde (backward-moving) transport vesicles. A new cis-Golgi cisterna with its cargo of proteins physically moves from the cis position (nearest the ER) to the trans position (farthest from the ER), successively becoming first a medial-Golgi cisterna and then a trans-Golgi cisterna. This process, known as cisternal progression, does not involve the budding off and fusion of anterograde transport vesicles. During cisternal progression, enzymes and other Golgi-resident proteins are constantly being retrieved from later to earlier Golgi cisternae by retrograde transport vesicles, thereby remaining localized to the cis-, medial-, or trans-Golgi cisternae.

Proteins in the secretory pathway that are destined for compartments other than the ER or Golgi eventually reach a complex network of membranes and vesicles termed the trans-Golgi network (TGN). From this major branch point in the secretory pathway, a protein can be loaded into one

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