Golgi apparatus lysosomal proteins

From the Golgi apparatus, the proteins are transported by vesicles to various targets in the cells—e.g., to lysosomes (4), the plasma membrane (6), and secretory vesicles (5) that release their contents into the extracellular space by fusion with the plasma membrane (exocytosis see p. 228). Protein transport can either proceed continuously (constitutive), or it can be regulated by chemical signals. The decision regarding which pathway a protein... 

Impelled by fascination with natnral NRs such as mitochondria, nucleus, Golgi apparatus, lysosomes, and the pores of channel proteins, and against the background of host-gnest chemistry, the concept of the NR emerged in the late 1990s [9], A NR is not a simple nanosize reaction vessel like bench-top or microreactor [10]. The size, shape, and microenvironment within a NR have a remarkable role in the chemical process and can dictate new activity and selectivity [7,11], The yield, kinetic, and mechanism of reaction taking place in the NR may be different from what is expected in the bulk solvent due to encapsulation effects (vide infra) [10],... 

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. 

In addition to their plasma membrane eukaryotic cells also contain internal membranes that define a variety of organelles (fig. 17.2). Each of these organelles is specialized for particular functions The nucleus synthesizes nucleic acids, mitochondria oxidize carbohydrates and lipids and make ATP, chloroplasts carry out photosynthesis, the endoplasmic reticulum and the Golgi apparatus synthesize and secrete proteins, and lysosomes digest proteins. Additional membranes divide mitochondria and chloroplasts into even finer, more specialized subcompartments. Like the plasma membrane, organellar membranes act as barriers to the leakage of proteins, metabolites, and ions they contain transport systems for import and export of materials, and they are the sites of enzymatic activities as diverse as cholesterol biosynthesis and oxidative phosphorylation. 

Both endocytosis of material at the plasma membrane and exocytosis from the Golgi apparatus involve the formation of clathrin-coated pits and vesicles. On the cytosolic side of the membrane these structures have an electron-dense coat consisting mainly of the protein clathrin, the polypeptides of which form a three-legged structure known as a triskelion. The clathrin triskelions assemble into a basket-like convex framework that causes the membrane to invaginate at that point and eventually to pinch off and form a vesicle. In endocytosis these clathrin-coated vesicles migrate into the cell where the clathrin coats are lost before delivering their contents to the lysosomes. 

Membranes serve to delineate cellular entities, in which case they are termed plasma membranes, and various subcellular particles, such as mitochondria, nuclei, and lysosomes. Such subcellular structures as endoplasmic reticulum and the Golgi apparatus also consist, in large part, of membranous materials. The basic structure of membranes is lipid in nature, and it is the amphipathic nature of such lipid that permits them to aggregate into bilayers. Lipid is not the only component of membranes, however there are protein and carbohydrate components as well. It is these components that function as the means by which hydrophilic substances can cross membranes or that serve as recognition beacons that permit various substances to affect the metabolic activities of cellular and subcellular particles. 

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