Trans Golgi network

Constitutive exocytosis/secretion takes place in all eukaryotic cells and is essential for cell viability and growth. Trafficking vesicles destined for constitutive exocytosis originate from the trans-Golgi-network and contain secretory macromolecules derived from the... 

The mannose 6-phosphate receptor is the cargo/coat-receptor for trans-Golgi network (TGN)-derived cla-thrin vesicles. The receptor recognizes the mannose 6-phosphate tag of lysosomal hydrolases on the luminal side and the adaptor-1 complex of clathrin on the cytoplasmic face. 

Membrane proteins inside transport vesicles bud off the endoplasmic reticulum on their way to the Golgi final sorting of many membrane proteins occurs in the trans-Golgi network. 

A number of glycosylation reactions occur in compartments of the Golgi, and proteins are further sorted in the trans-Golgi network... 

Plasma membrane proteins are sorted to their final destinations at the trans-Golgi network 150... 

FIGURE 18-11 Regulation of neuropeptide expression is exerted at several levels. ER, endoplasmic reticulum LDCV, large dense-core vesicle TGN, trans-Golgi network. 

In spite of the variety of appearances of eukaryotic cells, their intracellular structures are essentially the same. Because of their extensive internal membrane structure, however, the problem of precise protein sorting for eukaryotic cells becomes much more difficult than that for bacteria. Figure 4 schematically illustrates this situation. There are various membrane-bound compartments within the cell. Such compartments are called organelles. Besides the plasma membrane, a typical animal cell has the nucleus, the mitochondrion (which has two membranes see Fig. 6), the peroxisome, the ER, the Golgi apparatus, the lysosome, and the endosome, among others. As for the Golgi apparatus, there are more precise distinctions between the cis, medial, and trans cisternae, and the TGN trans Golgi network) (see Fig. 8). In typical plant cells, the chloroplast (which has three membranes see Fig. 7) and the cell wall are added, and the lysosome is replaced with the vacuole. 

In a simplified view, the total flow is as follows (Fig. 8). Both soluble and membrane proteins that are translated at the membrane-bound ribosome are first localized at the ER. Some of them are transported to the Golgi apparatus, whereas others remain at the ER. At the Golgi apparatus, including the trans Golgi network (TGN), the next selection occurs some are transported to the plasma membrane, others to the endosome and to the lysosome/vacuole finally, and still others remain there. The lysosome is also an important organelle for the other transport system, the endocytic pathway. In this pathway, proteins at the plasma membrane are internalized by endocytosis. The sorting to lysosomes is treated in the next section. 

T. V. Kurzchalia, P. Dupree, R. G. Parton, R. Kellner, H. Virta, M. Lehnert, and K. Simons. VIP21, a 21-kD membrane protein is an integral component of trans-Golgi- network-derived transport vesicles. J. Cell Biol. 118 1003-1014(1992). 

Figure 1 The mode of action for bacterial AB-type exotoxins. AB-toxins are enzymes that modify specific substrate molecules in the cytosol of eukaryotic cells. Besides the enzyme domain (A-domain), AB-toxins have a binding/translocation domain (B-domain) that specifically interacts with a cell-surface receptor and facilitates internalization of the toxin into cellular transport vesicles, such as endosomes. In many cases, the B-domain mediates translocation of the A-domain into the cytosol by pore formation in cellular membranes. By following receptor-mediated endocytosis, AB-type toxins exploit normal vesicle traffic pathways into cells. One type of toxin escapes from early acidified endosomes (EE) into the cytosol, thus they are referred to as short-trip-toxins . In contrast, the long-trip-toxins take a retrograde route from early endosomes (EE) through late endosomes (LE), trans-Golgi network (TGN), and Golgi apparatus into the endoplasmic reticulum (ER) from where the A-domains translocate into the cytosol to modify specific substrates.

The Golgi apparatus (3) is a complex network, also enclosed, consisting of flattened membrane saccules ( cisterns ), which are stacked on top of each other in layers. Proteins mature here and are sorted and packed. A distinction is made between the ds, medial, and trans Golgi regions, as well as a trans Golgi network (tGN). The post-translational modification of proteins, which starts in the ER, continues in these sections. 

Figure 10-8 Current version of protein synthesis and processing via ER, Golgi, and secretory vesicles. CGN, ds-Golgi network C, T, M are the cis, medial, and trans compartments of the Golgi stack TGN, trans Golgi network. Arrows indicate some of the movements of transport vesicles.

In Chapter 11 the effects of binding of hormones to cell surface receptors have been emphasized. Equally important are the mechanisms that control the secretion of hormones. The topic of exocytosis has been considered briefly in Chapter 8, Section C,6 and aspects of the Golgi in Fig. 20-8 and associated text. Both hormones and neurotransmitters are secreted by exocytosis of vesicles. Cells have two pathways for secretion.386 387 The constitutive pathway is utilized for continuous secretion of membrane constituents, enzymes, growth factors, viral proteins, and components of the extracellular matrix. This pathway carries small vesicles that originate in the trans-Golgi network (TGN Fig. 20-8). The regulated pathway is utilized for secretion of hormones and neurotransmitters in response to chemical, electrical, or other stimuli. 

Schematic diagram showing the relative locations of nucleus, endoplasmic reticulum (ER), Golgi complex, trans-Golgi network, and plasma membrane. Glycoproteins synthesized in the lumen of the ER pass to the cis cisterna of the Golgi complex by a sequential membrane budding and fusion mechanism. The Golgi cisternae are classified into cis, medial, and trans in the order of increasing...

Fucose, galactose, and sialic acid residues are added in the trans-Golgi and trans-Golgi network compartments. The membranes of the Golgi compartments contain nucleotide-sugar translocases specific for transferring each nucleotide sugar across the membrane. 

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