Several minutes after proteins are synthesized in the rough ER, most of them leave the organelle within small membrane-bounded transport vesicles. These vesicles, which bud from regions of the rough ER not coated with ribosomes, carry the proteins to another membrane-limited organelle, the Golgi complex (see Figure 5-22).
Three-dimensional reconstructions from serial sections of a Golgi complex reveal this organelle to be a series of flattened membrane vesicles or sacs (cisternae), surrounded by a number of more or less spherical membrane-limited vesicles (Figure 5-23). The stack of Golgi cisternae has three de fined regions—the cis, the medial, and the trans. Transport vesicles from the rough ER fuse with the cis region of the Golgi complex, where they deposit their protein contents. As detailed in Chapter 17, these proteins then progress from the cis to the medial to the trans region. Within each region are different enzymes that modify proteins to be secreted and membrane proteins differently, depending on their structures and their final destinations.
After proteins to be secreted and membrane proteins are modified in the Golgi complex, they are transported out of the complex by a second set of vesicles, which seem to bud from the trans side of the Golgi complex. Some vesicles carry membrane proteins destined for the plasma membrane or soluble proteins to be released from the cell surface; others
▲ FIGURE 5-23 Model of the Golgi complex based on three-dimensional reconstruction of electron microscopy images. Transport vesicles (white spheres) that have budded off the rough ER fuse with the cis membranes (light blue) of the Golgi complex. By mechanisms described in Chapter 17, proteins move from the cis region to the medial region and finally to the trans region of the Golgi complex. Eventually, vesicles bud off the trans-Golgi membranes (orange and red); some move to the cell surface and others move to lysosomes. The Golgi complex, like the rough endoplasmic reticulum, is especially prominent in secretory cells. [From B. J. Marsh et al., 2001, Proc Nat'l. Acad. Sci USA 98:2399.]
carry soluble or membrane proteins to lysosomes or other organelles. How intracellular transport vesicles "know" with which membranes to fuse and where to deliver their contents is also discussed in Chapter 17.
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